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C# Basic Level Interview Questions for all Levels

November 7, 2020 by admin

//#1: What will be the output of the following code

            int DoSomething(int a, int b)

            {

                return 0;

            }

            float DoSomething(int c, int d);

            {

                return 0;

            }

            int a = DoSomething(10, 20);

            float b = DoSomething(30, 40);

Ans: Method Overloading cannot be done by Return types

//#2: OutPut for following Program

  class A

    {

        public virtual void Func1()

        {

            Console.WriteLine(“A:Func1”);

        }

        public void Func2()

        {

            Console.WriteLine(“A:Func2”);

        }

    }

    class B : A

    {

        public override void Func1()

        {

            Console.WriteLine(“B:Func1”);

        }

        public new void Func2()

        {

            Console.WriteLine(“B:Func2”);

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            Console.WriteLine(“Hello World!”);

            A a = new B();

            a.Func1();//B:Func1

            a.Func2();//A:Func2

     B a = new B();

            a.Func1();//B:Func1

            a.Func2();//B:Func2

        }

    }

 // Note: B a = new A(); // Not allowed

#3: Ignore Case Code in C#

            string s1 = “SHArad”;

            string s2 = “sharad”;

            var result = String.Compare(s1, s2, StringComparison.OrdinalIgnoreCase);

#4: State difference in throw & throw Ex

try

{

}

catch (Exception ex)

{

       throw;

       throw ex;

}

private static void ThrowException1() {
    try {
        DivByZero(); // line 34
    } catch {
        throw; // line 36
    }
}
private static void ThrowException2() {
    try {
        DivByZero(); // line 41
    } catch (Exception ex) {
        throw ex; // line 43
    }
}

Exception 1:

   at UnitTester.Program.DivByZero() in <snip>\Dev\UnitTester\Program.cs:line 49

   at UnitTester.Program.ThrowException1() in <snip>\Dev\UnitTester\Program.cs:line 36

   at UnitTester.Program.TestExceptions() in <snip>\Dev\UnitTester\Program.cs:line 19

Exception 2:

   at UnitTester.Program.ThrowException2() in <snip>\Dev\UnitTester\Program.cs:line 43

   at UnitTester.Program.TestExceptions() in <snip>\Dev\UnitTester\Program.cs:line 25

Thus, “throw” maintains the full hierarchy in the stack trace and gives complete information about the exception occurred in the code. Whereas “throw ex” pretends that exceptions occurred on the line where “throw ex” was written and removes all the hierarchy above the method containing the “throw ex” expression.

//#5: Write the output of the following Program

public class Employee

{

        public string FirstName { get; set; }

}

Main(){

     Employee e1 = new Employee();

            e1.FirstName = “Suresh”;

            ProcessEmployee(e1);

            Console.WriteLine(e1.FirstName);

            void ProcessEmployee(Employee e1)

            {

                e1.FirstName = “Abhijeet”;

            }

}

Ans: Answer will be Abhijeet

//#6: OutPut of following Code

     int i = 0;

            int n;

            n = i++;

            Console.WriteLine(n);

            n = ++i;

            Console.WriteLine(n);

Ans:

0

2

//#7: Output of following Code

     var s = “Shashikant”;

            s = 10;

            Console.WriteLine(s);

Ans: Error : Cannot implicitly convert int to string

//#8: Write a program to find largest of 3 unique integers in a single statement.

//int a, b, c;

            int a = 5, b = 3, c = 2;

            Console.WriteLine((a > b ? a : b) > c ? (a > b ? a : b) : c);

//#9: Write a program to print unique elements in an array.

      int[] items = { 2, 3, 5, 3, 7, 5 };

  1. static void Main(string[] args)  
  2.         {  
  3.             int[] items = { 2, 3, 5, 3, 7, 5 };  
  4.             int n = items.Length;  
  5.   
  6.             Console.WriteLine(“Unique array elements: “);  
  7.   
  8.             for(int i=0; i<n;i++)  
  9.             {  
  10.                 bool isDuplicate = false;  
  11.                 for(int j=0;j<i;j++)  
  12.                 {  
  13.                     if(items[i] == items[j])  
  14.                     {  
  15.                         isDuplicate = true;  
  16.                         break;  
  17.                     }  
  18.                 }  
  19.   
  20.                 if(!isDuplicate)  
  21.                 {  
  22.                     Console.WriteLine(items[i]);  
  23.                       
  24.                 }  
  25.             }  
  26.   
  27.             Console.ReadLine();  
  28.         }  

//#10: Association, Composition, & Aggregation

Association

Association is a relationship among the objects. Association is “*a*” relationship among objects. In Association, the relationship among the objects determine what an object instance can cause another to perform an action on its behalf. We can also say that an association defines the multiplicity among the objects. We can define a one-to-one, one-to-many, many-to-one and many-to-many relationship among objects. Association is a more general term to define a relationship among objects. 

Aggregation

Aggregation is a special type of Association. Aggregation is “*the*” relationship among objects. We can say it is a direct association among the objects. In Aggregation, the direction specifies which object contains the other object. There are mutual dependencies among objects.

Composition

Composition is special type of Aggregation. It is a strong type of Aggregation. In this type of Aggregation the child object does not have their own life cycle. The child object’s life depends on the parent’s life cycle. Only the parent object has an independent life cycle. If we delete the parent object then the child object(s) will also be deleted. We can define the Composition as a “Part of” relationship. E.g. “Company Location” under “Company”

//#11: Shadowing vs Overriding

Shadowing (method hiding)

A method or function of the base class is available to the child (derived) class without the use of the “overriding” keyword. The compiler hides the function or method of the base class. This concept is known as shadowing or method hiding. In the shadowing or method hiding, the child (derived) class has its own version of the function, the same function is also available in the base class.

Example

  1. Public class BaseClass  
  2. {  
  3.     public string GetMethodOwnerName()  
  4.     {  
  5.        return “Base Class”;  
  6.     }  
  7. }  
  8. public class ChildClass : BaseClass  
  9. {  
  10.     public new string GetMethodOwnerName()  
  11.     {  
  12.        return “ChildClass”;  
  13.     }  
  14. } 

Test Code

  1. static void Main(string[] args)  
  2. {  
  3.     ChildClass c = new ChildClass();  
  4.     Console.WriteLine(c.GetMethodOwnerName());  
  5. } 

Output

ChildClass

Overriding

Method overriding is an important feature of OOP that allows us to re-write a base class function or method with a different definition. Overriding is also known as “Dynamic polymorphism” because overriding is resolved at runtime. Here the signature of the method or function must be the same. In other words both methods (base class method and child class method) have the same name, same number and same type of parameter in the same order with the same return type. The overridden base method must be virtual, abstract or override.

Example

  1. public class BaseClass  
  2. {  
  3.     public virtual string GetMethodOwnerName()  
  4.     {  
  5.        return “Base Class”;  
  6.     }  
  7. }  
  8. public class ChildClass : BaseClass  
  9. {  
  10.    public override string GetMethodOwnerName()  
  11.    {  
  12.        return “Child Class”;  
  13.    }  
  14. } 

In overriding, the base class can be accessed using the child object’s overridden method.

Overriding Example

  1. static void Main(string[] args)  
  2. {  
  3.     BaseClass c = new ChildClass();  
  4.     Console.WriteLine(c.GetMethodOwnerName());  
  5. } 

Output

ChildClass

//#12: Diamond Box Problem in C#

The “diamond problem” is an ambiguity that arises when two classes B and C inherit from A, and class D inherits from both B and C. If there is a method in A that B and C have overridden, and D does not override it, then which class of the method does D inherit: that of B, or that of C? So this is an ambiguity problem in multiple inheritances in c#. So that c# does not support multiple inheritances. It also called an ambiguity problem in c#.

namespace CSharpConApp.DiamondProblem1

{

    interface IMother

    {

        void Loan();

    }

    interface IFather

    {

        void Loan();

    }

    public class Mother:IMother

    {

        public void Loan()

        {

            Console.WriteLine(“Loan taken by mother.”);

        }

    }

    public class Father:IFather

    {

        public void Loan()

        {

            Console.WriteLine(“Loan taken by father”);

        }

    }

    public class Child:IMother,IFather

    {

        void IMother.Loan()

        {            

            Console.WriteLine(“Mother’s loan paid by child”);

        }

        void IFather.Loan()

        {

            Console.WriteLine(“Father’s loan paid by child”);

        }

    }

   public class DiamondProblem1

    {

        static void Main(string[] args)

        {

            Child child = new Child();

            ((IMother)child).Loan();

            ((IFather)child).Loan();

            Console.ReadLine();

        }

    }

}

                                         ***

Deploy Angular 7 App to GitHub

November 5, 2020 by admin

Article By: Nikhil Mittal

Angular 7 Code is running in Visual Studio Code

Web API and SQL Server are the Service & DB Layer

Step 1:

git remote add origin https://github.com/USERNAME/REPOSITORY_NAME.git

Check above:

Git remote –v

Above path should be present in output as fetch and push

Step 2:

npm install -g angular-cli-ghpages

Step 3:

In your routes configuration add

RouterModule.forRoot(routes, {useHash: true});

In your index.html do this;

<!– <base href=”/”> –>

<script>document.write(‘<base href=”‘ + document.location + ‘” />’);</script>

Step 4:

Edit the Output variable in config: dist/Project_Name to dist

Step 5:

ng build –prod –base-href ./

Step 6:

ngh –no-silent –email=<GitHub_RegisteredEmail> –name=<GitHub_UserName>

Step 7: (For Version Control Add Project into master)

git init

git add README.md

git commit -m “first commit”

git remote add origin https://github.com/<GitHub_UserName>/<RepositoryName>.git

git push -u origin master

Now you can browse your repository as well as website in Github pages

Step 8: <To Add Custom Domain>

Go to Repository settings

Enter Custom Domain URL > Save

Remove Enforce Https option

Change A and CNAME in your personal domain

The DNS propagation might take 48 Hrs. to reflect the website from GitHub Pages

Step 9:

Create another sub domain Say: apis.domainname.com in windows hosting

Host your Web API here and make sure the same API Path is included in Angular App before you start

with Step 1.

Step 10:

Bingo!!

All Done

Angular Interview Qs & Ans

October 17, 2020 by admin

  1. What is Angular Framework? Angular is a TypeScript-based open-source front-end platform that makes it easy to build applications with in web/mobile/desktop. The major features of this framework such as declarative templates, dependency injection, end to end tooling, and many more other features are used to ease the development.

  1. What is the difference between AngularJS and Angular? Angular is a completely revived component-based framework in which an application is a tree of individual components. Some of the major difference in tabular form AngularJS Angular It is based on MVC architecture This is based on Service/Controller This uses use JavaScript to build the application Introduced the typescript to write the application Based on controllers concept This is a component based UI approach Not a mobile friendly framework Developed considering mobile platform Difficulty in SEO friendly application development Ease to create SEO friendly applications

  1. What is TypeScript? TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. Angular built entirely in TypeScript and used as a primary language. You can install it globally as npm install -g typescript Let’s see a simple example of TypeScript usage, function greeter(person: string) { return “Hello, ” + person; } let user = “Sudheer”; document.body.innerHTML = greeter(user); The greeter method allows only string type as argument.

  1. Write a pictorial diagram of Angular architecture? The main building blocks of an Angular application is shown in the below diagram

  1. What are the key components of Angular? Angular has the below key components,
    1. Component: These are the basic building blocks of angular application to control HTML views.
    2. Modules: An angular module is set of angular basic building blocks like component, directives, services etc. An application is divided into logical pieces and each piece of code is called as “module” which perform a single task.
    3. Templates: This represent the views of an Angular application.
    4. Services: It is used to create components which can be shared across the entire application.
    5. Metadata: This can be used to add more data to an Angular class.

  1. What are directives? Directives add behaviour to an existing DOM element or an existing component instance. import { Directive, ElementRef, Input } from ‘@angular/core’; @Directive({ selector: ‘[myHighlight]’ }) export class HighlightDirective { constructor(el: ElementRef) { el.nativeElement.style.backgroundColor = ‘yellow’; } } Now this directive extends HTML element behavior with a yellow background as below <p myHighlight>Highlight me!</p>

  1. What are components? Components are the most basic UI building block of an Angular app which formed a tree of Angular components. These components are subset of directives. Unlike directives, components always have a template and only one component can be instantiated per an element in a template. Let’s see a simple example of Angular component import { Component } from ‘@angular/core’; @Component ({ selector: ‘my-app’, template: ` <div> <h1>{{title}}</h1> <div>Learn Angular6 with examples</div> </div> `, }) export class AppComponent { title: string = ‘Welcome to Angular world’; }

  1. What are the differences between Component and Directive? In a short note, A component(@component) is a directive-with-a-template. Some of the major differences are mentioned in a tabular form Component Directive To register a component we use @Component meta-data annotation To register directives we use @Directive meta-data annotation Components are typically used to create UI widgets Directive is used to add behavior to an existing DOM element Component is used to break up the application into smaller components Directive is use to design re-usable components Only one component can be present per DOM element Many directives can be used per DOM element @View decorator or templateurl/template are mandatory Directive doesn’t use View

  1. What is a template? A template is a HTML view where you can display data by binding controls to properties of an Angular component. You can store your component’s template in one of two places. You can define it inline using the template property, or you can define the template in a separate HTML file and link to it in the component metadata using the @Component decorator’s templateUrl property. Using inline template with template syntax, import { Component } from ‘@angular/core’; @Component ({ selector: ‘my-app’, template: ‘ <div> <h1>{{title}}</h1> <div>Learn Angular</div> </div> ‘ }) export class AppComponent { title: string = ‘Hello World’; } Using separate template file such as app.component.html import { Component } from ‘@angular/core’; @Component ({ selector: ‘my-app’, templateUrl: ‘app/app.component.html’ }) export class AppComponent { title: string = ‘Hello World’; }

  1. What is a module? Modules are logical boundaries in your application and the application is divided into separate modules to separate the functionality of your application. Lets take an example of app.module.ts root module declared with @NgModule decorator as below, import { NgModule } from ‘@angular/core’; import { BrowserModule } from ‘@angular/platform-browser’; import { AppComponent } from ‘./app.component’; @NgModule ({ imports: [ BrowserModule ], declarations: [ AppComponent ], bootstrap: [ AppComponent ], providers: [] }) export class AppModule { } The NgModule decorator has five important(among all) options
    1. The imports option is used to import other dependent modules. The BrowserModule is required by default for any web based angular application
    2. The declarations option is used to define components in the respective module
    3. The bootstrap option tells Angular which Component to bootstrap in the application
    4. The providers option is used to configure set of injectable objects that are available in the injector of this module.
    5. The entryComponents option is a set of components dynamically loaded into the view.

  1. What are lifecycle hooks available? Angular application goes through an entire set of processes or has a lifecycle right from its initiation to the end of the application. The representation of lifecycle in pictorial representation as follows, The description of each lifecycle method is as below,
    1. ngOnChanges: When the value of a data bound property changes, then this method is called.
    2. ngOnInit: This is called whenever the initialization of the directive/component after Angular first displays the data-bound properties happens.
    3. ngDoCheck: This is for the detection and to act on changes that Angular can’t or won’t detect on its own.
    4. ngAfterContentInit: This is called in response after Angular projects external content into the component’s view.
    5. ngAfterContentChecked: This is called in response after Angular checks the content projected into the component.
    6. ngAfterViewInit: This is called in response after Angular initializes the component’s views and child views.
    7. ngAfterViewChecked: This is called in response after Angular checks the component’s views and child views.
    8. ngOnDestroy: This is the cleanup phase just before Angular destroys the directive/component.

  1. What is a data binding? Data binding is a core concept in Angular and allows to define communication between a component and the DOM, making it very easy to define interactive applications without worrying about pushing and pulling data. There are four forms of data binding(divided as 3 categories) which differ in the way the data is flowing.
    1. From the Component to the DOM: Interpolation: {{ value }}: Adds the value of a property from the component <li>Name: {{ user.name }}</li> <li>Address: {{ user.address }}</li> Property binding: [property]=”value”: The value is passed from the component to the specified property or simple HTML attribute <input type=”email” [value]=”user.email”>
    2. From the DOM to the Component: Event binding: (event)=”function”: When a specific DOM event happens (eg.: click, change, keyup), call the specified method in the component <button (click)=”logout()”></button>
    3. Two-way binding: Two-way data binding: [(ngModel)]=”value”: Two-way data binding allows to have the data flow both ways. For example, in the below code snippet, both the email DOM input and component email property are in sync <input type=”email” [(ngModel)]=”user.email”>

  1. What is metadata? Metadata is used to decorate a class so that it can configure the expected behavior of the class. The metadata is represented by decorators
    1. Class decorators, e.g. @Component and @NgModule import { NgModule, Component } from ‘@angular/core’; @Component({ selector: ‘my-component’, template: ‘<div>Class decorator</div>’, }) export class MyComponent { constructor() { console.log(‘Hey I am a component!’); } } @NgModule({ imports: [], declarations: [], }) export class MyModule { constructor() { console.log(‘Hey I am a module!’); } }
    2. Property decorators Used for properties inside classes, e.g. @Input and @Output import { Component, Input } from ‘@angular/core’; @Component({ selector: ‘my-component’, template: ‘<div>Property decorator</div>’ }) export class MyComponent { @Input() title: string; }
    3. Method decorators Used for methods inside classes, e.g. @HostListener import { Component, HostListener } from ‘@angular/core’; @Component({ selector: ‘my-component’, template: ‘<div>Method decorator</div>’ }) export class MyComponent { @HostListener(‘click’, [‘$event’]) onHostClick(event: Event) { // clicked, `event` available } }
    4. Parameter decorators Used for parameters inside class constructors, e.g. @Inject, Optional import { Component, Inject } from ‘@angular/core’; import { MyService } from ‘./my-service’; @Component({ selector: ‘my-component’, template: ‘<div>Parameter decorator</div>’ }) export class MyComponent { constructor(@Inject(MyService) myService) { console.log(myService); // MyService } }

  1. What is angular CLI? Angular CLI(Command Line Interface) is a command line interface to scaffold and build angular apps using nodejs style (commonJs) modules. You need to install using below npm command, npm install @angular/cli@latest Below are the list of few commands, which will come handy while creating angular projects
    1. Creating New Project: ng new
    2. Generating Components, Directives & Services: ng generate/g The different types of commands would be,
      • ng generate class my-new-class: add a class to your application
      • ng generate component my-new-component: add a component to your application
      • ng generate directive my-new-directive: add a directive to your application
      • ng generate enum my-new-enum: add an enum to your application
      • ng generate module my-new-module: add a module to your application
      • ng generate pipe my-new-pipe: add a pipe to your application
      • ng generate service my-new-service: add a service to your application
    3. Running the Project: ng serve

  1. What is the difference between constructor and ngOnInit? TypeScript classes has a default method called constructor which is normally used for the initialization purpose. Whereas ngOnInit method is specific to Angular, especially used to define Angular bindings. Even though constructor getting called first, it is preferred to move all of your Angular bindings to ngOnInit method. In order to use ngOnInit, you need to implement OnInit interface as below, export class App implements OnInit{ constructor(){ //called first time before the ngOnInit() } ngOnInit(){ //called after the constructor and called after the first ngOnChanges() } }

  1. What is a service? A service is used when a common functionality needs to be provided to various modules. Services allow for greater separation of concerns for your application and better modularity by allowing you to extract common functionality out of components. Let’s create a repoService which can be used across components, import { Injectable } from ‘@angular/core’; import { Http } from ‘@angular/http’; @Injectable({ // The Injectable decorator is required for dependency injection to work // providedIn option registers the service with a specific NgModule providedIn: ‘root’, // This declares the service with the root app (AppModule) }) export class RepoService{ constructor(private http: Http){ } fetchAll(){ return this.http.get(‘https://api.github.com/repositories’); } } The above service uses Http service as a dependency.

  1. What is dependency injection in Angular? Dependency injection (DI), is an important application design pattern in which a class asks for dependencies from external sources rather than creating them itself. Angular comes with its own dependency injection framework for resolving dependencies( services or objects that a class needs to perform its function).So you can have your services depend on other services throughout your application.

  1. How is Dependency Hierarchy formed?

  1. What is the purpose of async pipe? The AsyncPipe subscribes to an observable or promise and returns the latest value it has emitted. When a new value is emitted, the pipe marks the component to be checked for changes. Let’s take a time observable which continuously updates the view for every 2 seconds with the current time. @Component({ selector: ‘async-observable-pipe’, template: `<div><code>observable|async</code>: Time: {{ time | async }}</div>` }) export class AsyncObservablePipeComponent { time = new Observable(observer => setInterval(() => observer.next(new Date().toString()), 2000) ); }

  1. What is the option to choose between inline and external template file? You can store your component’s template in one of two places. You can define it inline using the template property, or you can define the template in a separate HTML file and link to it in the component metadata using the @Component decorator’s templateUrl property. The choice between inline and separate HTML is a matter of taste, circumstances, and organization policy. But normally we use inline template for small portion of code and external template file for bigger views. By default, the Angular CLI generates components with a template file. But you can override that with the below command, ng generate component hero -it

  1. What is the purpose of ngFor directive? We use Angular ngFor directive in the template to display each item in the list. For example, here we iterate over list of users, <li *ngFor=”let user of users”> {{ user }} </li> The user variable in the ngFor double-quoted instruction is a template input variable

  1. What is the purpose of ngIf directive? Sometimes an app needs to display a view or a portion of a view only under specific circumstances. The Angular ngIf directive inserts or removes an element based on a truthy/falsy condition. Let’s take an example to display a message if the user age is more than 18, <p *ngIf=”user.age > 18″>You are not eligible for student pass!</p> Note: Angular isn’t showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in the larger projects with many data bindings.

  1. What happens if you use script tag inside template? Angular recognizes the value as unsafe and automatically sanitizes it, which removes the <script> tag but keeps safe content such as the text content of the <script> tag. This way it eliminates the risk of script injection attacks. If you still use it then it will be ignored and a warning appears in the browser console. Let’s take an example of innerHtml property binding which causes XSS vulnerability, export class InnerHtmlBindingComponent { // For example, a user/attacker-controlled value from a URL. htmlSnippet = ‘Template <script>alert(“0wned”)</script> <b>Syntax</b>’; }

  1. What is interpolation? Interpolation is a special syntax that Angular converts into property binding. It’s a convenient alternative to property binding. It is represented by double curly braces({{}}). The text between the braces is often the name of a component property. Angular replaces that name with the string value of the corresponding component property. Let’s take an example, <h3> {{title}} <img src=”{{url}}” style=”height:30px”> </h3> In the example above, Angular evaluates the title and url properties and fills in the blanks, first displaying a bold application title and then a URL.

  1. What are template expressions? A template expression produces a value similar to any Javascript expression. Angular executes the expression and assigns it to a property of a binding target; the target might be an HTML element, a component, or a directive. In the property binding, a template expression appears in quotes to the right of the = symbol as in [property]=”expression”. In interpolation syntax, the template expression is surrounded by double curly braces. For example, in the below interpolation, the template expression is {{username}}, <h3>{{username}}, welcome to Angular</h3> The below javascript expressions are prohibited in template expression
    1. assignments (=, +=, -=, …)
    2. new
    3. chaining expressions with ; or ,
    4. increment and decrement operators (++ and –)

  1. What are template statements? A template statement responds to an event raised by a binding target such as an element, component, or directive. The template statements appear in quotes to the right of the = symbol like (event)=”statement”. Let’s take an example of button click event’s statement <button (click)=”editProfile()”>Edit Profile</button> In the above expression, editProfile is a template statement. The below JavaScript syntax expressions are not allowed.
    1. new
    2. increment and decrement operators, ++ and —
    3. operator assignment, such as += and -=
    4. the bitwise operators | and &
    5. the template expression operators

  1. How do you categorize data binding types? Binding types can be grouped into three categories distinguished by the direction of data flow. They are listed as below,
    1. From the source-to-view
    2. From view-to-source
    3. View-to-source-to-view
    The possible binding syntax can be tabularized as below, Data direction Syntax Type From the source-to-view(One-way) 1. {{expression}} 2. [target]=”expression” 3. bind-target=”expression” Interpolation, Property, Attribute, Class, Style From view-to-source(One-way) 1. (target)=”statement” 2. on-target=”statement” Event View-to-source-to-view(Two-way) 1. [(target)]=”expression” 2. bindon-target=”expression” Two-way

  1. What are pipes? A pipe takes in data as input and transforms it to a desired output. For example, let us take a pipe to transform a component’s birthday property into a human-friendly date using date pipe. import { Component } from ‘@angular/core’; @Component({ selector: ‘app-birthday’, template: `<p>Birthday is {{ birthday | date }}</p>` }) export class BirthdayComponent { birthday = new Date(1987, 6, 18); // June 18, 1987 }

  1. What is a parameterized pipe? A pipe can accept any number of optional parameters to fine-tune its output. The parameterized pipe can be created by declaring the pipe name with a colon ( : ) and then the parameter value. If the pipe accepts multiple parameters, separate the values with colons. Let’s take a birthday example with a particular format(dd/MM/yyyy): import { Component } from ‘@angular/core’; @Component({ selector: ‘app-birthday’, template: `<p>Birthday is {{ birthday | date:’dd/MM/yyyy’}}</p>` // 18/06/1987 }) export class BirthdayComponent { birthday = new Date(1987, 6, 18); } Note: The parameter value can be any valid template expression, such as a string literal or a component property.

  1. How do you chain pipes? You can chain pipes together in potentially useful combinations as per the needs. Let’s take a birthday property which uses date pipe(along with parameter) and uppercase pipes as below import { Component } from ‘@angular/core’; @Component({ selector: ‘app-birthday’, template: `<p>Birthday is {{ birthday | date:’fullDate’ | uppercase}} </p>` // THURSDAY, JUNE 18, 1987 }) export class BirthdayComponent { birthday = new Date(1987, 6, 18); }

  1. What is a custom pipe? Apart from built-inn pipes, you can write your own custom pipe with the below key characteristics,
    1. A pipe is a class decorated with pipe metadata @Pipe decorator, which you import from the core Angular library For example, @Pipe({name: ‘myCustomPipe’})
    2. The pipe class implements the PipeTransform interface’s transform method that accepts an input value followed by optional parameters and returns the transformed value. The structure of pipeTransform would be as below, interface PipeTransform { transform(value: any, …args: any[]): any }
    3. The @Pipe decorator allows you to define the pipe name that you’ll use within template expressions. It must be a valid JavaScript identifier. template: `{{someInputValue | myCustomPipe: someOtherValue}}`

  1. Give an example of custom pipe? You can create custom reusable pipes for the transformation of existing value. For example, let us create a custom pipe for finding file size based on an extension, “`javascript import { Pipe, PipeTransform } from ‘@angular/core’; @Pipe({name: 'customFileSizePipe'}) export class FileSizePipe implements PipeTransform { transform(size: number, extension: string = 'MB'): string { return (size / (1024 * 1024)).toFixed(2) + extension; } } ``` Now you can use the above pipe in template expression as below, javascript template: ` <h2>Find the size of a file</h2> <p>Size: {{288966 | customFileSizePipe: 'GB'}}</p> `

  1. What is the difference between pure and impure pipe? A pure pipe is only called when Angular detects a change in the value or the parameters passed to a pipe. For example, any changes to a primitive input value (String, Number, Boolean, Symbol) or a changed object reference (Date, Array, Function, Object). An impure pipe is called for every change detection cycle no matter whether the value or parameters changes. i.e, An impure pipe is called often, as often as every keystroke or mouse-move.

  1. What is a bootstrapping module? Every application has at least one Angular module, the root module that you bootstrap to launch the application is called as bootstrapping module. It is commonly known as AppModule. The default structure of AppModule generated by AngularCLI would be as follows, ```javascript /* JavaScript imports */ import { BrowserModule } from '@angular/platform-browser'; import { NgModule } from '@angular/core'; import { FormsModule } from '@angular/forms'; import { HttpClientModule } from '@angular/common/http'; import { AppComponent } from './app.component'; /* the AppModule class with the @NgModule decorator */ @NgModule({ declarations: [ AppComponent ], imports: [ BrowserModule, FormsModule, HttpClientModule ], providers: [], bootstrap: [AppComponent] }) export class AppModule { } ```

  1. What are observables? Observables are declarative which provide support for passing messages between publishers and subscribers in your application. They are mainly used for event handling, asynchronous programming, and handling multiple values. In this case, you define a function for publishing values, but it is not executed until a consumer subscribes to it. The subscribed consumer then receives notifications until the function completes, or until they unsubscribe.

  1. What is HttpClient and its benefits? Most of the Front-end applications communicate with backend services over HTTP protocol using either XMLHttpRequest interface or the fetch() API. Angular provides a simplified client HTTP API known as HttpClient which is based on top of XMLHttpRequest interface. This client is avaialble from @angular/common/http package. You can import in your root module as below, import { HttpClientModule } from ‘@angular/common/http’; The major advantages of HttpClient can be listed as below,
    1. Contains testability features
    2. Provides typed request and response objects
    3. Intercept request and response
    4. Supports Observalbe APIs
    5. Supports streamlined error handling

  1. Explain on how to use HttpClient with an example? Below are the steps need to be followed for the usage of HttpClient.
    1. Import HttpClient into root module: import { HttpClientModule } from ‘@angular/common/http’; @NgModule({ imports: [ BrowserModule, // import HttpClientModule after BrowserModule. HttpClientModule, ], …… }) export class AppModule {}
    2. Inject the HttpClient into the application: Let’s create a userProfileService(userprofile.service.ts) as an example. It also defines get method of HttpClient import { Injectable } from ‘@angular/core’; import { HttpClient } from ‘@angular/common/http’; const userProfileUrl: string = ‘assets/data/profile.json’; @Injectable() export class UserProfileService { constructor(private http: HttpClient) { } getUserProfile() { return this.http.get(this.userProfileUrl); } }
    3. Create a component for subscribing service: Let’s create a component called UserProfileComponent(userprofile.component.ts) which inject UserProfileService and invokes the service method, fetchUserProfile() { this.userProfileService.getUserProfile() .subscribe((data: User) => this.user = { id: data[‘userId’], name: data[‘firstName’], city: data[‘city’] }); }
    Since the above service method returns an Observable which needs to be subscribed in the component.

  1. How can you read full response? The response body doesn’t may not return full response data because sometimes servers also return special headers or status code which which are important for the application workflow. Inorder to get full response, you should use observe option from HttpClient, getUserResponse(): Observable<HttpResponse<User>> { return this.http.get<User>( this.userUrl, { observe: ‘response’ }); } Now HttpClient.get() method returns an Observable of typed HttpResponse rather than just the JSON data.

  1. How do you perform Error handling? If the request fails on the server or failed to reach the server due to network issues then HttpClient will return an error object instead of a successful reponse. In this case, you need to handle in the component by passing error object as a second callback to subscribe() method. Let’s see how it can be handled in the component with an example, fetchUser() { this.userService.getProfile() .subscribe( (data: User) => this.userProfile = { …data }, // success path error => this.error = error // error path ); } It is always a good idea to give the user some meaningful feedback instead of displaying the raw error object returned from HttpClient.

  1. What is RxJS? RxJS is a library for composing asynchronous and callback-based code in a functional, reactive style using Observables. Many APIs such as HttpClient produce and consume RxJS Observables and also uses operators for processing observables. For example, you can import observables and operators for using HttpClient as below, import { Observable, throwError } from ‘rxjs’; import { catchError, retry } from ‘rxjs/operators’;

  1. What is subscribing? An Observable instance begins publishing values only when someone subscribes to it. So you need to subscribe by calling the subscribe() method of the instance, passing an observer object to receive the notifications. Let’s take an example of creating and subscribing to a simple observable, with an observer that logs the received message to the console. Creates an observable sequence of 5 integers, starting from 1 const source = range(1, 5); // Create observer object const myObserver = { next: x => console.log(‘Observer got a next value: ‘ + x), error: err => console.error(‘Observer got an error: ‘ + err), complete: () => console.log(‘Observer got a complete notification’), }; // Execute with the observer object and Prints out each item source.subscribe(myObserver); // => Observer got a next value: 1 // => Observer got a next value: 2 // => Observer got a next value: 3 // => Observer got a next value: 4 // => Observer got a next value: 5 // => Observer got a complete notification

  1. What is an observable? An Observable is a unique Object similar to a Promise that can help manage async code. Observables are not part of the JavaScript language so we need to rely on a popular Observable library called RxJS. The observables are created using new keyword. Let see the simple example of observable, import { Observable } from ‘rxjs’; const observable = new Observable(observer => { setTimeout(() => { observer.next(‘Hello from a Observable!’); }, 2000); });

  1. What is an observer? Observer is an interface for a consumer of push-based notifications delivered by an Observable. It has below structure, interface Observer<T> { closed?: boolean; next: (value: T) => void; error: (err: any) => void; complete: () => void; } A handler that implements the Observer interface for receiving observable notifications will be passed as a parameter for observable as below, myObservable.subscribe(myObserver); Note: If you don’t supply a handler for a notification type, the observer ignores notifications of that type.

  1. What is the difference between promise and observable? Below are the list of differences between promise and observable, Observable Promise Declarative: Computation does not start until subscription so that they can be run whenever you need the result Execute immediately on creation Provide multiple values over time Provide only one Subscribe method is used for error handling which makes centralized and predictable error handling Push errors to the child promises Provides chaining and subscription to handle complex applications Uses only .then() clause

  1. What is multicasting? Multi-casting is the practice of broadcasting to a list of multiple subscribers in a single execution. Let’s demonstrate the multi-casting feature, var source = Rx.Observable.from([1, 2, 3]); var subject = new Rx.Subject(); var multicasted = source.multicast(subject); // These are, under the hood, `subject.subscribe({…})`: multicasted.subscribe({ next: (v) => console.log(‘observerA: ‘ + v) }); multicasted.subscribe({ next: (v) => console.log(‘observerB: ‘ + v) }); // This is, under the hood, `s

  1. How do you perform error handling in observables? You can handle errors by specifying an error callback on the observer instead of relying on try/catch which are ineffective in asynchronous environment. For example, you can define error callback as below, myObservable.subscribe({ next(num) { console.log(‘Next num: ‘ + num)}, error(err) { console.log(‘Received an errror: ‘ + err)} });

  1. What is the short hand notation for subscribe method? The subscribe() method can accept callback function definitions in line, for next, error, and complete handlers is known as short hand notation or Subscribe method with positional arguments. For example, you can define subscribe method as below, myObservable.subscribe( x => console.log(‘Observer got a next value: ‘ + x), err => console.error(‘Observer got an error: ‘ + err), () => console.log(‘Observer got a complete notification’) );

  1. What are the utility functions provided by RxJS? The RxJS library also provides below utility functions for creating and working with observables.
    1. Converting existing code for async operations into observables
    2. Iterating through the values in a stream
    3. Mapping values to different types
    4. Filtering streams
    5. Composing multiple streams

  1. What are observable creation functions? RxJS provides creation functions for the process of creating observables from things such as promises, events, timers and Ajax requests. Let us explain each of them with an example,
    1. Create an observable from a promise import { from } from ‘rxjs’; // from function const data = from(fetch(‘/api/endpoint’)); //Created from Promise data.subscribe({ next(response) { console.log(response); }, error(err) { console.error(‘Error: ‘ + err); }, complete() { console.log(‘Completed’); } });
    2. Create an observable that creates an AJAX request import { ajax } from ‘rxjs/ajax’; // ajax function const apiData = ajax(‘/api/data’); // Created from AJAX request // Subscribe to create the request apiData.subscribe(res => console.log(res.status, res.response));
    3. Create an observable from a counter import { interval } from ‘rxjs’; // interval function const secondsCounter = interval(1000); // Created from Counter value secondsCounter.subscribe(n => console.log(`Counter value: ${n}`));
    4. Create an observable from an event import { fromEvent } from ‘rxjs’; const el = document.getElementById(‘custom-element’); const mouseMoves = fromEvent(el, ‘mousemove’); const subscription = mouseMoves.subscribe((e: MouseEvent) => { console.log(`Coordnitaes of mouse pointer: ${e.clientX} * ${e.clientY}`); });

  1. What will happen if you do not supply handler for observer? Normally an observer object can define any combination of next, error and complete notification type handlers. If you don’t supply a handler for a notification type, the observer just ignores notifications of that type.

  1. What are angular elements? Angular elements are Angular components packaged as custom elements(a web standard for defining new HTML elements in a framework-agnostic way). Angular Elements hosts an Angular component, providing a bridge between the data and logic defined in the component and standard DOM APIs, thus, providing a way to use Angular components in non-Angular environments.

  1. What is the browser support of Angular Elements? Since Angular elements are packaged as custom elements the browser support of angular elements is same as custom elements support. This feature is is currently supported natively in a number of browsers and pending for other browsers. Browser Angular Element Support Chrome Natively supported Opera Natively supported Safari Natively supported Firefox Natively supported from 63 version onwards. You need to enable dom.webcomponents.enabled and dom.webcomponents.customelements.enabled in older browsers Edge Currently it is in progress

  1. What are custom elements? Custom elements (or Web Components) are a Web Platform feature which extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code. The browser maintains a CustomElementRegistry of defined custom elements, which maps an instantiable JavaScript class to an HTML tag. Currently this feature is supported by Chrome, Firefox, Opera, and Safari, and available in other browsers through polyfills.

  1. Do I need to bootstrap custom elements? No, custom elements bootstrap (or start) automatically when they are added to the DOM, and are automatically destroyed when removed from the DOM. Once a custom element is added to the DOM for any page, it looks and behaves like any other HTML element, and does not require any special knowledge of Angular.

  1. Explain how custom elements works internally? Below are the steps in an order about custom elements functionality,
    1. App registers custom element with browser: Use the createCustomElement() function to convert a component into a class that can be registered with the browser as a custom element.
    2. App adds custom element to DOM: Add custom element just like a built-in HTML element directly into the DOM.
    3. Browser instantiate component based class: Browser creates an instance of the registered class and adds it to the DOM.
    4. Instance provides content with data binding and change detection: The content with in template is rendered using the component and DOM data. The flow chart of the custom elements functionality would be as follows,

  1. How to transfer components to custom elements? Transforming components to custom elements involves two major steps,
    1. Build custom element class: Angular provides the createCustomElement() function for converting an Angular component (along with its dependencies) to a custom element. The conversion process implements NgElementConstructor interface, and creates a constructor class which is used to produce a self-bootstrapping instance of Angular component.
    2. Register element class with browser: It uses customElements.define() JS function, to register the configured constructor and its associated custom-element tag with the browser’s CustomElementRegistry. When the browser encounters the tag for the registered element, it uses the constructor to create a custom-element instance.
    The detailed structure would be as follows,

  1. What are the mapping rules between Angular component and custom element? The Component properties and logic maps directly into HTML attributes and the browser’s event system. Let us describe them in two steps,
    1. The createCustomElement() API parses the component input properties with corresponding attributes for the custom element. For example, component @Input(‘myInputProp’) converted as custom element attribute my-input-prop.
    2. The Component outputs are dispatched as HTML Custom Events, with the name of the custom event matching the output name. For example, component @Output() valueChanged = new EventEmitter() converted as custom element with dispatch event as “valueChanged”.

  1. How do you define typings for custom elements? You can use the NgElement and WithProperties types exported from @angular/elements. Let’s see how it can be applied by comparing with Angular component.
    1. The simple container with input property would be as below, @Component(…) class MyContainer { @Input() message: string; }
    2. After applying types typescript validates input value and their types, const container = document.createElement('my-container') as NgElement & WithProperties<{message: string}>; container.message = 'Welcome to Angular elements!'; container.message = true; // <-- ERROR: TypeScript knows this should be a string. container.greet = 'News'; // <-- ERROR: TypeScript knows there is no `greet` property on `container`.

  1. What are dynamic components? Dynamic components are the components in which components location in the application is not defined at build time.i.e, They are not used in any angular template. But the component is instantiated and placed in the application at runtime.

  1. What are the various kinds of directives? There are mainly three kinds of directives,
    1. Components — These are directives with a template.
    2. Structural directives — These directives change the DOM layout by adding and removing DOM elements.
    3. Attribute directives — These directives change the appearance or behavior of an element, component, or another directive.

  1. How do you create directives using CLI? You can use CLI command ng generate directive to create the directive class file. It creates the source file(src/app/components/directivename.directive.ts), the respective test file(.spec.ts) and declare the directive class file in root module.

  1. Give an example for attribute directives? Let’s take simple highlighter behavior as a example directive for DOM element. You can create and apply the attribute directive using below steps,
    1. Create HighlightDirective class with the file name src/app/highlight.directive.ts. In this file, we need to import Directive from core library to apply the metadata and ElementRef in the directive’s constructor to inject a reference to the host DOM element , import { Directive, ElementRef } from ‘@angular/core’; @Directive({ selector: ‘[appHighlight]’ }) export class HighlightDirective { constructor(el: ElementRef) { el.nativeElement.style.backgroundColor = ‘red’; } }
    2. Apply the attribute directive as an attribute to the host element(for example, ) <p appHighlight>Highlight me!</p>
    3. Run the application to see the highlight behavior on paragraph element ng serve

  1. What is Angular Router? Angular Router is a mechanism in which navigation happens from one view to the next as users perform application tasks. It borrows the concepts or model of browser’s application navigation.

  1. What is the purpose of base href tag? The routing application should add element to the index.html as the first child in the tag in order to indicate how to compose navigation URLs. If app folder is the application root then you can set the href value as below <base href=”/”>

  1. What are the router imports? The Angular Router which represents a particular component view for a given URL is not part of Angular Core. It is available in library named @angular/router to import required router components. For example, we import them in app module as below, import { RouterModule, Routes } from ‘@angular/router’;

  1. What is router outlet? The RouterOutlet is a directive from the router library and it acts as a placeholder that marks the spot in the template where the router should display the components for that outlet. Router outlet is used like a component, <router-outlet></router-outlet> <!– Routed components go here –>

  1. What are router links? The RouterLink is a directive on the anchor tags give the router control over those elements. Since the navigation paths are fixed, you can assign string values to router-link directive as below, <h1>Angular Router</h1> <nav> <a routerLink=”/todosList” >List of todos</a> <a routerLink=”/completed” >Completed todos</a> </nav> <router-outlet></router-outlet>

  1. What are active router links? RouterLinkActive is a directive that toggles css classes for active RouterLink bindings based on the current RouterState. i.e, the Router will add CSS classes when this link is active and and remove when the link is inactive. For example, you can add them to RouterLinks as below <h1>Angular Router</h1> <nav> <a routerLink=”/todosList” routerLinkActive=”active”>List of todos</a> <a routerLink=”/completed” routerLinkActive=”active”>Completed todos</a> </nav> <router-outlet></router-outlet>

  1. What is router state? RouterState is a tree of activated routes. Every node in this tree knows about the “consumed” URL segments, the extracted parameters, and the resolved data. You can access the current RouterState from anywhere in the application using the Router service and the routerState property. @Component({templateUrl:’template.html’}) class MyComponent { constructor(router: Router) { const state: RouterState = router.routerState; const root: ActivatedRoute = state.root; const child = root.firstChild; const id: Observable<string> = child.params.map(p => p.id); //… } }

  1. What are router events? During each navigation, the Router emits navigation events through the Router.events property allowing you to track the lifecycle of the route. The sequence of router events is as below,
    1. NavigationStart,
    2. RouteConfigLoadStart,
    3. RouteConfigLoadEnd,
    4. RoutesRecognized,
    5. GuardsCheckStart,
    6. ChildActivationStart,
    7. ActivationStart,
    8. GuardsCheckEnd,
    9. ResolveStart,
    10. ResolveEnd,
    11. ActivationEnd
    12. ChildActivationEnd
    13. NavigationEnd,
    14. NavigationCancel,
    15. NavigationError
    16. Scroll

  1. What is activated route? ActivatedRoute contains the information about a route associated with a component loaded in an outlet. It can also be used to traverse the router state tree. The ActivatedRoute will be injected as a router service to access the information. In the below example, you can access route path and parameters, @Component({…}) class MyComponent { constructor(route: ActivatedRoute) { const id: Observable<string> = route.params.pipe(map(p => p.id)); const url: Observable<string> = route.url.pipe(map(segments => segments.join(”))); // route.data includes both `data` and `resolve` const user = route.data.pipe(map(d => d.user)); } }

  1. How do you define routes? A router must be configured with a list of route definitions. You configures the router with routes via the RouterModule.forRoot() method, and adds the result to the AppModule’s imports array. const appRoutes: Routes = [ { path: ‘todo/:id’, component: TodoDetailComponent }, { path: ‘todos’, component: TodosListComponent, data: { title: ‘Todos List’ } }, { path: ”, redirectTo: ‘/todos’, pathMatch: ‘full’ }, { path: ‘**’, component: PageNotFoundComponent } ]; @NgModule({ imports: [ RouterModule.forRoot( appRoutes, { enableTracing: true } // <– debugging purposes only ) // other imports here ], … }) export class AppModule { }

  1. What is the purpose of Wildcard route? If the URL doesn’t match any predefined routes then it causes the router to throw an error and crash the app. In this case, you can use wildcard route. A wildcard route has a path consisting of two asterisks to match every URL. For example, you can define PageNotFoundComponent for wildcard route as below { path: ‘**’, component: PageNotFoundComponent }

  1. Do I need a Routing Module always? No, the Routing Module is a design choice. You can skip routing Module (for example, AppRoutingModule) when the configuration is simple and merge the routing configuration directly into the companion module (for example, AppModule). But it is recommended when the configuration is complex and includes specialized guard and resolver services.

  1. What is Angular Universal? Angular Universal is a server-side rendering module for Angular applications in various scenarios. This is a community driven project and available under @angular/platform-server package. Recently Angular Universal is integrated with Angular CLI.

  1. What are different types of compilation in Angular? Angular offers two ways to compile your application,
    1. Just-in-Time (JIT)
    2. Ahead-of-Time (AOT)

  1. What is JIT? Just-in-Time (JIT) is a type of compilation that compiles your app in the browser at runtime. JIT compilation is the default when you run the ng build (build only) or ng serve (build and serve locally) CLI commands. i.e, the below commands used for JIT compilation, ng build ng serve

  1. What is AOT? Ahead-of-Time (AOT) is a type of compilation that compiles your app at build time. For AOT compilation, include the --aot option with the ng build or ng serve command as below, ng build –aot ng serve –aot Note: The ng build command with the –prod meta-flag (ng build --prod) compiles with AOT by default.

  1. Why do we need compilation process? The Angular components and templates cannot be understood by the browser directly. Due to that Angular applications require a compilation process before they can run in a browser. For example, In AOT compilation, both Angular HTML and TypeScript code converted into efficient JavaScript code during the build phase before browser runs it.

  1. What are the advantages with AOT? Below are the list of AOT benefits,
    1. Faster rendering: The browser downloads a pre-compiled version of the application. So it can render the application immediately without compiling the app.
    2. Fewer asynchronous requests: It inlines external HTML templates and CSS style sheets within the application javascript which eliminates separate ajax requests.
    3. Smaller Angular framework download size: Doesn’t require downloading the Angular compiler. Hence it dramatically reduces the application payload.
    4. Detect template errors earlier: Detects and reports template binding errors during the build step itself
    5. Better security: It compiles HTML templates and components into JavaScript. So there won’t be any injection attacks.

  1. What are the ways to control AOT compilation? You can control your app compilation in two ways,
    1. By providing template compiler options in the tsconfig.json file
    2. By configuring Angular metadata with decorators

  1. What are the restrictions of metadata? In Angular, You must write metadata with the following general constraints,
    1. Write expression syntax with in the supported range of javascript features
    2. The compiler can only reference symbols which are exported
    3. Only call the functions supported by the compiler
    4. Decorated and data-bound class members must be public.

  1. What are the two phases of AOT? The AOT compiler works in three phases,
    1. Code Analysis: The compiler records a representation of the source
    2. Code generation: It handles the interpretation as well as places restrictions on what it interprets.
    3. Validation: In this phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates.

  1. Can I use arrow functions in AOT? No, Arrow functions or lambda functions can’t be used to assign values to the decorator properties. For example, the following snippet is invalid: @Component({ providers: [{ provide: MyService, useFactory: () => getService() }] }) To fix this, it has to be changed as following exported function: function getService(){ return new MyService(); } @Component({ providers: [{ provide: MyService, useFactory: getService }] }) If you still use arrow function, it generates an error node in place of the function. When the compiler later interprets this node, it reports an error to turn the arrow function into an exported function. Note: From Angular5 onwards, the compiler automatically performs this rewriting while emitting the .js file.

  1. What is the purpose of metadata json files? The metadata.json file can be treated as a diagram of the overall structure of a decorator’s metadata, represented as an abstract syntax tree(AST). During the analysis phase, the AOT collector scan the metadata recorded in the Angular decorators and outputs metadata information in .metadata.json files, one per .d.ts file.

  1. Can I use any javascript feature for expression syntax in AOT? No, the AOT collector understands a subset of (or limited) JavaScript features. If an expression uses unsupported syntax, the collector writes an error node to the .metadata.json file. Later point of time, the compiler reports an error if it needs that piece of metadata to generate the application code.

  1. What is folding? The compiler can only resolve references to exported symbols in the metadata. Where as some of the non-exported members are folded while generating the code. i.e Folding is a process in which the collector evaluate an expression during collection and record the result in the .metadata.json instead of the original expression. For example, the compiler couldn’t refer selector reference because it is not exported let selector = ‘app-root’; @Component({ selector: selector }) Will be folded into inline selector @Component({ selector: ‘app-root’ }) Remember that the compiler can’t fold everything. For example, spread operator on arrays, objects created using new keywords and function calls.

  1. What are macros? The AOT compiler supports macros in the form of functions or static methods that return an expression in a single return expression. For example, let us take a below macro function, export function wrapInArray<T>(value: T): T[] { return [value]; } You can use it inside metadata as an expression, @NgModule({ declarations: wrapInArray(TypicalComponent) }) export class TypicalModule {} The compiler treats the macro expression as it written directly @NgModule({ declarations: [TypicalComponent] }) export class TypicalModule {}

  1. Give an example of few metadata errors? Below are some of the errors encountered in metadata,
    1. Expression form not supported: Some of the language features outside of the compiler’s restricted expression syntax used in angular metadata can produce this error. Let’s see some of these examples, 1. export class User { … } const prop = typeof User; // typeof is not valid in metadata 2. { provide: ‘token’, useValue: { [prop]: ‘value’ } }; // bracket notation is not valid in metadata
    2. Reference to a local (non-exported) symbol: The compiler encountered a referenced to a locally defined symbol that either wasn’t exported or wasn’t initialized. Let’s take example of this error, // ERROR let username: string; // neither exported nor initialized @Component({ selector: ‘my-component’, template: … , providers: [ { provide: User, useValue: username } ] }) export class MyComponent {} You can fix this by either exporting or initializing the value, export let username: string; // exported (or) let username = ‘John’; // initialized
    3. Function calls are not supported: The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider’s useFactory to an anonymous function or arrow function as below. providers: [ { provide: MyStrategy, useFactory: function() { … } }, { provide: OtherStrategy, useFactory: () => { … } } ] You can fix this with exported function export function myStrategy() { … } export function otherStrategy() { … } … // metadata providers: [ { provide: MyStrategy, useFactory: myStrategy }, { provide: OtherStrategy, useFactory: otherStrategy },
    4. Destructured variable or constant not supported: The compiler does not support references to variables assigned by destructuring. For example, you cannot write something like this: import { user } from ‘./user’; // destructured assignment to name and age const {name, age} = user; … //metadata providers: [ {provide: Name, useValue: name}, {provide: Age, useValue: age}, ] You can fix this by non-destructured values import { user } from './user'; ... //metadata providers: [ {provide: Name, useValue: user.name}, {provide: Age, useValue: user.age}, ]

  1. What is metadata rewriting? Metadata rewriting is the process in which the compiler converts the expression initializing the fields such as useClass, useValue, useFactory, and data into an exported variable, which replaces the expression. Remember that the compiler does this rewriting during the emit of the .js file but not in definition files( .d.ts file).

  1. How do you provide configuration inheritance? Angular Compiler supports configuration inheritance through extends in the tsconfig.json on angularCompilerOptions. i.e, The configuration from the base file(for example, tsconfig.base.json) are loaded first, then overridden by those in the inheriting config file. { “extends”: “../tsconfig.base.json”, “compilerOptions”: { “experimentalDecorators”: true, … }, “angularCompilerOptions”: { “fullTemplateTypeCheck”: true, “preserveWhitespaces”: true, … } }

  1. How do you specify angular template compiler options? The angular template compiler options are specified as members of the angularCompilerOptions object in the tsconfig.json file. These options will be specified adjecent to typescript compiler options. { “compilerOptions”: { “experimentalDecorators”: true, … }, “angularCompilerOptions”: { “fullTemplateTypeCheck”: true, “preserveWhitespaces”: true, … } }

  1. How do you enable binding expression validation? You can enable binding expression validation explicitly by adding the compiler option fullTemplateTypeCheck in the “angularCompilerOptions” of the project’s tsconfig.json. It produces error messages when a type error is detected in a template binding expression. For example, consider the following component: @Component({ selector: ‘my-component’, template: ‘{{user.contacts.email}}’ }) class MyComponent { user?: User; } This will produce the following error: my.component.ts.MyComponent.html(1,1): : Property ‘contacts’ does not exist on type ‘User’. Did you mean ‘contact’?

  1. What is the purpose of any type cast function? You can disable binding expression type checking using $any() type cast function(by surrounding the expression). In the following example, the error Property contacts does not exist is suppressed by casting user to the any type. template: ‘{{ $any(user).contacts.email }}’ The $any() cast function also works with this to allow access to undeclared members of the component. template: ‘{{ $any(this).contacts.email }}’

  1. What is Non null type assertion operator? You can use the non-null type assertion operator to suppress the Object is possibly ‘undefined’ error. In the following example, the user and contact properties are always set together, implying that contact is always non-null if user is non-null. The error is suppressed in the example by using contact!.email. @Component({ selector: ‘my-component’, template: ‘<span *ngIf=”user”> {{user.name}} contacted through {{contact!.email}} </span>’ }) class MyComponent { user?: User; contact?: Contact; setData(user: User, contact: Contact) { this.user = user; this.contact = contact; } }

  1. What is type narrowing? The expression used in an ngIf directive is used to narrow type unions in the Angular template compiler similar to if expression in typescript. So *ngIf allows the typeScript compiler to infer that the data used in the binding expression will never be undefined. @Component({ selector: ‘my-component’, template: ‘<span *ngIf=”user”> {{user.contact.email}} </span>’ }) class MyComponent { user?: User; }

  1. How do you describe various dependencies in angular application? The dependencies section of package.json with in an angular application can be divided as follow,
    1. Angular packages: Angular core and optional modules; their package names begin @angular/.
    2. Support packages: Third-party libraries that must be present for Angular apps to run.
    3. Polyfill packages: Polyfills plug gaps in a browser’s JavaScript implementation.

  1. What is zone? A Zone is an execution context that persists across async tasks. Angular relies on zone.js to run Angular’s change detection processes when native JavaScript operations raise events

  1. What is the purpose of common module? The commonly-needed services, pipes, and directives provided by @angular/common module. Apart from these HttpClientModule is available under @angular/common/http.

  1. What is codelyzer? Codelyzer provides set of tslint rules for static code analysis of Angular TypeScript projects. ou can run the static code analyzer over web apps, NativeScript, Ionic etc. Angular CLI has support for this and it can be use as below, ng new codelyzer ng lint

  1. What is angular animation? Angular’s animation system is built on CSS functionality in order to animate any property that the browser considers animatable. These properties includes positions, sizes, transforms, colors, borders etc. The Angular modules for animations are @angular/animations and @angular/platform-browser and these dependencies are automatically added to your project when you create a project using Angular CLI.

  1. What are the steps to use animation module? You need to follow below steps to implement animation in your angular project,
    1. Enabling the animations module: Import BrowserAnimationsModule to add animation capabilities into your Angular root application module(for example, src/app/app.module.ts). import { NgModule } from ‘@angular/core’; import { BrowserModule } from ‘@angular/platform-browser’; import { BrowserAnimationsModule } from ‘@angular/platform-browser/animations’; @NgModule({ imports: [ BrowserModule, BrowserAnimationsModule ], declarations: [ ], bootstrap: [ ] }) export class AppModule { }
    2. Importing animation functions into component files: Import required animation functions from @angular/animations in component files(for example, src/app/app.component.ts). import { trigger, state, style, animate, transition, // … } from ‘@angular/animations’;
    3. Adding the animation metadata property: add a metadata property called animations: within the @Component() decorator in component files(for example, src/app/app.component.ts) @Component({ selector: ‘app-root’, templateUrl: ‘app.component.html’, styleUrls: [‘app.component.css’], animations: [ // animation triggers go here ] })

  1. What is State function? Angular’s state() function is used to define different states to call at the end of each transition. This function takes two arguments: a unique name like open or closed and a style() function. For example, you can write a open state function state(‘open’, style({ height: ‘300px’, opacity: 0.5, backgroundColor: ‘blue’ })),

  1. What is Style function? The style function is used to define a set of styles to associate with a given state name. You need to use it along with state() function to set CSS style attributes. For example, in the close state, the button has a height of 100 pixels, an opacity of 0.8, and a background color of green. state(‘close’, style({ height: ‘100px’, opacity: 0.8, backgroundColor: ‘green’ })), Note: The style attributes must be in camelCase.

  1. What is the purpose of animate function? Angular Animations are a powerful way to implement sophisticated and compelling animations for your Angular single page web application. import { Component, OnInit, Input } from ‘@angular/core’; import { trigger, state, style, animate, transition } from ‘@angular/animations’; @Component({ selector: ‘app-animate’, templateUrl: `<div [@changeState]=”currentState” class=”myblock mx-auto”></div>`, styleUrls: `.myblock { background-color: green; width: 300px; height: 250px; border-radius: 5px; margin: 5rem; }`, animations: [ trigger(‘changeState’, [ state(‘state1’, style({ backgroundColor: ‘green’, transform: ‘scale(1)’ })), state(‘state2’, style({ backgroundColor: ‘red’, transform: ‘scale(1.5)’ })), transition(‘*=>state1’, animate(‘300ms’)), transition(‘*=>state2’, animate(‘2000ms’)) ]) ] }) export class AnimateComponent implements OnInit { @Input() currentState; constructor() { } ngOnInit() { } }

  1. What is transition function? The animation transition function is used to specify the changes that occur between one state and another over a period of time. It accepts two arguments: the first argument accepts an expression that defines the direction between two transition states, and the second argument accepts an animate() function. Let’s take an example state transition from open to closed with an half second transition between states. transition(‘open => closed’, [ animate(‘500ms’) ]),

  1. How to inject the dynamic script in angular? Using DomSanitizer we can inject the dynamic Html,Style,Script,Url. import { Component, OnInit } from '@angular/core'; import { DomSanitizer } from '@angular/platform-browser'; @Component({ selector: 'my-app', template: ` <div [innerHtml]="htmlSnippet"></div> `, }) export class App { constructor(protected sanitizer: DomSanitizer) {} htmlSnippet: string = this.sanitizer.bypassSecurityTrustScript("<script>safeCode()</script>"); }

  1. What is a service worker and its role in Angular? A service worker is a script that runs in the web browser and manages caching for an application. Starting from 5.0.0 version, Angular ships with a service worker implementation. Angular service worker is designed to optimize the end user experience of using an application over a slow or unreliable network connection, while also minimizing the risks of serving outdated content.

  1. What are the design goals of service workers? Below are the list of design goals of Angular’s service workers,
    1. It caches an application just like installing a native application
    2. A running application continues to run with the same version of all files without any incompatible files
    3. When you refresh the application, it loads the latest fully cached version
    4. When changes are published then it immediately updates in the background
    5. Service workers saves the bandwidth by downloading the resources only when they changed.

  1. What are the differences between AngularJS and Angular with respect to dependency injection? Dependency injection is a common component in both AngularJS and Angular, but there are some key differences between the two frameworks in how it actually works. AngularJS Angular Dependency injection tokens are always strings Tokens can have different types. They are often classes and sometimes can be strings. There is exactly one injector even though it is a multi-module applications There is a tree hierarchy of injectors, with a root injector and an additional injector for each component.

  1. What is Angular Ivy? Angular Ivy is a new rendering engine for Angular. You can choose to opt in a preview version of Ivy from Angular version 8.
    1. You can enable ivy in a new project by using the –enable-ivy flag with the ng new command ng new ivy-demo-app –enable-ivy
    2. You can add it to an existing project by adding enableIvy option in the angularCompilerOptions in your project’s tsconfig.app.json. { “compilerOptions”: { … }, “angularCompilerOptions”: { “enableIvy”: true } }

  1. What are the features included in ivy preview? You can expect below features with Ivy preview,
    1. Generated code that is easier to read and debug at runtime
    2. Faster re-build time
    3. Improved payload size
    4. Improved template type checking

  1. Can I use AOT compilation with Ivy? Yes, it is a recommended configuration. Also, AOT compilation with Ivy is faster. So you need set the default build options(with in angular.json) for your project to always use AOT compilation. { “projects”: { “my-project”: { “architect”: { “build”: { “options”: { … “aot”: true, } } } } } }

  1. What is Angular Language Service? The Angular Language Service is a way to get completions, errors, hints, and navigation inside your Angular templates whether they are external in an HTML file or embedded in annotations/decorators in a string. It has the ability to autodetect that you are opening an Angular file, reads your tsconfig.json file, finds all the templates you have in your application, and then provides all the language services.

  1. How do you install angular language service in the project? You can install Angular Language Service in your project with the following npm command, npm install –save-dev @angular/language-service After that add the following to the “compilerOptions” section of your project’s tsconfig.json “plugins”: [ {“name”: “@angular/language-service”} ] Note: The completion and diagnostic services works for .ts files only. You need to use custom plugins for supporting HTML files.

  1. Is there any editor support for Angular Language Service? Yes, Angular Language Service is currently available for Visual Studio Code and WebStorm IDEs. You need to install angular language service using an extension and devDependency respectively. In sublime editor, you need to install typescript which has has a language service plugin model.

  1. Explain the features provided by Angular Language Service? Basically there are 3 main features provided by Angular Language Service,
    1. Autocompletion: Autocompletion can speed up your development time by providing you with contextual possibilities and hints as you type with in an interpolation and elements.
    2. Error checking: It can also warn you of mistakes in your code.
    3. Navigation: Navigation allows you to hover a component, directive, module and then click and press F12 to go directly to its definition.

  1. How do you add web workers in your application? You can add web worker anywhere in your application. For example, If the file that contains your expensive computation is src/app/app.component.ts, you can add a Web Worker using ng generate web-worker app command which will create src/app/app.worker.ts web worker file. This command will perform below actions,
    1. Configure your project to use Web Workers
    2. Adds app.worker.ts to receive messages addEventListener(‘message’, ({ data }) => { const response = `worker response to ${data}`; postMessage(response); });
    3. The component app.component.ts file updated with web worker file if (typeof Worker !== ‘undefined’) { // Create a new const worker = new Worker(‘./app.worker’, { type: ‘module’ }); worker.onmessage = ({ data }) => { console.log(‘page got message: $\{data\}’); }; worker.postMessage(‘hello’); } else { // Web Workers are not supported in this environment. }
    Note: You may need to refactor your initial scaffolding web worker code for sending messages to and from.

  1. What are the limitations with web workers? You need to remember two important things when using Web Workers in Angular projects,
    1. Some environments or platforms(like @angular/platform-server) used in Server-side Rendering, don’t support Web Workers. In this case you need to provide a fallback mechanism to perform the computations to work in this environments.
    2. Running Angular in web worker using @angular/platform-webworker is not yet supported in Angular CLI.

  1. What is Angular CLI Builder? In Angular8, the CLI Builder API is stable and available to developers who want to customize the Angular CLI by adding or modifying commands. For example, you could supply a builder to perform an entirely new task, or to change which third-party tool is used by an existing command.

  1. What is a builder? A builder function ia a function that uses the Architect API to perform a complex process such as “build” or “test”. The builder code is defined in an npm package. For example, BrowserBuilder runs a webpack build for a browser target and KarmaBuilder starts the Karma server and runs a webpack build for unit tests.

  1. How do you invoke a builder? The Angular CLI command ng run is used to invoke a builder with a specific target configuration. The workspace configuration file, angular.json, contains default configurations for built-in builders.

  1. How do you create app shell in Angular? An App shell is a way to render a portion of your application via a route at build time. This is useful to first paint of your application that appears quickly because the browser can render static HTML and CSS without the need to initialize JavaScript. You can achieve this using Angular CLI which generates an app shell for running server-side of your app. ng generate appShell [options] (or) ng g appShell [options]

  1. What are the case types in Angular? Angular uses capitalization conventions to distinguish the names of various types. Angular follows the list of the below case types.
    1. camelCase : Symbols, properties, methods, pipe names, non-component directive selectors, constants uses lowercase on the first letter of the item. For example, “selectedUser”
    2. UpperCamelCase (or PascalCase): Class names, including classes that define components, interfaces, NgModules, directives, and pipes uses uppercase on the first letter of the item.
    3. dash-case (or “kebab-case”): The descriptive part of file names, component selectors uses dashes between the words. For example, “app-user-list”.
    4. UPPER_UNDERSCORE_CASE: All constants uses capital letters connected with underscores. For example, “NUMBER_OF_USERS”.

  1. What are the class decorators in Angular? A class decorator is a decorator that appears immediately before a class definition, which declares the class to be of the given type, and provides metadata suitable to the type The following list of decorators comes under class decorators,
    1. @Component()
    2. @Directive()
    3. @Pipe()
    4. @Injectable()
    5. @NgModule()

  1. What are class field decorators? The class field decorators are the statements declared immediately before a field in a class definition that defines the type of that field. Some of the examples are: @input and @output, @Input() myProperty; @Output() myEvent = new EventEmitter();

  1. What is declarable in Angular? Declarable is a class type that you can add to the declarations list of an NgModule. The class types such as components, directives, and pipes comes can be declared in the module. The structure of declarations would be, declarations: [ YourComponent, YourPipe, YourDirective ],

  1. What are the restrictions on declarable classes? Below classes shouldn’t be declared,
    1. A class that’s already declared in another NgModule
    2. Ngmodule classes
    3. Service classes
    4. Helper classes

  1. What is a DI token? A DI token is a lookup token associated with a dependency provider in dependency injection system. The injector maintains an internal token-provider map that it references when asked for a dependency and the DI token is the key to the map. Let’s take example of DI Token usage, const BASE_URL = new InjectionToken<string>(‘BaseUrl’); const injector = Injector.create({providers: [{provide: BASE_URL, useValue: ‘http://some-domain.com’}]}); const url = injector.get(BASE_URL);

  1. What is Angular DSL? A domain-specific language (DSL) is a computer language specialized to a particular application domain. Angular has its own Domain Specific Language (DSL) which allows us to write Angular specific html-like syntax on top of normal html. It has its own compiler that compiles this syntax to html that the browser can understand. This DSL is defined in NgModules such as animations, forms, and routing and navigation. Basically you will see 3 main syntax in Angular DSL.
    1. (): Used for Output and DOM events.
    2. []: Used for Input and specific DOM element attributes.
    3. *: Structural directives(*ngFor or *ngIf) will affect/change the DOM structure.

  1. what is an rxjs subject in Angular An RxJS Subject is a special type of Observable that allows values to be multicasted to many Observers. While plain Observables are unicast (each subscribed Observer owns an independent execution of the Observable), Subjects are multicast. A Subject is like an Observable, but can multicast to many Observers. Subjects are like EventEmitters: they maintain a registry of many listeners. import { Subject } from ‘rxjs’; const subject = new Subject<number>(); subject.subscribe({ next: (v) => console.log(`observerA: ${v}`) }); subject.subscribe({ next: (v) => console.log(`observerB: ${v}`) }); subject.next(1); subject.next(2);

  1. What is Bazel tool? Bazel is a powerful build tool developed and massively used by Google and it can keep track of the dependencies between different packages and build targets. In Angular8, you can build your CLI application with Bazel. Note: The Angular framework itself is built with Bazel.

  1. What are the advantages of Bazel tool? Below are the list of key advantages of Bazel tool,
    1. It creates the possibility of building your back-ends and front-ends with the same tool
    2. The incremental build and tests
    3. It creates the possibility to have remote builds and cache on a build farm.

  1. How do you use Bazel with Angular CLI? The @angular/bazel package provides a builder that allows Angular CLI to use Bazel as the build tool.
    1. Use in an existing applciation: Add @angular/bazel using CLI ng add @angular/bazel
    2. Use in a new application: Install the package and create the application with collection option npm install -g @angular/bazel ng new –collection=@angular/bazel
    When you use ng build and ng serve commands, Bazel is used behind the scenes and outputs the results in dist/bin folder.

  1. How do you run Bazel directly? Sometimes you may want to bypass the Angular CLI builder and run Bazel directly using Bazel CLI. You can install it globally using @bazel/bazel npm package. i.e, Bazel CLI is available under @bazel/bazel package. After you can apply the below common commands, bazel build [targets] // Compile the default output artifacts of the given targets. bazel test [targets] // Run the tests with *_test targets found in the pattern. bazel run [target]: Compile the program represented by target and then run it.

  1. What is platform in Angular? A platform is the context in which an Angular application runs. The most common platform for Angular applications is a web browser, but it can also be an operating system for a mobile device, or a web server. The runtime-platform is provided by the @angular/platform-* packages and these packages allow applications that make use of @angular/core and @angular/common to execute in different environments. i.e, Angular can be used as platform-independent framework in different environments, For example,
    1. While running in the browser, it uses platform-browser package.
    2. When SSR(server-side rendering ) is used, it uses platform-server package for providing web server implementation.

  1. What happens if I import the same module twice? If multiple modules imports the same module then angular evaluates it only once (When it encounters the module first time). It follows this condition even the module appears at any level in a hierarchy of imported NgModules.

  1. How do you select an element with in a component template? You can use @ViewChild directive to access elements in the view directly. Let’s take input element with a reference, <input #uname> and define view child directive and access it in ngAfterViewInit lifecycle hook @ViewChild(‘uname’) input; ngAfterViewInit() { console.log(this.input.nativeElement.value); }

  1. How do you detect route change in Angular? In Angular7, you can subscribe to router to detect the changes. The subscription for router events would be as below, this.router.events.subscribe((event: Event) => {}) Let’s take a simple component to detect router changes import { Component } from ‘@angular/core’; import { Router, Event, NavigationStart, NavigationEnd, NavigationError } from ‘@angular/router’; @Component({ selector: ‘app-root’, template: `<router-outlet></router-outlet>` }) export class AppComponent { constructor(private router: Router) { this.router.events.subscribe((event: Event) => { if (event instanceof NavigationStart) { // Show loading indicator and perform an action } if (event instanceof NavigationEnd) { // Hide loading indicator and perform an action } if (event instanceof NavigationError) { // Hide loading indicator and perform an action console.log(event.error); // It logs an error for debugging } }); } }

How do you pass headers for HTTP client?

You can directly pass object map for http client or create HttpHeaders class to supply the headers.

constructor(private _http: HttpClient) {}
this._http.get('someUrl',{
   headers: {'header1':'value1','header2':'value2'}
});

(or)
let headers = new HttpHeaders().set('header1', headerValue1); // create header object
headers = headers.append('header2', headerValue2); // add a new header, creating a new object
headers = headers.append('header3', headerValue3); // add another header

let params = new HttpParams().set('param1', value1); // create params object
params = params.append('param2', value2); // add a new param, creating a new object
params = params.append('param3', value3); // add another param

return this._http.get<any[]>('someUrl', { headers: headers, params: params })

What is the purpose of differential loading in CLI?

From Angular8 release onwards, the applications are built using differential loading strategy from CLI to build two separate bundles as part of your deployed application.

  1. The first build contains ES2015 syntax which takes the advantage of built-in support in modern browsers, ships less polyfills, and results in a smaller bundle size.
  2. The second build contains old ES5 syntax to support older browsers with all necessary polyfills. But this results in a larger bundle size.

Note: This strategy is used to support multiple browsers but it only load the code that the browser needs.

Is Angular supports dynamic imports?

Yes, Angular 8 supports dynamic imports in router configuration. i.e, You can use the import statement for lazy loading the module using loadChildren method and it will be understood by the IDEs(VSCode and WebStorm), webpack, etc. Previously, you have been written as below to lazily load the feature module. By mistake, if you have typo in the module name it still accepts the string and throws an error during build time.

{path: ‘user’, loadChildren: ‘./users/user.module#UserModulee’},

This problem is resolved by using dynamic imports and IDEs are able to find it during compile time itself.

{path: ‘user’, loadChildren: () => import(‘./users/user.module’).then(m => m.UserModule)};

What is lazy loading?

Lazy loading is one of the most useful concepts of Angular Routing. It helps us to download the web pages in chunks instead of downloading everything in a big bundle. It is used for lazy loading by asynchronously loading the feature module for routing whenever required using the property loadChildren. Let’s load both Customer and Order feature modules lazily as below,

const routes: Routes = [
  {
    path: 'customers',
    loadChildren: () => import('./customers/customers.module').then(module => module.CustomersModule)
  },
  {
    path: 'orders',
    loadChildren: () => import('./orders/orders.module').then(module => module.OrdersModule)
  },
  {
    path: '',
    redirectTo: '',
    pathMatch: 'full'
  }
];

What are workspace APIs?

Angular 8.0 release introduces Workspace APIs to make it easier for developers to read and modify the angular.json file instead of manually modifying it. Currently, the only supported storage3 format is the JSON-based format used by the Angular CLI. You can enable or add optimization option for build target as below,

import { NodeJsSyncHost } from '@angular-devkit/core/node';
import { workspaces } from '@angular-devkit/core';

async function addBuildTargetOption() {
    const host = workspaces.createWorkspaceHost(new NodeJsSyncHost());
    const workspace = await workspaces.readWorkspace('path/to/workspace/directory/', host);

    const project = workspace.projects.get('my-app');
    if (!project) {
      throw new Error('my-app does not exist');
    }

    const buildTarget = project.targets.get('build');
    if (!buildTarget) {
      throw new Error('build target does not exist');
    }

    buildTarget.options.optimization = true;

    await workspaces.writeWorkspace(workspace, host);
}

addBuildTargetOption();

How do you upgrade angular version?

The Angular upgrade is quite easier using Angular CLI ng update command as mentioned below. For example, if you upgrade from Angular 7 to 8 then your lazy loaded route imports will be migrated to the new import syntax automatically.

$ ng update @angular/cli @angular/core

What is Angular Material?

Angular Material is a collection of Material Design components for Angular framework following the Material Design spec. You can apply Material Design very easily using Angular Material. The installation can be done through npm or yarn,

npm install --save @angular/material @angular/cdk @angular/animations
(OR)
yarn add @angular/material @angular/cdk @angular/animations

It supports the most recent two versions of all major browsers. The latest version of Angular material is 8.1.1

How do you upgrade location service of angularjs?

If you are using $location service in your old AngularJS application, now you can use LocationUpgradeModule(unified location service) which puts the responsibilities of $location service to Location service in Angular. Let’s add this module to AppModule as below,

// Other imports ...
import { LocationUpgradeModule } from '@angular/common/upgrade';

@NgModule({
  imports: [
    // Other NgModule imports...
    LocationUpgradeModule.config()
  ]
})
export class AppModule {}

What is NgUpgrade?

NgUpgrade is a library put together by the Angular team, which you can use in your applications to mix and match AngularJS and Angular components and bridge the AngularJS and Angular dependency injection systems.

How do you test Angular application using CLI?

Angular CLI downloads and install everything needed with the Jasmine Test framework. You just need to run ng test to see the test results. By default this command builds the app in watch mode, and launches the Karma test runner. The output of test results would be as below,

10% building modules 1/1 modules 0 active
...INFO [karma]: Karma v1.7.1 server started at http://0.0.0.0:9876/
...INFO [launcher]: Launching browser Chrome ...
...INFO [launcher]: Starting browser Chrome
...INFO [Chrome ...]: Connected on socket ...
Chrome ...: Executed 3 of 3 SUCCESS (0.135 secs / 0.205 secs)

Note: A chrome browser also opens and displays the test output in the “Jasmine HTML Reporter”.

How to use polyfills in Angular application?

The Angular CLI provides support for polyfills officially. When you create a new project with the ng new command, a src/polyfills.ts configuration file is created as part of your project folder. This file includes the mandatory and many of the optional polyfills as JavaScript import statements. Let’s categorize the polyfills,

  1. Mandatory polyfills: These are installed automatically when you create your project with ng new command and the respective import statements enabled in ‘src/polyfills.ts’ file.
  2. Optional polyfills: You need to install its npm package and then create import statement in ‘src/polyfills.ts’ file. For example, first you need to install below npm package for adding web animations (optional) polyfill. bash npm install --save web-animations-js and create import statement in polyfill file. javascript import 'web-animations-js';

What are the ways to trigger change detection in Angular?

You can inject either ApplicationRef or NgZone, or ChangeDetectorRef into your component and apply below specific methods to trigger change detection in Angular. i.e, There are 3 possible ways,

  1. ApplicationRef.tick(): Invoke this method to explicitly process change detection and its side-effects. It check the full component tree.
  2. NgZone.run(callback): It evaluate the callback function inside the Angular zone.
  3. ChangeDetectorRef.detectChanges(): It detects only the components and it’s children.

What are the differences of various versions of Angular?

There are different versions of Angular framework. Let’s see the features of all the various versions,

  1. Angular 1:
    • Angular 1 (AngularJS) is the first angular framework released in the year 2010.
    • AngularJS is not built for mobile devices.
    • It is based on controllers with MVC architecture.
  2. Angular 2:
    • Angular 2 was released in the year 2016. Angular 2 is a complete rewrite of Angular1 version.
    • The performance issues that Angular 1 version had has been addressed in Angular 2 version.
    • Angular 2 is built from scratch for mobile devices unlike Angular 1 version.
    • Angular 2 is components based.
  3. Angular 3:
    • The following are the different package versions in Angular 2:
      • @angular/core v2.3.0
      • @angular/compiler v2.3.0
      • @angular/http v2.3.0
      • @angular/router v3.3.0
    • The router package is already versioned 3 so to avoid confusion switched to Angular 4 version and skipped 3 version.
  4. Angular 4:
    • The compiler generated code file size in AOT mode is very much reduced.
    • With Angular 4 the production bundles size is reduced by hundreds of KB’s.
    • Animation features are removed from angular/core and formed as a separate package.
    • Supports Typescript 2.1 and 2.2.
    • Angular Universal
    • New HttpClient
  5. Angular 5:
    • Angular 5 makes angular faster. It improved the loading time and execution time.
    • Shipped with new build optimizer.
    • Supports Typescript 2.5.
    • Service Worker
  6. Angular 6:
    • It is released in May 2018.
    • Includes Angular Command Line Interface (CLI), Component Development KIT (CDK), Angular Material Package, Angular Elements.
    • Service Worker bug fixes.
    • i18n
    • Experimental mode for Ivy.
    • RxJS 6.0
    • Tree Shaking
  7. Angular 7:
    • It is released in October 2018.
    • TypeScript 3.1
    • RxJS 6.3
    • New Angular CLI
    • CLI Prompts capability provide an ability to ask questions to the user before they run. It is like interactive dialog between the user and the CLI
    • With the improved CLI Prompts capability, it helps developers to make the decision. New ng commands ask users for routing and CSS styles types(SCSS) and ng add @angular/material asks for themes and gestures or animations.
  8. Angular 8:
    • It is released in May 2019.
    • TypeScript 3.4
  9. Angular 9:
    • It is released in February 2020.
    • TypeScript 3.7
    • Ivy enabled by default
  10. Angular 10:
    • It is released in June 2020.
    • TypeScript 3.9
    • TSlib 2.0

What are the security principles in angular?

Below are the list of security principles in angular,

  1. You should avoid direct use of the DOM APIs.
  2. You should enable Content Security Policy (CSP) and configure your web server to return appropriate CSP HTTP headers.
  3. You should Use the offline template compiler.
  4. You should Use Server Side XSS protection.
  5. You should Use DOM Sanitizer.
  6. You should Preventing CSRF or XSRF attacks.

What is the reason to deprecate Web Tracing Framework?

Angular has supported the integration with the Web Tracing Framework (WTF) for the purpose of performance testing. Since it is not well maintained and failed in majority of the applications, the support is deprecated in latest releases.

What is the reason to deprecate web worker packages?

Both @angular/platform-webworker and @angular/platform-webworker-dynamic are officially deprecated, the Angular team realized it’s not good practice to run the Angular application on Web worker

How do you find angular CLI version?

Angular CLI provides it’s installed version using below different ways using ng command,

ng v
ng version
ng -v
ng --version

and the output would be as below,

Angular CLI: 1.6.3
Node: 8.11.3
OS: darwin x64
Angular:
...

What is the browser support for Angular?

Angular supports most recent browsers which includes both desktop and mobile browsers.

BrowserVersion
Chromelatest
Firefoxlatest
Edge2 most recent major versions
IE11, 10, 9 (Compatibility mode is not supported)
Safari2 most recent major versions
IE Mobile11
iOS2 most recent major versions
Android7.0, 6.0, 5.0, 5.1, 4.4

What is schematic?

It’s a scaffolding library that defines how to generate or transform a programming project by creating, modifying, refactoring, or moving files and code. It defines rules that operate on a virtual file system called a tree.

What is rule in Schematics?

In schematics world, it’s a function that operates on a file tree to create, delete, or modify files in a specific manner.

What is Schematics CLI?

Schematics come with their own command-line tool known as Schematics CLI. It is used to install the schematics executable, which you can use to create a new schematics collection with an initial named schematic. The collection folder is a workspace for schematics. You can also use the schematics command to add a new schematic to an existing collection, or extend an existing schematic. You can install Schematic CLI globally as below,

npm install -g @angular-devkit/schematics-cli

What are the best practices for security in angular?

Below are the best practices of security in angular,

  1. Use the latest Angular library releases
  2. Don’t modify your copy of Angular
  3. Avoid Angular APIs marked in the documentation as “Security Risk.”

What is Angular security model for preventing XSS attacks?

Angular treats all values as untrusted by default. i.e, Angular sanitizes and escapes untrusted values When a value is inserted into the DOM from a template, via property, attribute, style, class binding, or interpolation.

What is the role of template compiler for prevention of XSS attacks?

The offline template compiler prevents vulnerabilities caused by template injection, and greatly improves application performance. So it is recommended to use offline template compiler in production deployments without dynamically generating any template.

What are the various security contexts in Angular?

Angular defines the following security contexts for sanitization,

  1. HTML: It is used when interpreting a value as HTML such as binding to innerHtml.
  2. Style: It is used when binding CSS into the style property.
  3. URL: It is used for URL properties such as <a href>.
  4. Resource URL: It is a URL that will be loaded and executed as code such as <script src>.

What is Sanitization? Is angular supports it?

Sanitization is the inspection of an untrusted value, turning it into a value that’s safe to insert into the DOM. Yes, Angular suppports sanitization. It sanitizes untrusted values for HTML, styles, and URLs but sanitizing resource URLs isn’t possible because they contain arbitrary code.

What is the purpose of innerHTML?

The innerHtml is a property of HTML-Elements, which allows you to set it’s html-content programmatically. Let’s display the below html code snippet in a <div> tag as below using innerHTML binding,

<div [innerHTML]="htmlSnippet"></div>

and define the htmlSnippet property from any component

export class myComponent {
  htmlSnippet: string = '<b>Hello World</b>, Angular';
}

Unfortunately this property could cause Cross Site Scripting (XSS) security bugs when improperly handled.

What is the difference between interpolated content and innerHTML?

The main difference between interpolated and innerHTML code is the behavior of code interpreted. Interpolated content is always escaped i.e, HTML isn’t interpreted and the browser displays angle brackets in the element’s text content. Where as in innerHTML binding, the content is interpreted i.e, the browser will convert < and > characters as HTMLEntities. For example, the usage in template would be as below,

<p>Interpolated value:</p>
<div >{{htmlSnippet}}</div>
<p>Binding of innerHTML:</p>
<div [innerHTML]="htmlSnippet"></div>

and the property defined in a component.

export class InnerHtmlBindingComponent {
  htmlSnippet = 'Template <script>alert("XSS Attack")</script> <b>Code attached</b>';
}

Even though innerHTML binding create a chance of XSS attack, Angular recognizes the value as unsafe and automatically sanitizes it.

How do you prevent automatic sanitization?

Sometimes the applications genuinely need to include executable code such as displaying <iframe> from an URL. In this case, you need to prevent automatic sanitization in Angular by saying that you inspected a value, checked how it was generated, and made sure it will always be secure. Basically it involves 2 steps,

  1. Inject DomSanitizer: You can inject DomSanitizer in component as parameter in constructor
  2. Mark the trusted value by calling some of the below methods
    1. bypassSecurityTrustHtml
    2. bypassSecurityTrustScript
    3. bypassSecurityTrustStyle
    4. bypassSecurityTrustUrl
    5. bypassSecurityTrustResourceUrl

For example,The usage of dangerous url to trusted url would be as below,

constructor(private sanitizer: DomSanitizer) {
  this.dangerousUrl = 'javascript:alert("XSS attack")';
  this.trustedUrl = sanitizer.bypassSecurityTrustUrl(this.dangerousUrl);

Is safe to use direct DOM API methods in terms of security?

No,the built-in browser DOM APIs or methods don’t automatically protect you from security vulnerabilities. In this case it is recommended to use Angular templates instead of directly interacting with DOM. If it is unavoidable then use the built-in Angular sanitization functions.

What is DOM sanitizer?

DomSanitizer is used to help preventing Cross Site Scripting Security bugs (XSS) by sanitizing values to be safe to use in the different DOM contexts.

How do you support server side XSS protection in Angular application?

The server-side XSS protection is supported in an angular application by using a templating language that automatically escapes values to prevent XSS vulnerabilities on the server. But don’t use a templating language to generate Angular templates on the server side which creates a high risk of introducing template-injection vulnerabilities.

Is angular prevents http level vulnerabilities?

Angular has built-in support for preventing http level vulnerabilities such as as cross-site request forgery (CSRF or XSRF) and cross-site script inclusion (XSSI). Even though these vulnerabilities need to be mitigated on server-side, Angular provides helpers to make the integration easier on the client side.

  1. HttpClient supports a token mechanism used to prevent XSRF attacks
  2. HttpClient library recognizes the convention of prefixed JSON responses(which non-executable js code with “)]}’,\n” characters) and automatically strips the string “)]}’,\n” from all responses before further parsing

What are Http Interceptors?

Http Interceptors are part of @angular/common/http, which inspect and transform HTTP requests from your application to the server and vice-versa on HTTP responses. These interceptors can perform a variety of implicit tasks, from authentication to logging.

The syntax of HttpInterceptor interface looks like as below,

interface HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>>
}

You can use interceptors by declaring a service class that implements the intercept() method of the HttpInterceptor interface.

@Injectable()
export class MyInterceptor implements HttpInterceptor {
    constructor() {}
    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        ...
    }
}

After that you can use it in your module,

@NgModule({
    ...
    providers: [
        {
            provide: HTTP_INTERCEPTORS,
            useClass: MyInterceptor,
            multi: true
        }
    ]
    ...
})
export class AppModule {}

What are the applications of HTTP interceptors?

The HTTP Interceptors can be used for different variety of tasks,

  1. Authentication
  2. Logging
  3. Caching
  4. Fake backend
  5. URL transformation
  6. Modifying headers

Is multiple interceptors supported in Angular?

Yes, Angular supports multiple interceptors at a time. You could define multiple interceptors in providers property:

providers: [
  { provide: HTTP_INTERCEPTORS, useClass: MyFirstInterceptor, multi: true },
  { provide: HTTP_INTERCEPTORS, useClass: MySecondInterceptor, multi: true }
],

The interceptors will be called in the order in which they were provided. i.e, MyFirstInterceptor will be called first in the above interceptors configuration.

How can I use interceptor for an entire application?

You can use same instance of HttpInterceptors for the entire app by importing the HttpClientModule only in your AppModule, and add the interceptors to the root application injector. For example, let’s define a class that is injectable in root application.

@Injectable()
export class MyInterceptor implements HttpInterceptor {
  intercept(
    req: HttpRequest<any>,
    next: HttpHandler
  ): Observable<HttpEvent<any>> {

    return next.handle(req).do(event => {
      if (eventt instanceof HttpResponse) {
           // Code goes here
      }
    });

  }
}

After that import HttpClientModule in AppModule

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule, HttpClientModule],
  providers: [
    { provide: HTTP_INTERCEPTORS, useClass: MyInterceptor, multi: true }
  ],
  bootstrap: [AppComponent]
})
export class AppModule {}

How does Angular simplifies Internationalization?

Angular simplifies the below areas of internationalization,

  1. Displaying dates, number, percentages, and currencies in a local format.
  2. Preparing text in component templates for translation.
  3. Handling plural forms of words.
  4. Handling alternative text.

How do you manually register locale data?

By default, Angular only contains locale data for en-US which is English as spoken in the United States of America . But if you want to set to another locale, you must import locale data for that new locale. After that you can register using registerLocaleData method and the syntax of this method looks like below,

registerLocaleData(data: any, localeId?: any, extraData?: any): void

For example, let us import German locale and register it in the application

import { registerLocaleData } from '@angular/common';
import localeDe from '@angular/common/locales/de';

registerLocaleData(localeDe, 'de');

What are the four phases of template translation?

The i18n template translation process has four phases:

  1. Mark static text messages in your component templates for translation: You can place i18n on every element tag whose fixed text is to be translated. For example, you need i18n attribue for heading as below, <h1 i18n>Hello i18n!</h1>
  2. Create a translation file: Use the Angular CLI xi18n command to extract the marked text into an industry-standard translation source file. i.e, Open terminal window at the root of the app project and run the CLI command xi18n. ng xi18n The above command creates a file named messages.xlf in your project’s root directory. Note: You can supply command options to change the format, the name, the location, and the source locale of the extracted file.
  3. Edit the generated translation file: Translate the extracted text into the target language. In this step, create a localization folder (such as locale)under root directory(src) and then create target language translation file by copying and renaming the default messages.xlf file. You need to copy source text node and provide the translation under target tag. For example, create the translation file(messages.de.xlf) for German language <trans-unit id=”greetingHeader” datatype=”html”> <source>Hello i18n!</source> <target>Hallo i18n !</target> <note priority=”1″ from=”description”>A welcome header for this sample</note> <note priority=”1″ from=”meaning”>welcome message</note> </trans-unit>
  4. Merge the completed translation file into the app: You need to use Angular CLI build command to compile the app, choosing a locale-specific configuration, or specifying the following command options.
    1. –i18nFile=path to the translation file
    2. –i18nFormat=format of the translation file
    3. –i18nLocale= locale id

What is the purpose of i18n attribute?

The Angular i18n attribute marks translatable content. It is a custom attribute, recognized by Angular tools and compilers. The compiler removes it after translation.

Note: Remember that i18n is not an Angular directive.

What is the purpose of custom id?

When you change the translatable text, the Angular extractor tool generates a new id for that translation unit. Because of this behavior, you must then update the translation file with the new id every time.

For example, the translation file messages.de.xlf.html has generated trans-unit for some text message as below

<trans-unit id="827wwe104d3d69bf669f823jjde888" datatype="html">

You can avoid this manual update of id attribute by specifying a custom id in the i18n attribute by using the prefix @@.

<h1 i18n="@@welcomeHeader">Hello i18n!</h1>

What happens if the custom id is not unique?

You need to define custom ids as unique. If you use the same id for two different text messages then only the first one is extracted. But its translation is used in place of both original text messages.

For example, let’s define same custom id myCustomId for two messages,

<h2 i18n="@@myCustomId">Good morning</h3>
<!-- ... -->
<h2 i18n="@@myCustomId">Good night</p>

and the translation unit generated for first text in for German language as

<trans-unit id="myId" datatype="html">
  <source>Good morning</source>
  <target state="new">Guten Morgen</target>
</trans-unit>

Since custom id is the same, both of the elements in the translation contain the same text as below

<h2>Guten Morgen</h2>
<h2>Guten Morgen</h2>

Can I translate text without creating an element?

Yes, you can achieve using <ng-container> attribute. Normally you need to wrap a text content with i18n attribute for the translation. But if you don’t want to create a new DOM element just for the sake of translation, you can wrap the text in an element.

<ng-container i18n>I'm not using any DOM element for translation</ng-container>

Remember that <ng-container> is transformed into an html comment

How can I translate attribute?

You can translate attributes by attaching i18n-x attribute where x is the name of the attribute to translate. For example, you can translate image title attribute as below,

<img [src]="example" i18n-title title="Internationlization" />

By the way, you can also assign meaning, description and id with the i18n-x=”|@@” syntax.

List down the pluralization categories?

Pluralization has below categories depending on the language.

  1. =0 (or any other number)
  2. zero
  3. one
  4. two
  5. few
  6. many
  7. other

What is select ICU expression?

ICU expression is is similar to the plural expressions except that you choose among alternative translations based on a string value instead of a number. Here you define those string values.

Let’s take component binding with residenceStatus property which has “citizen”, “permanent resident” and “foreigner” possible values and the message maps those values to the appropriate translations.

<span i18n>The person is {residenceStatus, select, citizen {citizen} permanent resident {permanentResident} foreigner {foreigner}}</span>

How do you report missing translations?

By default, When translation is missing, it generates a warning message such as “Missing translation for message ‘somekey'”. But you can configure with a different level of message in Angular compiler as below,

  1. Error: It throws an error. If you are using AOT compilation, the build will fail. But if you are using JIT compilation, the app will fail to load.
  2. Warning (default): It shows a ‘Missing translation’ warning in the console or shell.
  3. Ignore: It doesn’t do anything.

If you use AOT compiler then you need to perform changes in configurations section of your Angular CLI configuration file, angular.json.

"configurations": {
  ...
  "de": {
    ...
    "i18nMissingTranslation": "error"
  }
}

If you use the JIT compiler, specify the warning level in the compiler config at bootstrap by adding the ‘MissingTranslationStrategy’ property as below,

import { MissingTranslationStrategy } from '@angular/core';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { AppModule } from './app/app.module';

platformBrowserDynamic().bootstrapModule(AppModule, {
  missingTranslation: MissingTranslationStrategy.Error,
  providers: [
    // ...
  ]
});

How do you provide build configuration for multiple locales?

You can provide build configuration such as translation file path, name, format and application url in configuration settings of Angular.json file. For example, the German version of your application configured the build as follows,

"configurations": {
  "de": {
    "aot": true,
    "outputPath": "dist/my-project-de/",
    "baseHref": "/fr/",
    "i18nFile": "src/locale/messages.de.xlf",
    "i18nFormat": "xlf",
    "i18nLocale": "de",
    "i18nMissingTranslation": "error",
  }

What is an angular library?

An Angular library is an Angular project that differs from an app in that it cannot run on its own. It must be imported and used in an app. For example, you can import or add service worker library to an Angular application which turns an application into a Progressive Web App (PWA).

Note: You can create own third party library and publish it as npm package to be used in an Application.

What is AOT compiler?

The AOT compiler is part of a build process that produces a small, fast, ready-to-run application package, typically for production. It converts your Angular HTML and TypeScript code into efficient JavaScript code during the build phase before the browser downloads and runs that code.

How do you select an element in component template?

You can control any DOM element via ElementRef by injecting it into your component’s constructor. i.e, The component should have constructor with ElementRef parameter,

constructor(myElement: ElementRef) {
   el.nativeElement.style.backgroundColor = 'yellow';
}

What is TestBed?

TestBed is an api for writing unit tests for Angular applications and it’s libraries. Even though We still write our tests in Jasmine and run using Karma, this API provides an easier way to create components, handle injection, test asynchronous behaviour and interact with our application.

What is protractor?

Protractor is an end-to-end test framework for Angular and AngularJS applications. It runs tests against your application running in a real browser, interacting with it as a user would.

npm install -g protractor

What is collection?

Collection is a set of related schematics collected in an npm package. For example, @schematics/angular collection is used in Angular CLI to apply transforms to a web-app project. You can create your own schematic collection for customizing angular projects.

How do you create schematics for libraries?

You can create your own schematic collections to integrate your library with the Angular CLI. These collections are classified as 3 main schematics,

  1. Add schematics: These schematics are used to install library in an Angular workspace using ng add command. For example, @angular/material schematic tells the add command to install and set up Angular Material and theming.
  2. Generate schematics: These schematics are used to modify projects, add configurations and scripts, and scaffold artifacts in library using ng generate command. For example, @angular/material generation schematic supplies generation schematics for the UI components. Let’s say the table component is generated using ng generate @angular/material:table .
  3. Update schematics: These schematics are used to update library’s dependencies and adjust for breaking changes in a new library release using ng update command. For example, @angular/material update schematic updates material and cdk dependencies using ng update @angular/material command.

How do you use jquery in Angular?

You can use jquery in Angular using 3 simple steps,

  1. Install the dependency: At first, install the jquery dependency using npm npm install –save jquery
  2. Add the jquery script: In Angular-CLI project, add the relative path to jquery in the angular.json file. “scripts”: [ “node_modules/jquery/dist/jquery.min.js” ]
  3. Start using jquery: Define the element in template. Whereas declare the jquery variable and apply CSS classes on the element. <div id=”elementId”> <h1>JQuery integration</h1> </div> import {Component, OnInit} from ‘@angular/core’; declare var $: any; // (or) import * as $ from ‘jquery’; @Component({ selector: ‘app-root’, templateUrl: ‘./app.component.html’, styleUrls: [‘./app.component.css’] }) export class AppComponent implements OnInit { ngOnInit(): void { $(document).ready(() => { $(‘#elementId’).css({‘text-color’: ‘blue’, ‘font-size’: ‘150%’}); }); } }

What is the reason for No provider for HTTP exception?

This exception is due to missing HttpClientModule in your module. You just need to import in module as below,

import { HttpClientModule } from '@angular/common/http';

@NgModule({
  imports: [
    BrowserModule,
    HttpClientModule,
  ],
  declarations: [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

What is router state?

The RouteState is an interface which represents the state of the router as a tree of activated routes.

interface RouterState extends Tree {
  snapshot: RouterStateSnapshot
  toString(): string
}

You can access the current RouterState from anywhere in the Angular app using the Router service and the routerState property.

How can I use SASS in angular project?

When you are creating your project with angular cli, you can use ng newcommand. It generates all your components with predefined sass files.

ng new My_New_Project --style=sass

But if you are changing your existing style in your project then use ng set command,

ng set defaults.styleExt scss

What is the purpose of hidden property?

The hidden property is used to show or hide the associated DOM element, based on an expression. It can be compared close to ng-show directive in AngularJS. Let’s say you want to show user name based on the availability of user using hidden property.

<div [hidden]="!user.name">
  My name is: {{user.name}}
</div>

What is the difference between ngIf and hidden property?

The main difference is that *ngIf will remove the element from the DOM, while [hidden] actually plays with the CSS style by setting display:none. Generally it is expensive to add and remove stuff from the DOM for frequent actions.

What is slice pipe?

The slice pipe is used to create a new Array or String containing a subset (slice) of the elements. The syntax looks like as below,

{{ value_expression | slice : start [ : end ] }}

For example, you can provide ‘hello’ list based on a greeting array,

@Component({
  selector: 'list-pipe',
  template: `<ul>
    <li *ngFor="let i of greeting | slice:0:5">{{i}}</li>
  </ul>`
})
export class PipeListComponent {
  greeting: string[] = ['h', 'e', 'l', 'l', 'o', 'm','o', 'r', 'n', 'i', 'n', 'g'];
}

What is index property in ngFor directive?

The index property of the NgFor directive is used to return the zero-based index of the item in each iteration. You can capture the index in a template input variable and use it in the template.

For example, you can capture the index in a variable named indexVar and displays it with the todo’s name using ngFor directive as below.

<div *ngFor="let todo of todos; let i=index">{{i + 1}} - {{todo.name}}</div>

What is the purpose of ngFor trackBy?

The main purpose of using *ngFor with trackBy option is performance optimization. Normally if you use NgFor with large data sets, a small change to one item by removing or adding an item, can trigger a cascade of DOM manipulations. In this case, Angular sees only a fresh list of new object references and to replace the old DOM elements with all new DOM elements. You can help Angular to track which items added or removed by providing a trackBy function which takes the index and the current item as arguments and needs to return the unique identifier for this item.

For example, lets set trackBy to the trackByTodos() method

<div *ngFor="let todo of todos; trackBy: trackByTodos">
  ({{todo.id}}) {{todo.name}}
</div>

and define the trackByTodos method,

trackByTodos(index: number, item: Todo): number { return todo.id; }

What is the purpose of ngSwitch directive?

NgSwitch directive is similar to JavaScript switch statement which displays one element from among several possible elements, based on a switch condition. In this case only the selected element placed into the DOM. It has been used along with NgSwitch, NgSwitchCase and NgSwitchDefault directives.

For example, let’s display the browser details based on selected browser using ngSwitch directive.

<div [ngSwitch]="currentBrowser.name">
  <chrome-browser    *ngSwitchCase="'chrome'"    [item]="currentBrowser"></chrome-browser>
  <firefox-browser   *ngSwitchCase="'firefox'"     [item]="currentBrowser"></firefox-browser>
  <opera-browser     *ngSwitchCase="'opera'"  [item]="currentBrowser"></opera-browser>
  <safari-browser     *ngSwitchCase="'safari'"   [item]="currentBrowser"></safari-browser>
  <ie-browser  *ngSwitchDefault           [item]="currentItem"></ie-browser>
</div>

Is it possible to do aliasing for inputs and outputs?

Yes, it is possible to do aliasing for inputs and outputs in two ways.

  1. Aliasing in metadata: The inputs and outputs in the metadata aliased using a colon-delimited (:) string with the directive property name on the left and the public alias on the right. i.e. It will be in the format of propertyName:alias. inputs: [‘input1: buyItem’], outputs: [‘outputEvent1: completedEvent’]
  2. Aliasing with @Input()/@Output() decorator: The alias can be specified for the property name by passing the alias name to the @Input()/@Output() decorator.i.e. It will be in the form of @Input(alias) or @Output(alias). @Input(‘buyItem’) input1: string; @Output(‘completedEvent’) outputEvent1 = new EventEmitter<string>();

What is safe navigation operator?

The safe navigation operator(?)(or known as Elvis Operator) is used to guard against null and undefined values in property paths when you are not aware whether a path exists or not. i.e. It returns value of the object path if it exists, else it returns the null value.

For example, you can access nested properties of a user profile easily without null reference errors as below,

<p>The user firstName is: {{user?.fullName.firstName}}</p>

Using this safe navigation operator, Angular framework stops evaluating the expression when it hits the first null value and renders the view without any errors.

Is any special configuration required for Angular9?

You don’t need any special configuration. In Angular9, the Ivy renderer is the default Angular compiler. Even though Ivy is available Angular8 itself, you had to configure it in tsconfig.json file as below,

"angularCompilerOptions": {    "enableIvy": true  }

What are type safe TestBed API changes in Angular9?

Angular 9 provides type safe changes in TestBed API changes by replacing the old get function with the new inject method. Because TestBed.get method is not type-safe. The usage would be as below,

TestBed.get(ChangeDetectorRef) // returns any. It is deprecated now.

TestBed.inject(ChangeDetectorRef) // returns ChangeDetectorRef

Is mandatory to pass static flag for ViewChild?

In Angular 8, the static flag is required for ViewChild. Whereas in Angular9, you no longer need to pass this property. Once you updated to Angular9 using ng update, the migration will remove { static: false } script everywhere.

@ViewChild(ChildDirective) child: ChildDirective; // Angular9 usage
@ViewChild(ChildDirective, { static: false }) child: ChildDirective; //Angular8 usage

What are the list of template expression operators?

The Angular template expression language supports three special template expression operators.

  1. Pipe operator
  2. Safe navigation operator
  3. Non-null assertion operator

What is the precedence between pipe and ternary operators?

The pipe operator has a higher precedence than the ternary operator (?:). For example, the expression first ? second : third | fourth is parsed as first ? second : (third | fourth).

What is an entry component?

An entry component is any component that Angular loads imperatively(i.e, not referencing it in the template) by type. Due to this behavior, they can’t be found by the Angular compiler during compilation. These components created dynamically with ComponentFactoryResolver.

Basically, there are two main kinds of entry components which are following –

  1. The bootstrapped root component
  2. A component you specify in a route

What is a bootstrapped component?

A bootstrapped component is an entry component that Angular loads into the DOM during the bootstrap process or application launch time. Generally, this bootstrapped or root component is named as AppComponent in your root module using bootstrap property as below.

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpClientModule,
    AppRoutingModule
  ],
  providers: [],
  bootstrap: [AppComponent] // bootstrapped entry component need to be declared here
})

How do you manually bootstrap an application?

You can use ngDoBootstrap hook for a manual bootstrapping of the application instead of using bootstrap array in @NgModule annotation. This hook is part of DoBootstap interface.

interface DoBootstrap {
  ngDoBootstrap(appRef: ApplicationRef): void
}

The module needs to be implement the above interface to use the hook for bootstrapping.

class AppModule implements DoBootstrap {
  ngDoBootstrap(appRef: ApplicationRef) {
    appRef.bootstrap(AppComponent); // bootstrapped entry component need to be passed
  }
}

Is it necessary for bootstrapped component to be entry component?

Yes, the bootstrapped component needs to be an entry component. This is because the bootstrapping process is an imperative process.

What is a routed entry component?

The components referenced in router configuration are called as routed entry components. This routed entry component defined in a route definition as below,

const routes: Routes = [
  {
    path: '',
    component: TodoListComponent // router entry component
  }
];

Since router definition requires you to add the component in two places (router and entryComponents), these components are always entry components.

Note: The compilers are smart enough to recognize a router definition and automatically add the router component into entryComponents.

Why is not necessary to use entryComponents array every time?

Most of the time, you don’t need to explicity to set entry components in entryComponents array of ngModule decorator. Because angular adds components from both @NgModule.bootstrap and route definitions to entry components automatically.

Do I still need to use entryComponents array in Angular9?

No. In previous angular releases, the entryComponents array of ngModule decorator is used to tell the compiler which components would be created and inserted dynamically in the view. In Angular9, this is not required anymore with Ivy.

Is it all components generated in production build?

No, only the entry components and template components appears in production builds. If a component isn’t an entry component and isn’t found in a template, the tree shaker will throw it away. Due to this reason, make sure to add only true entry components to reduce the bundle size.

What is Angular compiler?

The Angular compiler is used to convert the application code into JavaScript code. It reads the template markup, combines it with the corresponding component class code, and emits component factories which creates JavaScript representation of the component along with elements of @Component metadata.

What is the role of ngModule metadata in compilation process?

The @NgModule metadata is used to tell the Angular compiler what components to be compiled for this module and how to link this module with other modules.

How does angular finds components, directives and pipes?

The Angular compiler finds a component or directive in a template when it can match the selector of that component or directive in that template. Whereas it finds a pipe if the pipe’s name appears within the pipe syntax of the template HTML.

Give few examples for NgModules?

The Angular core libraries and third-party libraries are available as NgModules.

  1. Angular libraries such as FormsModule, HttpClientModule, and RouterModule are NgModules.
  2. Many third-party libraries such as Material Design, Ionic, and AngularFire2 are NgModules.

What are feature modules?

Feature modules are NgModules, which are used for the purpose of organizing code. The feature module can be created with Angular CLI using the below command in the root directory,

ng generate module MyCustomFeature //

Angular CLI creates a folder called my-custom-feature with a file inside called my-custom-feature.module.ts with the following contents

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

@NgModule({
  imports: [
    CommonModule
  ],
  declarations: []
})
export class MyCustomFeature { }

Note: The “Module” suffix shouldn’t present in the name because the CLI appends it.

What are the imported modules in CLI generated feature modules?

In the CLI generated feature module, there are two JavaScript import statements at the top of the file

  1. NgModule: InOrder to use the @NgModule decorator
  2. CommonModule: It provides many common directives such as ngIf and ngFor.

What are the differences between ngmodule and javascript module?

Below are the main differences between Angular NgModule and javascript module,

NgModuleJavaScript module
NgModule bounds declarable classes onlyThere is no restriction classes
List the module’s classes in declarations array onlyCan define all member classes in one giant file
It only export the declarable classes it owns or imports from other modulesIt can export any classes
Extend the entire application with services by adding providers to provides arrayCan’t extend the application with services

What are the possible errors with declarations?

There are two common possible errors with declarations array,

  1. If you use a component without declaring it, Angular returns an error message.
  2. If you try to declare the same class in more than one module then compiler emits an error.

What are the steps to use declaration elements?

Below are the steps to be followed to use declaration elements.

  1. Create the element(component, directive and pipes) and export it from the file where you wrote it
  2. Import it into the appropriate module.
  3. Declare it in the @NgModule declarations array.

What happens if browserModule used in feature module?

If you do import BrowserModule into a lazy loaded feature module, Angular returns an error telling you to use CommonModule instead. Because BrowserModule’s providers are for the entire app so it should only be in the root module, not in feature module. Whereas Feature modules only need the common directives in CommonModule.

ScreenShot

What are the types of feature modules?

Below are the five categories of feature modules,

  1. Domain: Deliver a user experience dedicated to a particular application domain(For example, place an order, registration etc)
  2. Routed: These are domain feature modules whose top components are the targets of router navigation routes.
  3. Routing: It provides routing configuration for another module.
  4. Service: It provides utility services such as data access and messaging(For example, HttpClientModule)
  5. Widget: It makes components, directives, and pipes available to external modules(For example, third-party libraries such as Material UI)

What is a provider?

A provider is an instruction to the Dependency Injection system on how to obtain a value for a dependency(aka services created). The service can be provided using Angular CLI as below,

ng generate service my-service

The created service by CLI would be as below,

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root', //Angular provide the service in root injector
})
export class MyService {
}

What is the recommendation for provider scope?

You should always provide your service in the root injector unless there is a case where you want the service to be available only if you import a particular @NgModule.

How do you restrict provider scope to a module?

It is possible to restrict service provider scope to a specific module instead making available to entire application. There are two possible ways to do it.

  1. Using providedIn in service: import { Injectable } from ‘@angular/core’; import { SomeModule } from ‘./some.module’; @Injectable({ providedIn: SomeModule, }) export class SomeService { }
  2. Declare provider for the service in module: import { NgModule } from ‘@angular/core’; import { SomeService } from ‘./some.service’; @NgModule({ providers: [SomeService], }) export class SomeModule { }

How do you provide a singleton service?

There are two possible ways to provide a singleton service.

  1. Set the providedIn property of the @Injectable() to “root”. This is the preferred way(starting from Angular 6.0) of creating a singleton service since it makes your services tree-shakable. import { Injectable } from ‘@angular/core’; @Injectable({ providedIn: ‘root’, }) export class MyService { }
  2. Include the service in root module or in a module that is only imported by root module. It has been used to register services before Angular 6.0. @NgModule({ … providers: [MyService], … })

What are the different ways to remove duplicate service registration?

If a module defines provides and declarations then loading the module in multiple feature modules will duplicate the registration of the service. Below are the different ways to prevent this duplicate behavior.

  1. Use the providedIn syntax instead of registering the service in the module.
  2. Separate your services into their own module.
  3. Define forRoot() and forChild() methods in the module.

How does forRoot method helpful to avoid duplicate router instances?

If the RouterModule module didn’t have forRoot() static method then each feature module would instantiate a new Router instance, which leads to broken application due to duplicate instances. After using forRoot() method, the root application module imports RouterModule.forRoot(...) and gets a Router, and all feature modules import RouterModule.forChild(...) which does not instantiate another Router.

What is a shared module?

The Shared Module is the module in which you put commonly used directives, pipes, and components into one module that is shared(import it) throughout the application.

For example, the below shared module imports CommonModule, FormsModule for common directives and components, pipes and directives based on the need,

import { CommonModule } from '@angular/common';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { UserComponent } from './user.component';
import { NewUserDirective } from './new-user.directive';
import { OrdersPipe } from './orders.pipe';

@NgModule({
 imports:      [ CommonModule ],
 declarations: [ UserComponent, NewUserDirective, OrdersPipe ],
 exports:      [ UserComponent, NewUserDirective, OrdersPipe,
                 CommonModule, FormsModule ]
})
export class SharedModule { }

Can I share services using modules?

No, it is not recommended to share services by importing module. i.e Import modules when you want to use directives, pipes, and components only. The best approach to get a hold of shared services is through ‘Angular dependency injection’ because importing a module will result in a new service instance.

How do you get current direction for locales?

In Angular 9.1, the API method getLocaleDirection can be used to get the current direction in your app. This method is useful to support Right to Left locales for your Internationalization based applications.

import { getLocaleDirection, registerLocaleData } from '@angular/common';
import { LOCALE_ID } from '@angular/core';
import localeAr from '@angular/common/locales/ar';

  ...

  constructor(@Inject(LOCALE_ID) locale) {

    const directionForLocale = getLocaleDirection(locale); // Returns 'rtl' or 'ltr' based on the current locale
    registerLocaleData(localeAr, 'ar-ae');
    const direction = getLocaleDirection('ar-ae'); // Returns 'rtl'

    // Current direction is used to provide conditional logic here
  }

What is ngcc?

The ngcc(Angular Compatibility Compiler) is a tool which upgrades node_module compiled with non-ivy ngc into ivy compliant format. The postinstall script from package.json will make sure your node_modules will be compatible with the Ivy renderer.

"scripts": {
   "postinstall": "ngcc"
}

Whereas, Ivy compiler (ngtsc), which compiles Ivy-compatible code.

What classes should not be added to declarations?

The below class types shouldn’t be added to declarations

  1. A class which is already declared in any another module.
  2. Directives imported from another module.
  3. Module classes.
  4. Service classes.
  5. Non-Angular classes and objects, such as strings, numbers, functions, entity models, configurations, business logic, and helper classes.

What is NgZone?

Angular provides a service called NgZone which creates a zone named angular to automatically trigger change detection when the following conditions are satisfied.

  1. When a sync or async function is executed.
  2. When there is no microTask scheduled.

What is NoopZone?

Zone is loaded/required by default in Angular applications and it helps Angular to know when to trigger the change detection. This way, it make sures developers focus on application development rather core part of Angular. You can also use Angular without Zone but the change detection need to be implemented on your own and noop zone need to be configured in bootstrap process. Let’s follow the below two steps to remove zone.js,

  1. Remove the zone.js import from polyfills.ts. /*************************************************************************************************** * Zone JS is required by default for Angular itself. */ // import ‘zone.js/dist/zone’; // Included with Angular CLI.
  2. Bootstrap Angular with noop zone in src/main.ts. platformBrowserDynamic().bootstrapModule(AppModule, {ngZone: ‘noop’}) .catch(err => console.error(err));

How do you create displayBlock components?

By default, Angular CLI creates components in an inline displayed mode(i.e, display:inline). But it is possible to create components with display: block style using displayBlock option,

ng generate component my-component --displayBlock

(OR) the option can be turned on by default in Angular.json with schematics.@schematics/angular:component.displayBlock key value as true.

What are the possible data update scenarios for change detection?

The change detection works in the following scenarios where the data changes needs to update the application HTML.

  1. Component initialization: While bootstrapping the Angular application, Angular triggers the ApplicationRef.tick() to call change detection and View Rendering.
  2. Event listener: The DOM event listener can update the data in an Angular component and trigger the change detection too. @Component({ selector: ‘app-event-listener’, template: ` <button (click)=”onClick()”>Click</button> {{message}}` }) export class EventListenerComponent { message = ”; onClick() { this.message = ‘data updated’; } }
  3. HTTP Data Request: You can get data from a server through an HTTP request data = ‘default value’; constructor(private httpClient: HttpClient) {} ngOnInit() { this.httpClient.get(this.serverUrl).subscribe(response => { this.data = response.data; // change detection will happen automatically }); }
  4. Macro tasks setTimeout() or setInterval(): You can update the data in the callback function of setTimeout or setInterval data = ‘default value’; ngOnInit() { setTimeout(() => { this.data = ‘data updated’; // Change detection will happen automatically }); }
  5. Micro tasks Promises: You can update the data in the callback function of promise data = ‘initial value’; ngOnInit() { Promise.resolve(1).then(v => { this.data = v; // Change detection will happen automatically }); }
  6. Async operations like Web sockets and Canvas: The data can be updated asynchronously using WebSocket.onmessage() and Canvas.toBlob().

What is a zone context?

Execution Context is an abstract concept that holds information about the environment within the current code being executed. A zone provides an execution context that persists across asynchronous operations is called as zone context. For example, the zone context will be same in both outside and inside setTimeout callback function,

zone.run(() => {
  // outside zone
  expect(zoneThis).toBe(zone);
  setTimeout(function() {
    // the same outside zone exist here
    expect(zoneThis).toBe(zone);
  });
});

The current zone is retrieved through Zone.current.

What are the lifecycle hooks of a zone?

There are four lifecycle hooks for asynchronous operations from zone.js.

  1. onScheduleTask: This hook triggers when a new asynchronous task is scheduled. For example, when you call setTimeout() onScheduleTask: function(delegate, curr, target, task) { console.log(‘new task is scheduled:’, task.type, task.source); return delegate.scheduleTask(target, task); }
  2. onInvokeTask: This hook triggers when an asynchronous task is about to execute. For example, when the callback of setTimeout() is about to execute. onInvokeTask: function(delegate, curr, target, task, applyThis, applyArgs) { console.log(‘task will be invoked:’, task.type, task.source); return delegate.invokeTask(target, task, applyThis, applyArgs); }
  3. onHasTask: This hook triggers when the status of one kind of task inside a zone changes from stable(no tasks in the zone) to unstable(a new task is scheduled in the zone) or from unstable to stable. onHasTask: function(delegate, curr, target, hasTaskState) { console.log(‘task state changed in the zone:’, hasTaskState); return delegate.hasTask(target, hasTaskState); }
  4. onInvoke: This hook triggers when a synchronous function is going to execute in the zone. onInvoke: function(delegate, curr, target, callback, applyThis, applyArgs) { console.log(‘the callback will be invoked:’, callback); return delegate.invoke(target, callback, applyThis, applyArgs); }

What are the methods of NgZone used to control change detection?

NgZone service provides a run() method that allows you to execute a function inside the angular zone. This function is used to execute third party APIs which are not handled by Zone and trigger change detection automatically at the correct time.

export class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    // use ngZone.run() to make the asynchronous operation in the angular zone
    this.ngZone.run(() => {
      someNewAsyncAPI(() => {
        // update the data of the component
      });
    });
  }
}

Whereas runOutsideAngular() method is used when you don’t want to trigger change detection.

export class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    // Use this method when you know no data will be updated
    this.ngZone.runOutsideAngular(() => {
      setTimeout(() => {
        // update component data and don't trigger change detection
      });
    });
  }
}

How do you change the settings of zonejs?

You can change the settings of zone by configuring them in a separate file and import it just after zonejs import. For example, you can disable the requestAnimationFrame() monkey patch to prevent change detection for no data update as one setting and prevent DOM events(a mousemove or scroll event) to trigger change detection. Let’s say the new file named zone-flags.js,

// disable patching requestAnimationFrame
(window as any).__Zone_disable_requestAnimationFrame = true;

// disable patching specified eventNames
(window as any).__zone_symbol__UNPATCHED_EVENTS = ['scroll', 'mousemove'];

The above configuration file can be imported in a polyfill.ts file as below,

/***************************************************************************************************
 * Zone JS is required by default for Angular.
 */
import `./zone-flags`;
import 'zone.js/dist/zone';  // Included with Angular CLI.

How do you trigger an animation?

Angular provides a trigger() function for animation in order to collect the states and transitions with a specific animation name, so that you can attach it to the triggering element in the HTML template. This function watch for changes and trigger initiates the actions when a change occurs. For example, let’s create trigger named upDown, and attach it to the button element.

content_copy
@Component({
  selector: 'app-up-down',
  animations: [
    trigger('upDown', [
      state('up', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('down', style({
        height: '100px',
        opacity: 0.5,
        backgroundColor: 'green'
      })),
      transition('up => down', [
        animate('1s')
      ]),
      transition('down => up', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'up-down.component.html',
  styleUrls: ['up-down.component.css']
})
export class UpDownComponent {
  isUp = true;

  toggle() {
    this.isUp = !this.isUp;
  }

How do you configure injectors with providers at different levels?

You can configure injectors with providers at different levels of your application by setting a metadata value. The configuration can happen in one of three places,

  1. In the @Injectable() decorator for the service itself
  2. In the @NgModule() decorator for an NgModule
  3. In the @Component() decorator for a component

Is it mandatory to use injectable on every service class?

No. The @Injectable() decorator is not strictly required if the class has other Angular decorators on it or does not have any dependencies. But the important thing here is any class that is going to be injected with Angular is decorated. i.e, If we add the decorator, the metadata design:paramtypes is added, and the dependency injection can do it’s job. That is the exact reason to add the @Injectable() decorator on a service if this service has some dependencies itself. For example, Let’s see the different variations of AppService in a root component,

  1. The below AppService can be injected in AppComponent without any problems. This is because there are no dependency services inside AppService. export class AppService { constructor() { console.log(‘A new app service’); } }
  2. The below AppService with dummy decorator and httpService can be injected in AppComponent without any problems. This is because meta information is generated with dummy decorator. function SomeDummyDecorator() { return (constructor: Function) => console.log(constructor); } @SomeDummyDecorator() export class AppService { constructor(http: HttpService) { console.log(http); } }

and the generated javascript code of above service has meta information about HttpService, js var AppService = (function () { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], AppService); return AppService; }()); exports.AppService = AppService; 3. The below AppService with @injectable decorator and httpService can be injected in AppComponent without any problems. This is because meta information is generated with Injectable decorator. js @Injectable({ providedIn: 'root', }) export class AppService { constructor(http: HttpService) { console.log(http); } }

What is an optional dependency?

The optional dependency is a parameter decorator to be used on constructor parameters, which marks the parameter as being an optional dependency. Due to this, the DI framework provides null if the dependency is not found. For example, If you don’t register a logger provider anywhere, the injector sets the value of logger(or logger service) to null in the below class.

import { Optional } from '@angular/core';

constructor(@Optional() private logger?: Logger) {
  if (this.logger) {
    this.logger.log('This is an optional dependency message');
  } else {
    console.log('The logger is not registered');
  }
}

What are the types of injector hierarchies?

There are two types of injector hierarchies in Angular

  1. ModuleInjector hierarchy: It configure on a module level using an @NgModule() or @Injectable() annotation.
  2. ElementInjector hierarchy: It created implicitly at each DOM element. Also it is empty by default unless you configure it in the providers property on @Directive() or @Component().

What are reactive forms?

Reactive forms is a model-driven approach for creating forms in a reactive style(form inputs changes over time). These are built around observable streams, where form inputs and values are provided as streams of input values. Let’s follow the below steps to create reactive forms,

  1. Register the reactive forms module which declares reactive-form directives in your app import { ReactiveFormsModule } from ‘@angular/forms’; @NgModule({ imports: [ // other imports … ReactiveFormsModule ], }) export class AppModule { }
  2. Create a new FormControl instance and save it in the component. import { Component } from ‘@angular/core’; import { FormControl } from ‘@angular/forms’; @Component({ selector: ‘user-profile’, styleUrls: [‘./user-profile.component.css’] }) export class UserProfileComponent { userName = new FormControl(”); }
  3. Register the FormControl in the template. <label> User name: <input type=”text” [formControl]=”userName”> </label>

Finally, the component with reactive form control appears as below, “`js import { Component } from ‘@angular/core’; import { FormControl } from ‘@angular/forms’;

@Component({
  selector: 'user-profile',
  styleUrls: ['./user-profile.component.css']
  template: `
     <label>
       User name:
       <input type="text" [formControl]="userName">
     </label>
  `
})
export class UserProfileComponent {
  userName = new FormControl('');
}
```

What are dynamic forms?

Dynamic forms is a pattern in which we build a form dynamically based on metadata that describes a business object model. You can create them based on reactive form API.

What are template driven forms?

Template driven forms are model-driven forms where you write the logic, validations, controls etc, in the template part of the code using directives. They are suitable for simple scenarios and uses two-way binding with [(ngModel)] syntax. For example, you can create register form easily by following the below simple steps,

  1. Import the FormsModule into the Application module’s imports array import { BrowserModule } from ‘@angular/platform-browser’; import { NgModule } from ‘@angular/core’; import {FormsModule} from ‘@angular/forms’ import { RegisterComponent } from ‘./app.component’; @NgModule({ declarations: [ RegisterComponent, ], imports: [ BrowserModule, FormsModule ], providers: [], bootstrap: [RegisterComponent] }) export class AppModule { }
  2. Bind the form from template to the component using ngModel syntax <input type=”text” class=”form-control” id=”name” required [(ngModel)]=”model.name” name=”name”>
  3. Attach NgForm directive to the tag in order to create FormControl instances and register them <form #registerForm=”ngForm”>
  4. Apply the validation message for form controls <label for=”name”>Name</label> <input type=”text” class=”form-control” id=”name” required [(ngModel)]=”model.name” name=”name” #name=”ngModel”> <div [hidden]=”name.valid || name.pristine” class=”alert alert-danger”> Please enter your name </div>
  5. Let’s submit the form with ngSubmit directive and add type=”submit” button at the bottom of the form to trigger form submit. <form (ngSubmit)=”onSubmit()” #heroForm=”ngForm”> // Form goes here <button type=”submit” class=”btn btn-success” [disabled]=”!registerForm.form.valid”>Submit</button>

Finally, the completed template-driven registration form will be appeared as follow.

```html
<div class="container">
    <h1>Registration Form</h1>
    <form (ngSubmit)="onSubmit()" #registerForm="ngForm">
      <div class="form-group">
        <label for="name">Name</label>
        <input type="text" class="form-control" id="name"
               required
               [(ngModel)]="model.name" name="name"
               #name="ngModel">
        <div [hidden]="name.valid || name.pristine"
             class="alert alert-danger">
          Please enter your name
        </div>
      </div>
            <button type="submit" class="btn btn-success" [disabled]="!registerForm.form.valid">Submit</button>
    </form>
</div>
```

What are the differences between reactive forms and template driven forms?

Below are the main differences between reactive forms and template driven forms

FeatureReactiveTemplate-Driven
Form model setupCreated(FormControl instance) in component explicitlyCreated by directives
Data updatesSynchronousAsynchronous
Form custom validationDefined as FunctionsDefined as Directives
TestingNo interaction with change detection cycleNeed knowledge of the change detection process
MutabilityImmutable(by always returning new value for FormControl instance)Mutable(Property always modified to new value)
ScalabilityMore scalable using low-level APIsLess scalable using due to abstraction on APIs

What are the different ways to group form controls?

Reactive forms provide two ways of grouping multiple related controls.

  1. FormGroup: It defines a form with a fixed set of controls those can be managed together in an one object. It has same properties and methods similar to a FormControl instance. This FormGroup can be nested to create complex forms as below. import { Component } from ‘@angular/core’; import { FormGroup, FormControl } from ‘@angular/forms’; @Component({ selector: ‘user-profile’, templateUrl: ‘./user-profile.component.html’, styleUrls: [‘./user-profile.component.css’] }) export class UserProfileComponent { userProfile = new FormGroup({ firstName: new FormControl(”), lastName: new FormControl(”), address: new FormGroup({ street: new FormControl(”), city: new FormControl(”), state: new FormControl(”), zip: new FormControl(”) }) }); onSubmit() { // Store this.userProfile.value in DB } } <form [formGroup]=”userProfile” (ngSubmit)=”onSubmit()”> <label> First Name: <input type=”text” formControlName=”firstName”> </label> <label> Last Name: <input type=”text” formControlName=”lastName”> </label> <div formGroupName=”address”> <h3>Address</h3> <label> Street: <input type=”text” formControlName=”street”> </label> <label> City: <input type=”text” formControlName=”city”> </label> <label> State: <input type=”text” formControlName=”state”> </label> <label> Zip Code: <input type=”text” formControlName=”zip”> </label> </div> <button type=”submit” [disabled]=”!userProfile.valid”>Submit</button> </form>
  2. FormArray: It defines a dynamic form in an array format, where you can add and remove controls at run time. This is useful for dynamic forms when you don’t know how many controls will be present within the group. import { Component } from ‘@angular/core’; import { FormArray, FormControl } from ‘@angular/forms’; @Component({ selector: ‘order-form’, templateUrl: ‘./order-form.component.html’, styleUrls: [‘./order-form.component.css’] }) export class OrderFormComponent { constructor () { this.orderForm = new FormGroup({ firstName: new FormControl(‘John’, Validators.minLength(3)), lastName: new FormControl(‘Rodson’), items: new FormArray([ new FormControl(null) ]) }); } onSubmitForm () { // Save the items this.orderForm.value in DB } onAddItem () { this.orderForm.controls .items.push(new FormControl(null)); } onRemoveItem (index) { this.orderForm.controls[‘items’].removeAt(index); } } <form [formControlName]=”orderForm” (ngSubmit)=”onSubmit()”> <label> First Name: <input type=”text” formControlName=”firstName”> </label> <label> Last Name: <input type=”text” formControlName=”lastName”> </label> <div> <p>Add items</p> <ul formArrayName=”items”> <li *ngFor=”let item of orderForm.controls.items.controls; let i = index”> <input type=”text” formControlName=”{{i}}”> <button type=”button” title=”Remove Item” (click)=”onRemoveItem(i)”>Remove</button> </li> </ul> <button type=”button” (click)=”onAddItem”> Add an item </button> </div>

How do you update specific properties of a form model?

You can use patchValue() method to update specific properties defined in the form model. For example,you can update the name and street of certain profile on click of the update button as shown below.

updateProfile() {
  this.userProfile.patchValue({
    firstName: 'John',
    address: {
      street: '98 Crescent Street'
    }
  });
}
  <button (click)="updateProfile()">Update Profile</button>

You can also use setValue method to update properties.

Note: Remember to update the properties against the exact model structure.

What is the purpose of FormBuilder?

FormBuilder is used as syntactic sugar for easily creating instances of a FormControl, FormGroup, or FormArray. This is helpful to reduce the amount of boilerplate needed to build complex reactive forms. It is available as an injectable helper class of the @angular/forms package.

For example, the user profile component creation becomes easier as shown here.

export class UserProfileComponent {
  profileForm = this.formBuilder.group({
    firstName: [''],
    lastName: [''],
    address: this.formBuilder.group({
      street: [''],
      city: [''],
      state: [''],
      zip: ['']
    }),
  });
  constructor(private formBuilder: FormBuilder) { }
  }

How do you verify the model changes in forms?

You can add a getter property(let’s say, diagnostic) inside component to return a JSON representation of the model during the development. This is useful to verify whether the values are really flowing from the input box to the model and vice versa or not.

export class UserProfileComponent {

  model = new User('John', 29, 'Writer');

  // TODO: Remove after the verification
  get diagnostic() { return JSON.stringify(this.model); }
}

and add diagnostic binding near the top of the form

{{diagnostic}}
<div class="form-group">
  // FormControls goes here
</div>

What are the state CSS classes provided by ngModel?

The ngModel directive updates the form control with special Angular CSS classes to reflect it’s state. Let’s find the list of classes in a tabular format,

Form control stateIf trueIf false
Visitedng-touchedng-untouched
Value has changedng-dirtyng-pristine
Value is validng-validng-invalid

How do you reset the form?

In a model-driven form, you can reset the form just by calling the function reset() on our form model. For example, you can reset the form model on submission as follows,

onSubmit() {
  if (this.myform.valid) {
    console.log("Form is submitted");
    // Perform business logic here
    this.myform.reset();
  }
}

Now, your form model resets the form back to its original pristine state.

What are the types of validator functions?

In reactive forms, the validators can be either synchronous or asynchronous functions,

  1. Sync validators: These are the synchronous functions which take a control instance and immediately return either a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are simple checks like whether a field is empty, whether it exceeds a maximum length etc.
  2. Async validators: These are the asynchronous functions which take a control instance and return a Promise or Observable that later emits a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are complex validations like hitting a server to check the availability of a username or email.

The representation of these validators looks like below

this.myForm = formBuilder.group({
    firstName: ['value'],
    lastName: ['value', *Some Sync validation function*],
    email: ['value', *Some validation function*, *Some asynchronous validation function*]
});

Can you give an example of built-in validators?

In reactive forms, you can use built-in validator like required and minlength on your input form controls. For example, the registration form can have these validators on name input field

this.registrationForm = new FormGroup({
    'name': new FormControl(this.hero.name, [
      Validators.required,
      Validators.minLength(4),
    ])
  });

Whereas in template-driven forms, both required and minlength validators available as attributes.

How do you optimize the performance of async validators?

Since all validators run after every form value change, it creates a major impact on performance with async validators by hitting the external API on each keystroke. This situation can be avoided by delaying the form validity by changing the updateOn property from change (default) to submit or blur. The usage would be different based on form types,

  1. Template-driven forms: Set the property on ngModelOptions directive <input [(ngModel)]=”name” [ngModelOptions]=”{updateOn: ‘blur’}”>
  2. Reactive-forms: Set the property on FormControl instance name = new FormControl(”, {updateOn: ‘blur’});

How to set ngFor and ngIf on the same element?

Sometimes you may need to both ngFor and ngIf on the same element but unfortunately you are going to encounter below template error.

 Template parse errors: Can't have multiple template bindings on one element.

In this case, You need to use either ng-container or ng-template. Let’s say if you try to loop over the items only when the items are available, the below code throws an error in the browser

<ul *ngIf="items" *ngFor="let item of items">
  <li></li>
</ul>

and it can be fixed by

<ng-container *ngIf="items">
  <ul *ngFor="let item of items">
    <li></li>
  </ul>
</ng-container>

What is host property in css?

The :host pseudo-class selector is used to target styles in the element that hosts the component. Since the host element is in a parent component’s template, you can’t reach the host element from inside the component by other means. For example, you can create a border for parent element as below,

//Other styles for app.component.css
//...
:host {
  display: block;
  border: 1px solid black;
  padding: 20px;
}

How do you get the current route?

In Angular, there is an url property of router package to get the current route. You need to follow the below few steps,

  1. Import Router from @angular/router
  import { Router } from '@angular/router';
  1. Inject router inside constructor
constructor(private router: Router ) {

}
  1. Access url parameter
  console.log(this.router.url); //  /routename

JS/ES6 – Advanced Interview Qs & Ans

October 17, 2020 by admin

  1. What are the possible ways to create objects in JavaScript
    There are many ways to create objects in javascript as below
    i. Object constructor:
    The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.
    var object = new Object();
    ii. Object’s create method:
    The create method of Object creates a new object by passing the prototype object as a parameter
    var object = Object.create(null);
    iii. Object literal syntax:
    The object literal syntax is equivalent to create method when it passes null as parameter
    var object = {};
    iv. Function constructor:
    Create any function and apply the new operator to create object instances,
    function Person(name){
    var object = {};
    object.name=name;
    object.age=21;
    return object;
    }
    var object = new Person(“Sudheer”);
    v. Function constructor with prototype:
    This is similar to function constructor but it uses prototype for their properties and methods,
    function Person(){}
    Person.prototype.name = “Sudheer”;
    var object = new Person();
    This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.
    function func {};

new func(x, y, z);
(OR)
// Create a new instance using function prototype.
var newInstance = Object.create(func.prototype)

// Call the function
var result = func.call(newInstance, x, y, z),

// If the result is a non-null object then use it otherwise just use the new instance.
console.log(result && typeof result === ‘object’ ? result : newInstance);
vi. ES6 Class syntax:
ES6 introduces class feature to create the objects
class Person {
constructor(name) {
this.name = name;
}
}

var object = new Person(“Sudheer”);
vii. Singleton pattern:
A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don’t accidentally create multiple instances.
var object = new function(){
this.name = “Sudheer”;
}

  1. What is a prototype chain
    Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language. The prototype on object instance is available through Object.getPrototypeOf(object) or proto property whereas prototype on constructors function is available through object.prototype.
  2. What is the difference between Call, Apply and Bind
    The difference between Call, Apply and Bind can be explained with below examples,
    Call: The call() method invokes a function with a given this value and arguments provided one by one
    var employee1 = {firstName: ‘John’, lastName: ‘Rodson’};
    var employee2 = {firstName: ‘Jimmy’, lastName: ‘Baily’};

function invite(greeting1, greeting2) {
console.log(greeting1 + ‘ ‘ + this.firstName + ‘ ‘ + this.lastName+ ‘, ‘+ greeting2);
}

invite.call(employee1, ‘Hello’, ‘How are you?’); // Hello John Rodson, How are you?
invite.call(employee2, ‘Hello’, ‘How are you?’); // Hello Jimmy Baily, How are you?
Apply: Invokes the function and allows you to pass in arguments as an array
var employee1 = {firstName: ‘John’, lastName: ‘Rodson’};
var employee2 = {firstName: ‘Jimmy’, lastName: ‘Baily’};

function invite(greeting1, greeting2) {
console.log(greeting1 + ‘ ‘ + this.firstName + ‘ ‘ + this.lastName+ ‘, ‘+ greeting2);
}

invite.apply(employee1, [‘Hello’, ‘How are you?’]); // Hello John Rodson, How are you?
invite.apply(employee2, [‘Hello’, ‘How are you?’]); // Hello Jimmy Baily, How are you?
bind: returns a new function, allowing you to pass in an array and any number of arguments
var employee1 = {firstName: ‘John’, lastName: ‘Rodson’};
var employee2 = {firstName: ‘Jimmy’, lastName: ‘Baily’};

function invite(greeting1, greeting2) {
console.log(greeting1 + ‘ ‘ + this.firstName + ‘ ‘ + this.lastName+ ‘, ‘+ greeting2);
}

var inviteEmployee1 = invite.bind(employee1);
var inviteEmployee2 = invite.bind(employee2);
inviteEmployee1(‘Hello’, ‘How are you?’); // Hello John Rodson, How are you?
inviteEmployee2(‘Hello’, ‘How are you?’); // Hello Jimmy Baily, How are you?
Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array. Whereas Bind creates a new function that will have this set to the first parameter passed to bind().

  1. What is JSON and its common operations
    JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json Parsing: Converting a string to a native object
    JSON.parse(text)
    Stringification: **converting a native object to a string so it can be transmitted across the network
    JSON.stringify(object)
  2. What is the purpose of the array slice method
    The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end. Some of the examples of this method are,
    let arrayIntegers = [1, 2, 3, 4, 5];
    let arrayIntegers1 = arrayIntegers.slice(0,2); // returns [1,2]
    let arrayIntegers2 = arrayIntegers.slice(2,3); // returns [3]
    let arrayIntegers3 = arrayIntegers.slice(4); //returns [5]
    Note: Slice method won’t mutate the original array but it returns the subset as a new array.
  3. What is the purpose of the array splice method
    The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the option second argument indicates the number of elements to be deleted. Each additional argument is added to the array. Some of the examples of this method are,
    let arrayIntegersOriginal1 = [1, 2, 3, 4, 5];
    let arrayIntegersOriginal2 = [1, 2, 3, 4, 5];
    let arrayIntegersOriginal3 = [1, 2, 3, 4, 5];

let arrayIntegers1 = arrayIntegersOriginal1.splice(0,2); // returns [1, 2]; original array: [3, 4, 5]
let arrayIntegers2 = arrayIntegersOriginal2.splice(3); // returns [4, 5]; original array: [1, 2, 3]
let arrayIntegers3 = arrayIntegersOriginal3.splice(3, 1, “a”, “b”, “c”); //returns [4]; original array: [1, 2, 3, “a”, “b”, “c”, 5]
Note: Splice method modifies the original array and returns the deleted array.

  1. What is the difference between slice and splice
    Some of the major difference in a tabular form
    Slice Splice
    Doesn’t modify the original array(immutable) Modifies the original array(mutable)
    Returns the subset of original array Returns the deleted elements as array
    Used to pick the elements from array Used to insert or delete elements to/from array
  2. How do you compare Object and Map
    Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.
    i. The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
    ii. The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
    iii. You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
    iv. A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
    v. An Object has a prototype, so there are default keys in the map that could collide with your keys if you’re not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
    vi. A Map may perform better in scenarios involving frequent addition and removal of key pairs.
  3. What is the difference between == and === operators
    JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,
    i. Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
    ii. Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value. There are two special cases in this,
    a. NaN is not equal to anything, including NaN.
    b. Positive and negative zeros are equal to one another.
    iii. Two Boolean operands are strictly equal if both are true or both are false.
    iv. Two objects are strictly equal if they refer to the same Object.
    v. Null and Undefined types are not equal with ===, but equal with ==. i.e, null===undefined –> false but null==undefined –> true
    Some of the example which covers the above cases,
    0 == false // true
    0 === false // false
    1 == “1” // true
    1 === “1” // false
    null == undefined // true
    null === undefined // false
    ‘0’ == false // true
    ‘0’ === false // false
    []==[] or []===[] //false, refer different objects in memory
    {}=={} or {}==={} //false, refer different objects in memory
  4. What are lambda or arrow functions
    An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.
  5. What is a first class function
    In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.
    For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener
    const handler = () => console.log (‘This is a click handler function’);
    document.addEventListener (‘click’, handler);
  6. What is a first order function
    First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.
    const firstOrder = () => console.log (‘I am a first order function!’);
  7. What is a higher order function
    Higher-order function is a function that accepts another function as an argument or returns a function as a return value.
    const firstOrderFunc = () => console.log (‘Hello I am a First order function’);
    const higherOrder = ReturnFirstOrderFunc => ReturnFirstOrderFunc ();
    higherOrder (firstOrderFunc);
  8. What is a unary function
    Unary function (i.e. monadic) is a function that accepts exactly one argument. Let us take an example of unary function. It stands for a single argument accepted by a function.
    const unaryFunction = a => console.log (a + 10); // Add 10 to the given argument and display the value
  9. What is the currying function
    Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function. Let’s take an example of n-ary function and how it turns into a currying function
    const multiArgFunction = (a, b, c) => a + b + c;
    const curryUnaryFunction = a => b => c => a + b + c;
    curryUnaryFunction (1); // returns a function: b => c => 1 + b + c
    curryUnaryFunction (1) (2); // returns a function: c => 3 + c
    curryUnaryFunction (1) (2) (3); // returns the number 6
    Curried functions are great to improve code reusability and functional composition.
  10. What is a pure function
    A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments ‘n’ number of times and ‘n’ number of places in the application then it will always return the same value. Let’s take an example to see the difference between pure and impure functions,
    //Impure
    let numberArray = [];
    const impureAddNumber = number => numberArray.push (number);
    //Pure
    const pureAddNumber = number => argNumberArray =>
    argNumberArray.concat ([number]);

//Display the results
console.log (impureAddNumber (6)); // returns 1
console.log (numberArray); // returns [6]
console.log (pureAddNumber (7) (numberArray)); // returns [6, 7]
console.log (numberArray); // returns [6]
As per above code snippets, Push function is impure itself by altering the array and returning an push number index which is independent of parameter value. Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array. Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.

  1. What is the purpose of the let keyword
    The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope. Let’s take an example to demonstrate the usage,
    let counter = 30;
    if (counter === 30) {
    let counter = 31;
    console.log(counter); // 31
    }
    console.log(counter); // 30 (because if block variable won’t exist here)
  2. What is the difference between let and var
    You can list out the differences in a tabular format
    var let
    It is been available from the beginning of JavaScript Introduced as part of ES6
    It has function scope It has block scope
    Variables will be hoisted Hoisted but not initialized
    Let’s take an example to see the difference,
    function userDetails(username) {
    if(username) {
    console.log(salary); // undefined(due to hoisting)
    console.log(age); // error: age is not defined
    let age = 30;
    var salary = 10000;
    }
    console.log(salary); //10000 (accessible to due function scope)
    console.log(age); //error: age is not defined(due to block scope)
    }
  3. What is the reason to choose the name let as a keyword
    Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible.
  4. How do you redeclare variables in switch block without an error
    If you try to redeclare variables in a switch block then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,
    let counter = 1;
    switch(x) {
    case 0:
    let name;
    break; case 1:
    let name; // SyntaxError for redeclaration.
    break;
    }
    To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.
    let counter = 1;
    switch(x) {
    case 0: {
    let name;
    break;
    }
    case 1: {
    let name; // No SyntaxError for redeclaration.
    break;
    }
    }
  5. What is the Temporal Dead Zone
    The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a let or const variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone. Let’s see this behavior with an example,
    function somemethod() {
    console.log(counter1); // undefined
    console.log(counter2); // ReferenceError
    var counter1 = 1;
    let counter2 = 2;
    }
  6. What is IIFE(Immediately Invoked Function Expression)
    IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,
    (function ()
    {
    // logic here
    }
    )
    ();
    The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below,
    (function ()
    {
    var message = “IIFE”;
    console.log(message);
    }
    )
    ();
    console.log(message); //Error: message is not defined
  7. What is the benefit of using modules
    There are a lot of benefits to using modules in favour of a sprawling. Some of the benefits are,
    i. Maintainability
    ii. Reusability
    iii. Namespacing
  8. What is memoization
    Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let’s take an example of adding function with memoization,
    const memoizAddition = () => {
    let cache = {};
    return (value) => {
    if (value in cache) {
    console.log(‘Fetching from cache’);
    return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript identifier. Hence, can only be accessed using the square bracket notation.
    }
    else {
    console.log(‘Calculating result’);
    let result = value + 20;
    cache[value] = result;
    return result;
    }
    }
    }
    // returned function from memoizAddition
    const addition = memoizAddition();
    console.log(addition(20)); //output: 40 calculated
    console.log(addition(20)); //output: 40 cached
  9. What is Hoisting
    Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let’s take a simple example of variable hoisting,
    console.log(message); //output : undefined
    var message = ‘The variable Has been hoisted’;
    The above code looks like as below to the interpreter,
    var message;
    console.log(message);
    message = ‘The variable Has been hoisted’;
  10. What are classes in ES6
    In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance. For example, the prototype based inheritance written in function expression as below,
    function Bike(model,color) {
    this.model = model;
    this.color = color;
    }

Bike.prototype.getDetails = function() {
return this.model + ‘ bike has’ + this.color + ‘ color’;
};
Whereas ES6 classes can be defined as an alternative
class Bike{
constructor(color, model) {
this.color= color;
this.model= model;
}

getDetails() {
return this.model + ‘ bike has’ + this.color + ‘ color’;
}
}

  1. What are closures
    A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains
    i. Own scope where variables defined between its curly brackets
    ii. Outer function’s variables
    iii. Global variables Let’s take an example of closure concept,
    function Welcome(name){
    var greetingInfo = function(message){
    console.log(message+’ ‘+name);
    }
    return greetingInfo;
    }
    var myFunction = Welcome(‘John’);
    myFunction(‘Welcome ‘); //Output: Welcome John
    myFunction(‘Hello Mr.’); //output: Hello Mr.John
    As per the above code, the inner function(greetingInfo) has access to the variables in the outer function scope(Welcome) even after the outer function has returned.
  2. What are modules
    Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor
  3. Why do you need modules
    Below are the list of benefits using modules in javascript ecosystem
    i. Maintainability
    ii. Reusability
    iii. Namespacing
  4. What is scope in javascript
    Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.
  5. What is a service worker
    A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don’t need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.
  6. How do you manipulate DOM using a service worker
    Service worker can’t access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the postMessage interface, and those pages can manipulate the DOM.
  7. How do you reuse information across service worker restarts
    The problem with service worker is that it gets terminated when not in use, and restarted when it’s next needed, so you cannot rely on global state within a service worker’s onfetch and onmessage handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.
  8. What is IndexedDB
    IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.
  9. What is web storage
    Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user’s browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.
    i. Local storage: It stores data for current origin with no expiration date.
    ii. Session storage: It stores data for one session and the data is lost when the browser tab is closed.
  10. What is a post message
    Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).
  11. What is a Cookie
    A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs. For example, you can create a cookie named username as below,
    document.cookie = “username=John”;
  12. Why do you need a Cookie
    Cookies are used to remember information about the user profile(such as username). It basically involves two steps,
    i. When a user visits a web page, the user profile can be stored in a cookie.
    ii. Next time the user visits the page, the cookie remembers the user profile.
  13. What are the options in a cookie
    There are few below options available for a cookie,
    i. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time).
    document.cookie = “username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC”;
    ii. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter.
    document.cookie = “username=John; path=/services”;
  14. How do you delete a cookie
    You can delete a cookie by setting the expiry date as a passed date. You don’t need to specify a cookie value in this case. For example, you can delete a username cookie in the current page as below.
    document.cookie = “username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;”;
    Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn’t allow to delete a cookie unless you specify a path parameter.
  15. What are the differences between cookie, local storage and session storage
    Below are some of the differences between cookie, local storage and session storage,
    Feature Cookie Local storage Session storage
    Accessed on client or server side Both server-side & client-side client-side only client-side only
    Lifetime As configured using Expires option until deleted until tab is closed
    SSL support Supported Not supported Not supported
    Maximum data size 4KB 5 MB 5MB
  16. What is the main difference between localStorage and sessionStorage
    LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.
  17. How do you access web storage
    The Window object implements the WindowLocalStorage and WindowSessionStorage objects which has localStorage(window.localStorage) and sessionStorage(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local). For example, you can read and write on local storage objects as below
    localStorage.setItem(‘logo’, document.getElementById(‘logo’).value);
    localStorage.getItem(‘logo’);
  18. What are the methods available on session storage
    The session storage provided methods for reading, writing and clearing the session data
    // Save data to sessionStorage
    sessionStorage.setItem(‘key’, ‘value’);

// Get saved data from sessionStorage
let data = sessionStorage.getItem(‘key’);

// Remove saved data from sessionStorage
sessionStorage.removeItem(‘key’);

// Remove all saved data from sessionStorage
sessionStorage.clear();

  1. What is a storage event and its event handler
    The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below
    window.onstorage = functionRef;
    Let’s take the example usage of onstorage event handler which logs the storage key and it’s values
    window.onstorage = function(e) {
    console.log(‘The ‘ + e.key +
    ‘ key has been changed from ‘ + e.oldValue +
    ‘ to ‘ + e.newValue + ‘.’);
    };
  2. Why do you need web storage
    Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.
  3. How do you check web storage browser support
    You need to check browser support for localStorage and sessionStorage before using web storage,
    if (typeof(Storage) !== “undefined”) {
    // Code for localStorage/sessionStorage.
    } else {
    // Sorry! No Web Storage support..
    }
  4. How do you check web workers browser support
    You need to check browser support for web workers before using it
    if (typeof(Worker) !== “undefined”) {
    // code for Web worker support.
    } else {
    // Sorry! No Web Worker support..
    }
  5. Give an example of a web worker
    You need to follow below steps to start using web workers for counting example
    i. Create a Web Worker File: You need to write a script to increment the count value. Let’s name it as counter.js
    let i = 0;

function timedCount() {
i = i + 1;
postMessage(i);
setTimeout(“timedCount()”,500);
}

timedCount();
Here postMessage() method is used to post a message back to the HTML page
ii. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let’s name this file as web_worker_example.js
if (typeof(w) == “undefined”) {
w = new Worker(“counter.js”);
}
and we can receive messages from web worker
w.onmessage = function(event){
document.getElementById(“message”).innerHTML = event.data;
};
iii. Terminate a Web Worker: Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.
w.terminate();
iv. Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code
w = undefined;

  1. What are the restrictions of web workers on DOM
    WebWorkers don’t have access to below javascript objects since they are defined in an external files
    i. Window object
    ii. Document object
    iii. Parent object
  2. What is a promise
    A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.
    The syntax of Promise creation looks like below,
    const promise = new Promise(function(resolve, reject) {
    // promise description
    })
    The usage of a promise would be as below,
    const promise = new Promise(resolve => {
    setTimeout(() => {
    resolve(“I’m a Promise!”);
    }, 5000);
    }, reject => {

});

promise.then(value => console.log(value));
The action flow of a promise will be as below,

  1. Why do you need a promise
    Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.
  2. What are the three states of promise
    Promises have three states:
    i. Pending: This is an initial state of the Promise before an operation begins
    ii. Fulfilled: This state indicates that the specified operation was completed.
    iii. Rejected: This state indicates that the operation did not complete. In this case an error value will be thrown.
  3. What is a callback function
    A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action. Let’s take a simple example of how to use callback function
    function callbackFunction(name) {
    console.log(‘Hello ‘ + name);
    }

function outerFunction(callback) {
let name = prompt(‘Please enter your name.’);
callback(name);
}

outerFunction(callbackFunction);

  1. Why do we need callbacks
    The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events. Let’s take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.
    function firstFunction(){
    // Simulate a code delay
    setTimeout( function(){
    console.log(‘First function called’);
    }, 1000 );
    }
    function secondFunction(){
    console.log(‘Second function called’);
    }
    firstFunction();
    secondFunction();

Output
// Second function called
// First function called
As observed from the output, javascript didn’t wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.

  1. What is a callback hell
    Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,
    async1(function(){
    async2(function(){
    async3(function(){
    async4(function(){
    ….
    });
    });
    });
    });
  2. What are server-sent events
    Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel – events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.
  3. How do you receive server-sent event notifications
    The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,
    if(typeof(EventSource) !== “undefined”) {
    var source = new EventSource(“sse_generator.js”);
    source.onmessage = function(event) {
    document.getElementById(“output”).innerHTML += event.data + “
    “;
    };
    }
  4. How do you check browser support for server-sent events
    You can perform browser support for server-sent events before using it as below,
    if(typeof(EventSource) !== “undefined”) {
    // Server-sent events supported. Let’s have some code here!
    } else {
    // No server-sent events supported
    }
  5. What are the events available for server sent events
    Below are the list of events available for server sent events
    Event Description
    onopen It is used when a connection to the server is opened
    onmessage This event is used when a message is received
    onerror It happens when an error occurs
  6. What are the main rules of promise
    A promise must follow a specific set of rules,
    i. A promise is an object that supplies a standard-compliant .then() method
    ii. A pending promise may transition into either fulfilled or rejected state
    iii. A fulfilled or rejected promise is settled and it must not transition into any other state.
    iv. Once a promise is settled, the value must not change.
  7. What is callback in callback
    You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.
    loadScript(‘/script1.js’, function(script) {
    console.log(‘first script is loaded’); loadScript(‘/script2.js’, function(script) { console.log(‘second script is loaded’); loadScript(‘/script3.js’, function(script) { console.log('third script is loaded'); // after all scripts are loaded
    }); })

});

  1. What is promise chaining
    The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let’s take an example of promise chaining for calculating the final result,
    new Promise(function(resolve, reject) { setTimeout(() => resolve(1), 1000);

}).then(function(result) {

console.log(result); // 1
return result * 2;

}).then(function(result) {

console.log(result); // 2
return result * 3;

}).then(function(result) {

console.log(result); // 6
return result * 4;

});
In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,
i. The initial promise resolves in 1 second,
ii. After that .then handler is called by logging the result(1) and then return a promise with the value of result * 2.
iii. After that the value passed to the next .then handler by logging the result(2) and return a promise with result * 3.
iv. Finally the value passed to the last .then handler by logging the result(6) and return a promise with result * 4.

  1. What is promise.all
    Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,
    Promise.all([Promise1, Promise2, Promise3]) .then(result) => { console.log(result) }) .catch(error => console.log(Error in promises ${error}))
    Note: Remember that the order of the promises(output the result) is maintained as per input order.
  2. What is the purpose of the race method in promise
    Promise.race() method will return the promise instance which is firstly resolved or rejected. Let’s take an example of race() method where promise2 is resolved first
    var promise1 = new Promise(function(resolve, reject) {
    setTimeout(resolve, 500, ‘one’);
    });
    var promise2 = new Promise(function(resolve, reject) {
    setTimeout(resolve, 100, ‘two’);
    });

Promise.race([promise1, promise2]).then(function(value) {
console.log(value); // “two” // Both promises will resolve, but promise2 is faster
});

  1. What is a strict mode in javascript
    Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression “use strict”; instructs the browser to use the javascript code in the Strict mode.
  2. Why do you need strict mode
    Strict mode is useful to write “secure” JavaScript by notifying “bad syntax” into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.
  3. How do you declare strict mode
    The strict mode is declared by adding “use strict”; to the beginning of a script or a function. If declared at the beginning of a script, it has global scope.
    “use strict”;
    x = 3.14; // This will cause an error because x is not declared
    and if you declare inside a function, it has local scope
    x = 3.14; // This will not cause an error.
    myFunction();

function myFunction() {
“use strict”;
y = 3.14; // This will cause an error
}

  1. What is the purpose of double exclamation
    The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true. For example, you can test IE version using this expression as below,
    let isIE8 = false;
    isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
    console.log(isIE8); // returns true or false
    If you don’t use this expression then it returns the original value.
    console.log(navigator.userAgent.match(/MSIE 8.0/)); // returns either an Array or null
    Note: The expression !! is not an operator, but it is just twice of ! operator.
  2. What is the purpose of the delete operator
    The delete keyword is used to delete the property as well as its value.
    var user= {name: “John”, age:20};
    delete user.age;

console.log(user); // {name: “John”}

  1. What is the typeof operator
    You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.
    typeof “John Abraham” // Returns “string”
    typeof (1 + 2) // Returns “number”
  2. What is undefined property
    The undefined property indicates that a variable has not been assigned a value, or not declared at all. The type of undefined value is undefined too.
    var user; // Value is undefined, type is undefined
    console.log(typeof(user)) //undefined
    Any variable can be emptied by setting the value to undefined.
    user = undefined
  3. What is null value
    The value null represents the intentional absence of any object value. It is one of JavaScript’s primitive values. The type of null value is object. You can empty the variable by setting the value to null.
    var user = null;
    console.log(typeof(user)) //object
  4. What is the difference between null and undefined
    Below are the main differences between null and undefined,
    Null Undefined
    It is an assignment value which indicates that variable points to no object. It is not an assignment value where a variable has been declared but has not yet been assigned a value.
    Type of null is object Type of undefined is undefined
    The null value is a primitive value that represents the null, empty, or non-existent reference. The undefined value is a primitive value used when a variable has not been assigned a value.
    Indicates the absence of a value for a variable Indicates absence of variable itself
    Converted to zero (0) while performing primitive operations Converted to NaN while performing primitive operations
  5. What is eval
    The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.
    console.log(eval(‘1 + 2’)); // 3
  6. What is the difference between window and document
    Below are the main differences between window and document,
    Window Document
    It is the root level element in any web page It is the direct child of the window object. This is also known as Document Object Model(DOM)
    By default window object is available implicitly in the page You can access it via window.document or document.
    It has methods like alert(), confirm() and properties like document, location It provides methods like getElementById, getElementByTagName, createElement etc
  7. How do you access history in javascript
    The window.history object contains the browser’s history. You can load previous and next URLs in the history using back() and next() methods.
    function goBack() {
    window.history.back()
    }
    function goForward() {
    window.history.forward()
    }
    Note: You can also access history without window prefix.
  8. What are the javascript data types
    Below are the list of javascript data types available
    i. Number
    ii. String
    iii. Boolean
    iv. Object
    v. Undefined
  9. What is isNaN
    The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.
    isNaN(‘Hello’) //true
    isNaN(‘100’) //false
  10. What are the differences between undeclared and undefined variables
    Below are the major differences between undeclared and undefined variables,
    undeclared undefined
    These variables do not exist in a program and are not declared These variables declared in the program but have not assigned any value
    If you try to read the value of an undeclared variable, then a runtime error is encountered If you try to read the value of an undefined variable, an undefined value is returned.
  11. What are global variables
    Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable
    msg = “Hello” // var is missing, it becomes global variable
  12. What are the problems with global variables
    The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.
  13. What is NaN property
    The NaN property is a global property that represents “Not-a-Number” value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases
    Math.sqrt(-1)
    parseInt(“Hello”)
  14. What is the purpose of isFinite function
    The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.
    isFinite(Infinity); // false
    isFinite(NaN); // false
    isFinite(-Infinity); // false

isFinite(100); // true

  1. What is an event flow
    Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object. There are two ways of event flow
    i. Top to Bottom(Event Capturing)
    ii. Bottom to Top (Event Bubbling)
  2. What is event bubbling
    Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.
  3. What is event capturing
    Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.
  4. How do you submit a form using JavaScript
    You can submit a form using JavaScript use document.form[0].submit(). All the form input’s information is submitted using onsubmit event handler
    function submit() {
    document.form[0].submit();
    }
  5. How do you find operating system details
    The window.navigator object contains information about the visitor’s browser OS details. Some of the OS properties are available under platform property,
    console.log(navigator.platform);
  6. What is the difference between document load and DOMContentLoaded events
    The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).
  7. What is the difference between native, host and user objects
    Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec. Host objects are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects. User objects are objects defined in the javascript code. For example, User objects created for profile information.
  8. What are the tools or techniques used for debugging JavaScript code
    You can use below tools or techniques for debugging javascript
    i. Chrome Devtools
    ii. debugger statement
    iii. Good old console.log statement
  9. What are the pros and cons of promises over callbacks
    Below are the list of pros and cons of promises over callbacks,
    Pros:
    i. It avoids callback hell which is unreadable
    ii. Easy to write sequential asynchronous code with .then()
    iii. Easy to write parallel asynchronous code with Promise.all()
    iv. Solves some of the common problems of callbacks(call the callback too late, too early, many times and swallow errors/exceptions)
    Cons:
    v. It makes little complex code
    vi. You need to load a polyfill if ES6 is not supported
  10. What is the difference between an attribute and a property
    Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,

    You can retrieve the attribute value as below,
    const input = document.querySelector(‘input’);
    console.log(input.getAttribute(‘value’)); // Good morning
    console.log(input.value); // Good morning
    And after you change the value of the text field to “Good evening”, it becomes like
    console.log(input.getAttribute(‘value’)); // Good morning
    console.log(input.value); // Good evening
  11. What is same-origin policy
    The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).
  12. What is the purpose of void 0
    Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href=”JavaScript:Void(0);” within an element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression. For example, the below link notify the message without reloading the page Click Me!
  13. Is JavaScript a compiled or interpreted language
    JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.
  14. Is JavaScript a case-sensitive language
    Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.
  15. Is there any relation between Java and JavaScript
    No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).
  16. What are events
    Events are “things” that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can react on these events. Some of the examples of HTML events are,
    i. Web page has finished loading
    ii. Input field was changed
    iii. Button was clicked
    Let’s describe the behavior of click event for button element,



Click me

  1. Who created javascript
    JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name Mocha, but later the language was officially called LiveScript when it first shipped in beta releases of Netscape.
  2. What is the use of preventDefault method
    The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.
    document.getElementById(“link”).addEventListener(“click”, function(event){
    event.preventDefault();
    });
    Note: Remember that not all events are cancelable.
  3. What is the use of stopPropagation method
    The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)

Click DIV1 Element DIV 2 DIV 1

  1. What are the steps involved in return false usage
    The return false statement in event handlers performs the below steps,
    i. First it stops the browser’s default action or behaviour.
    ii. It prevents the event from propagating the DOM
    iii. Stops callback execution and returns immediately when called.
  2. What is BOM
    The Browser Object Model (BOM) allows JavaScript to “talk to” the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.
  3. What is the use of setTimeout
    The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let’s log a message after 2 seconds using setTimeout method,
    setTimeout(function(){ console.log(“Good morning”); }, 2000);
  4. What is the use of setInterval
    The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let’s log a message after 2 seconds using setInterval method,
    setInterval(function(){ console.log(“Good morning”); }, 2000);
  5. Why is JavaScript treated as Single threaded
    JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.
  6. What is an event delegation
    Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.
    For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique,
    var form = document.querySelector(‘#registration-form’);

// Listen for changes to fields inside the form
form.addEventListener(‘input’, function (event) {

// Log the field that was changed
console.log(event.target);

}, false);

  1. What is ECMAScript
    ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.
  2. What is JSON
    JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.
  3. What are the syntax rules of JSON
    Below are the list of syntax rules of JSON
    i. The data is in name/value pairs
    ii. The data is separated by commas
    iii. Curly braces hold objects
    iv. Square brackets hold arrays
  4. What is the purpose JSON stringify
    When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.
    var userJSON = {‘name’: ‘John’, age: 31}
    var userString = JSON.stringify(user);
    console.log(userString); //”{“name”:”John”,”age”:31}”
  5. How do you parse JSON string
    When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.
    var userString = ‘{“name”:”John”,”age”:31}’;
    var userJSON = JSON.parse(userString);
    console.log(userJSON);// {name: “John”, age: 31}
  6. Why do you need JSON
    When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.
  7. What are PWAs
    Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.
  8. What is the purpose of clearTimeout method
    The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.
    For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.
  9. What is the purpose of clearInterval method
    The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.
    For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.
  1. How do you redirect new page in javascript
    In vanilla javascript, you can redirect to a new page using the location property of window object. The syntax would be as follows,
    function redirect() {
    window.location.href = ‘newPage.html’;
    }
  2. How do you check whether a string contains a substring
    There are 3 possible ways to check whether a string contains a substring or not,
    i. Using includes: ES6 provided String.prototype.includes method to test a string contains a substring
    var mainString = “hello”, subString = “hell”;
    mainString.includes(subString)
    ii. Using indexOf: In an ES5 or older environment, you can use String.prototype.indexOf which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.
    var mainString = “hello”, subString = “hell”;
    mainString.indexOf(subString) !== -1
    iii. Using RegEx: The advanced solution is using Regular expression’s test method(RegExp.test), which allows for testing for against regular expressions
    var mainString = “hello”, regex = “/hell/”;
    regex.test(mainString)
  3. How do you validate an email in javascript
    You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.
    function validateEmail(email) {
    var re = /^(([^<>()[]\.,;:\s@”]+(.[^<>()[]\.,;:\s@”]+)*)|(“.+”))@(([[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}])|(([a-zA-Z-0-9]+.)+[a-zA-Z]{2,}))$/;
    return re.test(String(email).toLowerCase());
    }

The above regular expression accepts unicode characters.

  1. How do you get the current url with javascript
    You can use window.location.href expression to get the current url path and you can use the same expression for updating the URL too. You can also use document.URL for read-only purposes but this solution has issues in FF.
    console.log(‘location.href’, window.location.href); // Returns full URL
  2. What are the various url properties of location object
    The below Location object properties can be used to access URL components of the page,
    i. href – The entire URL
    ii. protocol – The protocol of the URL
    iii. host – The hostname and port of the URL
    iv. hostname – The hostname of the URL
    v. port – The port number in the URL
    vi. pathname – The path name of the URL
    vii. search – The query portion of the URL
    viii. hash – The anchor portion of the URL
  3. How do get query string values in javascript
    You can use URLSearchParams to get query string values in javascript. Let’s see an example to get the client code value from URL query string,
    const urlParams = new URLSearchParams(window.location.search);
    const clientCode = urlParams.get(‘clientCode’);
  4. How do you check if a key exists in an object
    You can check whether a key exists in an object or not using three approaches,
    i. Using in operator: You can use the in operator whether a key exists in an object or not
    “key” in obj
    and If you want to check if a key doesn’t exist, remember to use parenthesis,
    !(“key” in obj)
    ii. Using hasOwnProperty method: You can use hasOwnProperty to particularly test for properties of the object instance (and not inherited properties)
    obj.hasOwnProperty(“key”) // true
    iii. Using undefined comparison: If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property.
    const user = {
    name: ‘John’
    };

console.log(user.name !== undefined); // true
console.log(user.nickName !== undefined); // false

  1. How do you loop through or enumerate javascript object
    You can use the for-in loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn’t come from the prototype using hasOwnProperty method.
    var object = {
    “k1”: “value1”,
    “k2”: “value2”,
    “k3”: “value3”
    };

for (var key in object) {
if (object.hasOwnProperty(key)) {
console.log(key + ” -> ” + object[key]); // k1 -> value1 …
}
}

  1. How do you test for an empty object
    There are different solutions based on ECMAScript versions
    i. Using Object entries(ECMA 7+): You can use object entries length along with constructor type.
    Object.entries(obj).length === 0 && obj.constructor === Object // Since date object length is 0, you need to check constructor check as well
    ii. Using Object keys(ECMA 5+): You can use object keys length along with constructor type.
    Object.keys(obj).length === 0 && obj.constructor === Object // Since date object length is 0, you need to check constructor check as well
    iii. Using for-in with hasOwnProperty(Pre-ECMA 5): You can use a for-in loop along with hasOwnProperty.
    function isEmpty(obj) {
    for(var prop in obj) {
    if(obj.hasOwnProperty(prop)) {
    return false;
    }
    } return JSON.stringify(obj) === JSON.stringify({});
    }
  2. What is an arguments object
    The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let’s see how to use arguments object inside sum function,
    function sum() {
    var total = 0;
    for (var i = 0, len = arguments.length; i < len; ++i) {
    total += arguments[i];
    }
    return total;
    }

sum(1, 2, 3) // returns 6
Note: You can’t apply array methods on arguments object. But you can convert into a regular array as below.
var argsArray = Array.prototype.slice.call(arguments);

  1. How do you make first letter of the string in an uppercase
    You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.
    function capitalizeFirstLetter(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
    }
  2. What are the pros and cons of for loop
    The for-loop is a commonly used iteration syntax in javascript. It has both pros and cons ####Pros
    i. Works on every environment
    ii. You can use break and continue flow control statements

Cons

iii. Too verbose
iv. Imperative
v. You might face one-by-off errors

  1. How do you display the current date in javascript
    You can use new Date() to generate a new Date object containing the current date and time. For example, let’s display the current date in mm/dd/yyyy
    var today = new Date();
    var dd = String(today.getDate()).padStart(2, ‘0’);
    var mm = String(today.getMonth() + 1).padStart(2, ‘0’); //January is 0!
    var yyyy = today.getFullYear();

today = mm + ‘/’ + dd + ‘/’ + yyyy;
document.write(today);

  1. How do you compare two date objects
    You need to use date.getTime() method to compare date values instead of comparison operators (==, !=, ===, and !== operators)
    var d1 = new Date();
    var d2 = new Date(d1);
    console.log(d1.getTime() === d2.getTime()); //True
    console.log(d1 === d2); // False
  2. How do you check if a string starts with another string
    You can use ECMAScript 6’s String.prototype.startsWith() method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let’s see an example to see this usage,
    “Good morning”.startsWith(“Good”); // true
    “Good morning”.startsWith(“morning”); // false
  3. How do you trim a string in javascript
    JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.
    ” Hello World “.trim(); //Hello World
    If your browser(<IE9) doesn’t support this method then you can use below polyfill.
    if (!String.prototype.trim) {
    (function() {
    // Make sure we trim BOM and NBSP
    var rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
    String.prototype.trim = function() {
    return this.replace(rtrim, ”);
    };
    })();
    }
  4. How do you add a key value pair in javascript
    There are two possible solutions to add new properties to an object. Let’s take a simple object to explain these solutions.
    var object = {
    key1: value1,
    key2: value2
    };
    i. Using dot notation: This solution is useful when you know the name of the property
    object.key3 = “value3”;
    ii. Using square bracket notation: This solution is useful when the name of the property is dynamically determined.
    obj[“key3”] = “value3”;
  5. Is the !– notation represents a special operator
    No,that’s not a special operator. But it is a combination of 2 standard operators one after the other,
    i. A logical not (!)
    ii. A prefix decrement (–)
    At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.
  6. How do you assign default values to variables
    You can use the logical or operator || in an assignment expression to provide a default value. The syntax looks like as below,
    var a = b || c;
    As per the above expression, variable ‘a ‘will get the value of ‘c’ only if ‘b’ is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise ‘a’ will get the value of ‘b’.
  7. How do you define multiline strings
    You can define multiline string literals using the ” character followed by line terminator.
    var str = “This is a \
    very lengthy \
    sentence!”;
    But if you have a space after the ” character, the code will look exactly the same, but it will raise a SyntaxError.
  8. What is an app shell model
    An application shell (or app shell) architecture is one way to build a Progressive Web App that reliably and instantly loads on your users’ screens, similar to what you see in native applications. It is useful for getting some initial HTML to the screen fast without a network.
  9. Can we define properties for functions
    Yes, We can define properties for functions because functions are also objects.
    fn = function(x) {
    //Function code goes here
    }

fn.name = “John”;

fn.profile = function(y) {
//Profile code goes here
}

  1. What is the way to find the number of parameters expected by a function
    You can use function.length syntax to find the number of parameters expected by a function. Let’s take an example of sum function to calculate the sum of numbers,
    function sum(num1, num2, num3, num4){
    return num1 + num2 + num3 + num4;
    }
    sum.length // 4 is the number of parameters expected.
  2. What is a polyfill
    A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.
  3. What are break and continue statements
    The break statement is used to “jump out” of a loop. i.e, It breaks the loop and continues executing the code after the loop.
    for (i = 0; i < 10; i++) { if (i === 5) { break; } text += “Number: ” + i + “
    “;
    }
    The continue statement is used to “jump over” one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
    for (i = 0; i < 10; i++) { if (i === 5) { continue; } text += “Number: ” + i + “
    “;
    }
  4. What are js labels
    The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,
    var i, j;

loop1:
for (i = 0; i < 3; i++) {
loop2:
for (j = 0; j < 3; j++) {
if (i === j) {
continue loop1;
}
console.log(‘i = ‘ + i + ‘, j = ‘ + j);
}
}

// Output is:
// “i = 1, j = 0”
// “i = 2, j = 0”
// “i = 2, j = 1”

  1. What are the benefits of keeping declarations at the top
    It is recommended to keep all declarations at the top of each script or function. The benefits of doing this are,
    i. Gives cleaner code
    ii. It provides a single place to look for local variables
    iii. Easy to avoid unwanted global variables
    iv. It reduces the possibility of unwanted re-declarations
  2. What are the benefits of initializing variables
    It is recommended to initialize variables because of the below benefits,
    i. It gives cleaner code
    ii. It provides a single place to initialize variables
    iii. Avoid undefined values in the code
  3. What are the recommendations to create new object
    It is recommended to avoid creating new objects using new Object(). Instead you can initialize values based on it’s type to create the objects.
    i. Assign {} instead of new Object()
    ii. Assign “” instead of new String()
    iii. Assign 0 instead of new Number()
    iv. Assign false instead of new Boolean()
    v. Assign [] instead of new Array()
    vi. Assign /()/ instead of new RegExp()
    vii. Assign function (){} instead of new Function()
    You can define them as an example,
    var v1 = {};
    var v2 = “”;
    var v3 = 0;
    var v4 = false;
    var v5 = [];
    var v6 = /()/;
    var v7 = function(){};
  4. How do you define JSON arrays
    JSON arrays are written inside square brackets and arrays contain javascript objects. For example, the JSON array of users would be as below,
    “users”:[
    {“firstName”:”John”, “lastName”:”Abrahm”},
    {“firstName”:”Anna”, “lastName”:”Smith”},
    {“firstName”:”Shane”, “lastName”:”Warn”}
    ]
  5. How do you generate random integers
    You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,
    Math.floor(Math.random() * 10) + 1; // returns a random integer from 1 to 10
    Math.floor(Math.random() * 100) + 1; // returns a random integer from 1 to 100
    Note: Math.random() returns a random number between 0 (inclusive), and 1 (exclusive)
  6. Can you write a random integers function to print integers with in a range
    Yes, you can create a proper random function to return a random number between min and max (both included)
    function randomInteger(min, max) {
    return Math.floor(Math.random() * (max – min + 1) ) + min;
    }
    randomInteger(1, 100); // returns a random integer from 1 to 100
    randomInteger(1, 1000); // returns a random integer from 1 to 1000
  7. What is tree shaking
    Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler rollup.
  8. What is the need of tree shaking
    Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.
  9. Is it recommended to use eval
    No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.
  10. What is a Regular Expression
    A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let’s see the syntax format now,
    /pattern/modifiers;
    For example, the regular expression or search pattern with case-insensitive username would be,
    /John/i
  11. What are the string methods available in Regular expression
    Regular Expressions has two string methods: search() and replace(). The search() method uses an expression to search for a match, and returns the position of the match.
    var msg = “Hello John”;
    var n = msg.search(/John/i); // 6
    The replace() method is used to return a modified string where the pattern is replaced.
    var msg = “Hello John”;
    var n = msg.replace(/John/i, “Buttler”); // Hello Buttler
  12. What are modifiers in regular expression
    Modifiers can be used to perform case-insensitive and global searches. Let’s list down some of the modifiers,
    Modifier Description
    i Perform case-insensitive matching
    g Perform a global match rather than stops at first match
    m Perform multiline matching
    Let’s take an example of global modifier,
    var text = “Learn JS one by one”;
    var pattern = /one/g;
    var result = text.match(pattern); // one,one
  13. What are regular expression patterns
    Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types,
    i. Brackets: These are used to find a range of characters. For example, below are some use cases,
    a. [abc]: Used to find any of the characters between the brackets(a,b,c)
    b. [0-9]: Used to find any of the digits between the brackets
    c. (a|b): Used to find any of the alternatives separated with |
    ii. Metacharacters: These are characters with a special meaning For example, below are some use cases,
    a. \d: Used to find a digit
    b. \s: Used to find a whitespace character
    c. \b: Used to find a match at the beginning or ending of a word
    iii. Quantifiers: These are useful to define quantities For example, below are some use cases,
    a. n+: Used to find matches for any string that contains at least one n
    b. n*: Used to find matches for any string that contains zero or more occurrences of n
    c. n?: Used to find matches for any string that contains zero or one occurrences of n
  14. What is a RegExp object
    RegExp object is a regular expression object with predefined properties and methods. Let’s see the simple usage of RegExp object,
    var regexp = new RegExp(‘\w+’);
    console.log(regexp);
    // expected output: /\w+/
  15. How do you search a string for a pattern
    You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result.
    var pattern = /you/;
    console.log(pattern.test(“How are you?”)); //true
  16. What is the purpose of exec method
    The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.
    var pattern = /you/;
    console.log(pattern.exec(“How are you?”)); //[“you”, index: 8, input: “How are you?”, groups: undefined]
  17. How do you change the style of a HTML element
    You can change inline style or classname of a HTML element using javascript
    i. Using style property: You can modify inline style using style property
    document.getElementById(“title”).style.fontSize = “30px”;
    ii. Using ClassName property: It is easy to modify element class using className property
    document.getElementById(“title”).style.className = “custom-title”;
  18. What would be the result of 1+2+’3′
    The output is going to be 33. Since 1 and 2 are numeric values, the result of the first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.
  19. What is a debugger statement
    The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect. For example, in the below function a debugger statement has been inserted. So execution is paused at the debugger statement just like a breakpoint in the script source.
    function getProfile() {
    // code goes here
    debugger;
    // code goes here
    }
  20. What is the purpose of breakpoints in debugging
    You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.
  21. Can I use reserved words as identifiers
    No, you cannot use the reserved words as variables, labels, object or function names. Let’s see one simple example,
    var else = “hello”; // Uncaught SyntaxError: Unexpected token else
  22. How do you detect a mobile browser
    You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.
    window.mobilecheck = function() {
    var mobileCheck = false;
    (function(a){if(/(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up.(browser|link)|vodafone|wap|windows ce|xda|xiino/i.test(a)||/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw-(n|u)|c55\/|capi|ccwa|cdm-|cell|chtm|cldc|cmd-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc-s|devi|dica|dmob|do(c|p)o|ds(12|-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(-|)|g1 u|g560|gene|gf-5|g-mo|go(.w|od)|gr(ad|un)|haie|hcit|hd-(m|p|t)|hei-|hi(pt|ta)|hp( i|ip)|hs-c|ht(c(-| ||a|g|p|s|t)|tp)|hu(aw|tc)|i-(20|go|ma)|i230|iac( |-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|-[a-w])|libw|lynx|m1-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|-([1-8]|c))|phil|pire|pl(ay|uc)|pn-2|po(ck|rt|se)|prox|psio|pt-g|qa-a|qc(07|12|21|32|60|-[2-7]|i-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h-|oo|p-)|sdk\/|se(c(-|0|1)|47|mc|nd|ri)|sgh-|shar|sie(-|m)|sk-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h-|v-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl-|tdg-|tel(i|m)|tim-|t-mo|to(pl|sh)|ts(70|m-|m3|m5)|tx-9|up(.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas-|your|zeto|zte-/i.test(a.substr(0,4))) mobileCheck = true;})(navigator.userAgent||navigator.vendor||window.opera);
    return mobileCheck;
    };
  23. How do you detect a mobile browser without regexp
    You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage,
    function detectmob() {
    if( navigator.userAgent.match(/Android/i)
    || navigator.userAgent.match(/webOS/i)
    || navigator.userAgent.match(/iPhone/i)
    || navigator.userAgent.match(/iPad/i)
    || navigator.userAgent.match(/iPod/i)
    || navigator.userAgent.match(/BlackBerry/i)
    || navigator.userAgent.match(/Windows Phone/i)
    ){
    return true;
    }
    else {
    return false;
    }
    }
  24. How do you get the image width and height using JS
    You can programmatically get the image and check the dimensions(width and height) using Javascript.
    var img = new Image();
    img.onload = function() {
    console.log(this.width + ‘x’ + this.height);
    }
    img.src = ‘http://www.google.com/intl/en_ALL/images/logo.gif’;
  25. How do you make synchronous HTTP request
    Browsers provide an XMLHttpRequest object which can be used to make synchronous HTTP requests from JavaScript
    function httpGet(theUrl)
    {
    var xmlHttpReq = new XMLHttpRequest();
    xmlHttpReq.open( “GET”, theUrl, false ); // false for synchronous request
    xmlHttpReq.send( null );
    return xmlHttpReq.responseText;
    }
  26. How do you make asynchronous HTTP request
    Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.
    function httpGetAsync(theUrl, callback)
    {
    var xmlHttpReq = new XMLHttpRequest();
    xmlHttpReq.onreadystatechange = function() {
    if (xmlHttpReq.readyState == 4 && xmlHttpReq.status == 200)
    callback(xmlHttpReq.responseText);
    }
    xmlHttp.open(“GET”, theUrl, true); // true for asynchronous
    xmlHttp.send(null);
    }
  27. How do you convert date to another timezone in javascript
    You can use the toLocaleString() method to convert dates in one timezone to another. For example, let’s convert current date to British English timezone as below,
    console.log(event.toLocaleString(‘en-GB’, { timeZone: ‘UTC’ })); //29/06/2019, 09:56:00
  28. What are the properties used to get size of window
    You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let’s use them combination of these properties to calculate the size of a window or document,
    var width = window.innerWidth
    || document.documentElement.clientWidth
    || document.body.clientWidth;

var height = window.innerHeight
|| document.documentElement.clientHeight
|| document.body.clientHeight;

  1. What is a conditional operator in javascript
    The conditional (ternary) operator is the only JavaScript operator that takes three operands which acts as a shortcut for if statements.
    var isAuthenticated = false;
    console.log(isAuthenticated ? ‘Hello, welcome’ : ‘Sorry, you are not authenticated’); //Sorry, you are not authenticated
  2. Can you apply chaining on conditional operator
    Yes, you can apply chaining on conditional operators similar to if … else if … else if … else chain. The syntax is going to be as below,
    function traceValue(someParam) {
    return condition1 ? value1
    : condition2 ? value2
    : condition3 ? value3
    : value4;
    }

// The above conditional operator is equivalent to:

function traceValue(someParam) {
if (condition1) { return value1; }
else if (condition2) { return value2; }
else if (condition3) { return value3; }
else { return value4; }
}

  1. What are the ways to execute javascript after page load
    You can execute javascript after page load in many different ways,
    i. window.onload:
    window.onload = function …
    ii. document.onload:
    document.onload = function …
    iii. body onload:
  2. What is the difference between proto and prototype
    The proto object is the actual object that is used in the lookup chain to resolve methods, etc. Whereas prototype is the object that is used to build proto when you create an object with new
    ( new Employee ).proto === Employee.prototype;
    ( new Employee ).prototype === undefined;
  3. Give an example where do you really need semicolon
    It is recommended to use semicolons after every statement in JavaScript. For example, in the below case it throws an error “.. is not a function” at runtime due to missing semicolon.
    // define a function
    var fn = function () {
    //…
    } // semicolon missing at this line

// then execute some code inside a closure
(function () {
//…
})();
and it will be interpreted as
var fn = function () {
//…
}(function () {
//…
})();
In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a “… is not a function” error at runtime.

  1. What is a freeze method
    The freeze() method is used to freeze an object. Freezing an object does not allow adding new properties to an object,prevents from removing and prevents changing the enumerability, configurability, or writability of existing properties. i.e, It returns the passed object and does not create a frozen copy.
    const obj = {
    prop: 100
    };

Object.freeze(obj);
obj.prop = 200; // Throws an error in strict mode

console.log(obj.prop); //100
Note: It causes a TypeError if the argument passed is not an object.

  1. What is the purpose of freeze method
    Below are the main benefits of using freeze method,
    i. It is used for freezing objects and arrays.
    ii. It is used to make an object immutable.
  2. Why do I need to use freeze method
    In the Object-oriented paradigm, an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. Hence it works as the final keyword which is used in various languages.
  3. How do you detect a browser language preference
    You can use navigator object to detect a browser language preference as below,
    var language = navigator.languages && navigator.languages[0] || // Chrome / Firefox
    navigator.language || // All browsers
    navigator.userLanguage; // IE <= 10

console.log(language);

  1. How to convert string to title case with javascript
    Title case means that the first letter of each word is capitalized. You can convert a string to title case using the below function,
    function toTitleCase(str) {
    return str.replace(
    /\w\S*/g,
    function(txt) {
    return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
    }
    );
    }
    toTitleCase(“good morning john”); // Good Morning John
  2. How do you detect javascript disabled in the page
    You can use the
  1. What are various operators supported by javascript
    An operator is capable of manipulating(mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below,
    i. Arithmetic Operators: Includes + (Addition),– (Subtraction), * (Multiplication), / (Division), % (Modulus), + + (Increment) and – – (Decrement)
    ii. Comparison Operators: Includes = =(Equal),!= (Not Equal), ===(Equal with type), > (Greater than),> = (Greater than or Equal to),< (Less than),<= (Less than or Equal to)
    iii. Logical Operators: Includes &&(Logical AND),||(Logical OR),!(Logical NOT)
    iv. Assignment Operators: Includes = (Assignment Operator), += (Add and Assignment Operator), – = (Subtract and Assignment Operator), *= (Multiply and Assignment), /= (Divide and Assignment), %= (Modules and Assignment)
    v. Ternary Operators: It includes conditional(: ?) Operator
    vi. typeof Operator: It uses to find type of variable. The syntax looks like typeof variable
  2. What is a rest parameter
    Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below,
    function f(a, b, …theArgs) {
    // …
    }
    For example, let’s take a sum example to calculate on dynamic number of parameters,
    function total(…args){
    let sum = 0;
    for(let i of args){
    sum+=i;
    }
    return sum;
    }
    console.log(fun(1,2)); //3
    console.log(fun(1,2,3)); //6
    console.log(fun(1,2,3,4)); //13
    console.log(fun(1,2,3,4,5)); //15
    Note: Rest parameter is added in ES2015 or ES6
  3. What happens if you do not use rest parameter as a last argument
    The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.
    function someFunc(a,…b,c){
    //You code goes here
    return;
    }
  4. What are the bitwise operators available in javascript
    Below are the list of bitwise logical operators used in JavaScript
    i. Bitwise AND ( & )
    ii. Bitwise OR ( | )
    iii. Bitwise XOR ( ^ )
    iv. Bitwise NOT ( ~ )
    v. Left Shift ( << ) vi. Sign Propagating Right Shift ( >> )
    vii. Zero fill Right Shift ( >>> )
  5. What is a spread operator
    Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. Let’s take an example to see this behavior,
    function calculateSum(x, y, z) {
    return x + y + z;
    }

const numbers = [1, 2, 3];

console.log(calculateSum(…numbers)); // 6

  1. How do you determine whether object is frozen or not
    Object.isFrozen() method is used to determine if an object is frozen or not.An object is frozen if all of the below conditions hold true,
    i. If it is not extensible.
    ii. If all of its properties are non-configurable.
    iii. If all its data properties are non-writable. The usage is going to be as follows,
    const object = {
    property: ‘Welcome JS world’
    };
    Object.freeze(object);
    console.log(Object.isFrozen(object));
  2. How do you determine two values same or not using object
    The Object.is() method determines whether two values are the same value. For example, the usage with different types of values would be,
    Object.is(‘hello’, ‘hello’); // true
    Object.is(window, window); // true
    Object.is([], []) // false
    Two values are the same if one of the following holds:
    i. both undefined
    ii. both null
    iii. both true or both false
    iv. both strings of the same length with the same characters in the same order
    v. both the same object (means both object have same reference)
    vi. both numbers and both +0 both -0 both NaN both non-zero and both not NaN and both have the same value.
  3. What is the purpose of using object is method
    Some of the applications of Object’s is method are follows,
    i. It is used for comparison of two strings.
    ii. It is used for comparison of two numbers.
    iii. It is used for comparing the polarity of two numbers.
    iv. It is used for comparison of two objects.
  4. How do you copy properties from one object to other
    You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the target object. The syntax would be as below,
    Object.assign(target, …sources)
    Let’s take example with one source and one target object,
    const target = { a: 1, b: 2 };
    const source = { b: 3, c: 4 };

const returnedTarget = Object.assign(target, source);

console.log(target); // { a: 1, b: 3, c: 4 }

console.log(returnedTarget); // { a: 1, b: 3, c: 4 }
As observed in the above code, there is a common property(b) from source to target so it’s value has been overwritten.

  1. What are the applications of assign method
    Below are the some of main applications of Object.assign() method,
    i. It is used for cloning an object.
    ii. It is used to merge objects with the same properties.
  2. What is a proxy object
    The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows,
    var p = new Proxy(target, handler);
    Let’s take an example of proxy object,
    var handler = {
    get: function(obj, prop) {
    return prop in obj ?
    obj[prop] :
    100;
    }
    };

var p = new Proxy({}, handler);
p.a = 10;
p.b = null;

console.log(p.a, p.b); // 10, null
console.log(‘c’ in p, p.c); // false, 100
In the above code, it uses get handler which define the behavior of the proxy when an operation is performed on it

  1. What is the purpose of seal method
    The Object.seal() method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let’s see the below example to understand more about seal() method
    const object = {
    property: ‘Welcome JS world’
    };
    Object.seal(object);
    object.property = ‘Welcome to object world’;
    console.log(Object.isSealed(object)); // true
    delete object.property; // You cannot delete when sealed
    console.log(object.property); //Welcome to object world
  2. What are the applications of seal method
    Below are the main applications of Object.seal() method,
    i. It is used for sealing objects and arrays.
    ii. It is used to make an object immutable.
  3. What are the differences between freeze and seal methods
    If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.
  4. How do you determine if an object is sealed or not
    The Object.isSealed() method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true
    i. If it is not extensible.
    ii. If all of its properties are non-configurable.
    iii. If it is not removable (but not necessarily non-writable). Let’s see it in the action
    const object = {
    property: ‘Hello, Good morning’
    };

Object.seal(object); // Using seal() method to seal the object

console.log(Object.isSealed(object)); // checking whether the object is sealed or not

  1. How do you get enumerable key and value pairs
    The Object.entries() method is used to return an array of a given object’s own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for…in loop. Let’s see the functionality of object.entries() method in an example,
    const object = {
    a: ‘Good morning’,
    b: 100
    };

for (let [key, value] of Object.entries(object)) {
console.log(${key}: ${value}); // a: ‘Good morning’
// b: 100
}
Note: The order is not guaranteed as object defined.

  1. What is the main difference between Object.values and Object.entries method
    The Object.values() method’s behavior is similar to Object.entries() method but it returns an array of values instead [key,value] pairs.
    const object = {
    a: ‘Good morning’,
    b: 100
    }; for (let value of Object.values(object)) {
    console.log(${value}); // ‘Good morning’
    100
    }
  2. How can you get the list of keys of any object
    You can use the Object.keys() method which is used to return an array of a given object’s own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,
    const user = {
    name: ‘John’,
    gender: ‘male’,
    age: 40
    };

console.log(Object.keys(user)); //[‘name’, ‘gender’, ‘age’]

  1. How do you create an object with prototype
    The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.
    const user = {
    name: ‘John’,
    printInfo: function () {
    console.log(My name is ${this.name}.);
    }
    }; const admin = Object.create(user); admin.name = “Nick”; // Remember that “name” is a property set on “admin” but not on “user” object admin.printInfo(); // My name is Nick
  2. What is a WeakSet
    WeakSet is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,
    new WeakSet([iterable]);
    Let’s see the below example to explain it’s behavior,
    var ws = new WeakSet();
    var user = {};
    ws.add(user);
    ws.has(user); // true
    ws.delete(user); // removes user from the set
    ws.has(user); // false, user has been removed
  3. What are the differences between WeakSet and Set
    The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it. Other differences are,
    i. Sets can store any value Whereas WeakSets can store only collections of objects
    ii. WeakSet does not have size property unlike Set
    iii. WeakSet does not have methods such as clear, keys, values, entries, forEach.
    iv. WeakSet is not iterable.
  4. List down the collection of methods available on WeakSet
    Below are the list of methods available on WeakSet,
    i. add(value): A new object is appended with the given value to the weakset
    ii. delete(value): Deletes the value from the WeakSet collection.
    iii. has(value): It returns true if the value is present in the WeakSet Collection, otherwise it returns false.
    iv. length(): It returns the length of weakSetObject Let’s see the functionality of all the above methods in an example,
    var weakSetObject = new WeakSet();
    var firstObject = {};
    var secondObject = {};
    // add(value)
    weakSetObject.add(firstObject);
    weakSetObject.add(secondObject);
    console.log(weakSetObject.has(firstObject)); //true
    console.log(weakSetObject.length()); //2
    weakSetObject.delete(secondObject);
  5. What is a WeakMap
    The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below,
    new WeakMap([iterable])
    Let’s see the below example to explain it’s behavior,
    var ws = new WeakMap();
    var user = {};
    ws.set(user);
    ws.has(user); // true
    ws.delete(user); // removes user from the map
    ws.has(user); // false, user has been removed
  6. What are the differences between WeakMap and Map
    The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it. Other differences are,
    i. Maps can store any key type Whereas WeakMaps can store only collections of key objects
    ii. WeakMap does not have size property unlike Map
    iii. WeakMap does not have methods such as clear, keys, values, entries, forEach.
    iv. WeakMap is not iterable.
  7. List down the collection of methods available on WeakMap
    Below are the list of methods available on WeakMap,
    i. set(key, value): Sets the value for the key in the WeakMap object. Returns the WeakMap object.
    ii. delete(key): Removes any value associated to the key.
    iii. has(key): Returns a Boolean asserting whether a value has been associated to the key in the WeakMap object or not.
    iv. get(key): Returns the value associated to the key, or undefined if there is none. Let’s see the functionality of all the above methods in an example,
    var weakMapObject = new WeakMap();
    var firstObject = {};
    var secondObject = {};
    // set(key, value)
    weakMapObject.set(firstObject, ‘John’);
    weakMapObject.set(secondObject, 100);
    console.log(weakMapObject.has(firstObject)); //true
    console.log(weakMapObject.get(firstObject)); // John
    weakMapObject.delete(secondObject);
  8. What is the purpose of uneval
    The uneval() is an inbuilt function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let’s see the below example to know more about it’s functionality,
    var a = 1;
    uneval(a); // returns a String containing 1
    uneval(function user() {}); // returns “(function user(){})”
  9. How do you encode an URL
    The encodeURI() function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.
    var uri = ‘https://mozilla.org/?x=шеллы’;
    var encoded = encodeURI(uri);
    console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
  10. How do you decode an URL
    The decodeURI() function is used to decode a Uniform Resource Identifier (URI) previously created by encodeURI().
    var uri = ‘https://mozilla.org/?x=шеллы’;
    var encoded = encodeURI(uri);
    console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
    try {
    console.log(decodeURI(encoded)); // “https://mozilla.org/?x=шеллы”
    } catch(e) { // catches a malformed URI
    console.error(e);
    }
  11. How do you print the contents of web page
    The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let’s see the usage of print method in an example,

    Note: In most browsers, it will block while the print dialog is open.
  12. What is the difference between uneval and eval
    The uneval function returns the source of a given object; whereas the eval function does the opposite, by evaluating that source code in a different memory area. Let’s see an example to clarify the difference,
    var msg = uneval(function greeting() { return ‘Hello, Good morning’; });
    var greeting = eval(msg);
    greeting(); // returns “Hello, Good morning”
  13. What is an anonymous function
    An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,
    function (optionalParameters) {
    //do something
    }

const myFunction = function(){ //Anonymous function assigned to a variable
//do something
};

[1, 2, 3].map(function(element){ //Anonymous function used as a callback function
//do something
});
Let’s see the above anonymous function in an example,
var x = function (a, b) {return a * b};
var z = x(5, 10);
console.log(z); // 50

  1. What is the precedence order between local and global variables
    A local variable takes precedence over a global variable with the same name. Let’s see this behavior in an example.
    var msg = “Good morning”;
    function greeting() {
    msg = “Good Evening”;
    console.log(msg);
    }
    greeting();
  2. What are javascript accessors
    ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the get keyword whereas Setters uses the set keyword.
    var user = {
    firstName: “John”,
    lastName : “Abraham”,
    language : “en”,
    get lang() {
    return this.language;
    }
    set lang(lang) {
    this.language = lang;
    }
    };
    console.log(user.lang); // getter access lang as en
    user.lang = ‘fr’;
    console.log(user.lang); // setter used to set lang as fr
  3. How do you define property on Object constructor
    The Object.defineProperty() static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let’s see an example to know how to define property,
    const newObject = {};

Object.defineProperty(newObject, ‘newProperty’, {
value: 100,
writable: false
});

console.log(newObject.newProperty); // 100

newObject.newProperty = 200; // It throws an error in strict mode due to writable setting

  1. What is the difference between get and defineProperty
    Both have similar results until unless you use classes. If you use get the property will be defined on the prototype of the object whereas using Object.defineProperty() the property will be defined on the instance it is applied to.
  2. What are the advantages of Getters and Setters
    Below are the list of benefits of Getters and Setters,
    i. They provide simpler syntax
    ii. They are used for defining computed properties, or accessors in JS.
    iii. Useful to provide equivalence relation between properties and methods
    iv. They can provide better data quality
    v. Useful for doing things behind the scenes with the encapsulated logic.
  3. Can I add getters and setters using defineProperty method
    Yes, You can use the Object.defineProperty() method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,
    var obj = {counter : 0};

// Define getters
Object.defineProperty(obj, “increment”, {
get : function () {this.counter++;}
});
Object.defineProperty(obj, “decrement”, {
get : function () {this.counter–;}
});

// Define setters
Object.defineProperty(obj, “add”, {
set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, “subtract”, {
set : function (value) {this.counter -= value;}
});

obj.add = 10;
obj.subtract = 5;
console.log(obj.increment); //6
console.log(obj.decrement); //5

  1. What is the purpose of switch-case
    The switch case statement in JavaScript is used for decision making purposes. In a few cases, using the switch case statement is going to be more convenient than if-else statements. The syntax would be as below,
    switch (expression)
    {
    case value1:
    statement1;
    break;
    case value2:
    statement2;
    break;
    .
    .
    case valueN:
    statementN;
    break;
    default:
    statementDefault;
    }
    The above multi-way branch statement provides an easy way to dispatch execution to different parts of code based on the value of the expression.
  2. What are the conventions to be followed for the usage of switch case
    Below are the list of conventions should be taken care,
    i. The expression can be of type either number or string.
    ii. Duplicate values are not allowed for the expression.
    iii. The default statement is optional. If the expression passed to switch does not match with any case value then the statement within default case will be executed.
    iv. The break statement is used inside the switch to terminate a statement sequence.
    v. The break statement is optional. But if it is omitted, the execution will continue on into the next case.
  3. What are primitive data types
    A primitive data type is data that has a primitive value (which has no properties or methods). There are 5 types of primitive data types.
    i. string
    ii. number
    iii. boolean
    iv. null
    v. undefined
  4. What are the different ways to access object properties
    There are 3 possible ways for accessing the property of an object.
    i. Dot notation: It uses dot for accessing the properties
    objectName.property
    ii. Square brackets notation: It uses square brackets for property access
    objectName[“property”]
    iii. Expression notation: It uses expression in the square brackets
    objectName[expression]
  5. What are the function parameter rules
    JavaScript functions follow below rules for parameters,
    i. The function definitions do not specify data types for parameters.
    ii. Do not perform type checking on the passed arguments.
    iii. Do not check the number of arguments received. i.e, The below function follows the above rules,
    function functionName(parameter1, parameter2, parameter3) {
    console.log(parameter1); // 1
    }
    functionName(1);
  6. What is an error object
    An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,
    try {
    greeting(“Welcome”);
    }
    catch(err) {
    console.log(err.name + “
    ” + err.message);
    }
  7. When you get a syntax error
    A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error
    try {
    eval(“greeting(‘welcome)”); // Missing ‘ will produce an error
    }
    catch(err) {
    console.log(err.name);
    }
  8. What are the different error names from error object
    There are 6 different types of error names returned from error object,
    Error Name Description
    EvalError An error has occurred in the eval() function
    RangeError An error has occurred with a number “out of range”
    ReferenceError An error due to an illegal reference
    SyntaxError An error due to a syntax error
    TypeError An error due to a type error
    URIError An error due to encodeURI()
  9. What are the various statements in error handling
    Below are the list of statements used in an error handling,
    i. try: This statement is used to test a block of code for errors
    ii. catch: This statement is used to handle the error
    iii. throw: This statement is used to create custom errors.
    iv. finally: This statement is used to execute code after try and catch regardless of the result.
  10. What are the two types of loops in javascript
    i. Entry Controlled loops: In this kind of loop type, the test condition is tested before entering the loop body. For example, For Loop and While Loop comes under this category.
    ii. Exit Controlled Loops: In this kind of loop type, the test condition is tested or evaluated at the end of the loop body. i.e, the loop body will execute at least once irrespective of test condition true or false. For example, do-while loop comes under this category.
  11. What is nodejs
    Node.js is a server-side platform built on Chrome’s JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google’s V8 JavaScript engine and libuv library.
  12. What is an Intl object
    The Intl object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.
  13. How do you perform language specific date and time formatting
    You can use the Intl.DateTimeFormat object which is a constructor for objects that enable language-sensitive date and time formatting. Let’s see this behavior with an example,
    var date = new Date(Date.UTC(2019, 07, 07, 3, 0, 0));
    console.log(new Intl.DateTimeFormat(‘en-GB’).format(date)); // 07/08/2019
    console.log(new Intl.DateTimeFormat(‘en-AU’).format(date)); // 07/08/2019
  14. What is an Iterator
    An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a next() method which returns an object with two properties: value (the next value in the sequence) and done (which is true if the last value in the sequence has been consumed).
  15. How does synchronous iteration works
    Synchronous iteration was introduced in ES6 and it works with below set of components,
    Iterable: It is an object which can be iterated over via a method whose key is Symbol.iterator. Iterator: It is an object returned by invoking [Symbol.iterator]() on an iterable. This iterator object wraps each iterated element in an object and returns it via next() method one by one. IteratorResult: It is an object returned by next() method. The object contains two properties; the value property contains an iterated element and the done property determines whether the element is the last element or not.
    Let’s demonstrate synchronous iteration with an array as below,
    const iterable = [‘one’, ‘two’, ‘three’];
    const iterator = iterable[Symbol.iterator]();
    console.log(iterator.next()); // { value: ‘one’, done: false }
    console.log(iterator.next()); // { value: ‘two’, done: false }
    console.log(iterator.next()); // { value: ‘three’, done: false }
    console.log(iterator.next()); // { value: ‘undefined, done: true }
  16. What is an event loop
    The Event Loop is a queue of callback functions. When an async function executes, the callback function is pushed into the queue. The JavaScript engine doesn’t start processing the event loop until the async function has finished executing the code. Note: It allows Node.js to perform non-blocking I/O operations even though JavaScript is single-threaded.
  17. What is call stack
    Call Stack is a data structure for javascript interpreters to keep track of function calls in the program. It has two major actions,
    i. Whenever you call a function for its execution, you are pushing it to the stack.
    ii. Whenever the execution is completed, the function is popped out of the stack.
    Let’s take an example and it’s state representation in a diagram format
    function hungry() {
    eatFruits();
    }
    function eatFruits() {
    return “I’m eating fruits”;
    }

// Invoke the hungry function
hungry();
The above code processed in a call stack as below,
iii. Add the hungry() function to the call stack list and execute the code.
iv. Add the eatFruits() function to the call stack list and execute the code.
v. Delete the eatFruits() function from our call stack list.
vi. Delete the hungry() function from the call stack list since there are no items anymore.

  1. What is an event queue
  2. What is a decorator
    A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let’s define admin decorator for user class at design time,
    function admin(isAdmin) {
    return function(target) {
    target.isAdmin = isAdmin;
    }
    }

@admin(true)
class User() {
}
console.log(User.isAdmin); //true

@admin(false)
class User() {
}
console.log(User.isAdmin); //false

  1. What are the properties of Intl object
    Below are the list of properties available on Intl object,
    i. Collator: These are the objects that enable language-sensitive string comparison.
    ii. DateTimeFormat: These are the objects that enable language-sensitive date and time formatting.
    iii. ListFormat: These are the objects that enable language-sensitive list formatting.
    iv. NumberFormat: Objects that enable language-sensitive number formatting.
    v. PluralRules: Objects that enable plural-sensitive formatting and language-specific rules for plurals.
    vi. RelativeTimeFormat: Objects that enable language-sensitive relative time formatting.
  2. What is an Unary operator
    The unary(+) operator is used to convert a variable to a number.If the variable cannot be converted, it will still become a number but with the value NaN. Let’s see this behavior in an action.
    var x = “100”;
    var y = + x;
    console.log(typeof x, typeof y); // string, number

var a = “Hello”;
var b = + a;
console.log(typeof a, typeof b, b); // string, number, NaN

  1. How do you sort elements in an array
    The sort() method is used to sort the elements of an array in place and returns the sorted array. The example usage would be as below,
    var months = [“Aug”, “Sep”, “Jan”, “June”];
    months.sort();
    console.log(months); // [“Aug”, “Jan”, “June”, “Sep”]
  2. What is the purpose of compareFunction while sorting arrays
    The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character’s Unicode code point value. Let’s take an example to see the usage of compareFunction,
    let numbers = [1, 2, 5, 3, 4];
    numbers.sort((a, b) => b – a);
    console.log(numbers); // [5, 4, 3, 2, 1]
  3. How do you reversing an array
    You can use the reverse() method to reverse the elements in an array. This method is useful to sort an array in descending order. Let’s see the usage of reverse() method in an example,
    let numbers = [1, 2, 5, 3, 4];
    numbers.sort((a, b) => b – a);
    numbers.reverse();
    console.log(numbers); // [1, 2, 3, 4 ,5]
  4. How do you find min and max value in an array
    You can use Math.min and Math.max methods on array variables to find the minimum and maximum elements within an array. Let’s create two functions to find the min and max value with in an array,
    var marks = [50, 20, 70, 60, 45, 30];
    function findMin(arr) {
    return Math.min.apply(null, arr);
    }
    function findMax(arr) {
    return Math.max.apply(null, arr);
    }

console.log(findMin(marks));
console.log(findMax(marks));

  1. How do you find min and max values without Math functions
    You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let’s create those functions to find min and max values,
    var marks = [50, 20, 70, 60, 45, 30];
    function findMin(arr) {
    var length = arr.length
    var min = Infinity;
    while (length–) {
    if (arr[length] < min) {
    min = arr[len];
    }
    }
    return min;
    } function findMax(arr) {
    var length = arr.length
    var max = -Infinity;
    while (len–) {
    if (arr[length] > max) {
    max = arr[length];
    }
    }
    return max;
    } console.log(findMin(marks));
    console.log(findMax(marks));
  2. What is an empty statement and purpose of it
    The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one. Since there is no action with an empty statement you might think that it’s usage is quite less, but the empty statement is occasionally useful when you want to create a loop that has an empty body. For example, you can initialize an array with zero values as below,
    // Initialize an array a
    for(int i=0; i < a.length; a[i++] = 0) ;
  3. How do you get metadata of a module
    You can use the import.meta object which is a meta-property exposing context-specific meta data to a JavaScript module. It contains information about the current module, such as the module’s URL. In browsers, you might get different meta data than NodeJS.

console.log(import.meta); // { url: “file:///home/user/welcome-module.js” }

  1. What is a comma operator
    The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below,
    var x = 1;
    x = (x++, x);

console.log(x); // 2

  1. What is the advantage of a comma operator
    It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a for loop. For example, the below for loop uses multiple expressions in a single location using comma operator,
    for (var a = 0, b =10; a <= 10; a++, b–)
    You can also use the comma operator in a return statement where it processes before returning.
    function myFunction() {
    var a = 1;
    return (a += 10, a); // 11
    }
  2. What is typescript
    TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. Angular built entirely in TypeScript and used as a primary language. You can install it globally as
    npm install -g typescript
    Let’s see a simple example of TypeScript usage,
    function greeting(name: string): string {
    return “Hello, ” + name;
    }

let user = “Sudheer”;

console.log(greeting(user));
The greeting method allows only string type as argument.

  1. What are the differences between javascript and typescript
    Below are the list of differences between javascript and typescript,
    feature typescript javascript
    Language paradigm Object oriented programming language Scripting language
    Typing support Supports static typing It has dynamic typing
    Modules Supported Not supported
    Interface It has interfaces concept Doesn’t support interfaces
    Optional parameters Functions support optional parameters No support of optional parameters for functions
  2. What are the advantages of typescript over javascript
    Below are some of the advantages of typescript over javascript,
    i. TypeScript is able to find compile time errors at the development time only and it makes sures less runtime errors. Whereas javascript is an interpreted language.
    ii. TypeScript is strongly-typed or supports static typing which allows for checking type correctness at compile time. This is not available in javascript.
    iii. TypeScript compiler can compile the .ts files into ES3,ES4 and ES5 unlike ES6 features of javascript which may not be supported in some browsers.
  3. What is an object initializer
    An object initializer is an expression that describes the initialization of an Object. The syntax for this expression is represented as a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). This is also known as literal notation. It is one of the ways to create an object.
    var initObject = {a: ‘John’, b: 50, c: {}};

console.log(initObject.a); // John

  1. What is a constructor method
    The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below,
    class Employee {
    constructor() {
    this.name = “John”;
    }
    }

var employeeObject = new Employee();

console.log(employeeObject.name); // John

  1. What happens if you write constructor more than once in a class
    The “constructor” in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a SyntaxError error.
    class Employee {
    constructor() {
    this.name = “John”;
    }
    constructor() { // Uncaught SyntaxError: A class may only have one constructor
    this.age = 30;
    }
    } var employeeObject = new Employee(); console.log(employeeObject.name);
  2. How do you call the constructor of a parent class
    You can use the super keyword to call the constructor of a parent class. Remember that super() must be called before using ‘this’ reference. Otherwise it will cause a reference error. Let’s the usage of it,
    class Square extends Rectangle {
    constructor(length) {
    super(length, length);
    this.name = ‘Square’;
    } get area() {
    return this.width * this.height;
    } set area(value) {
    this.area = value;
    }
    }
  3. How do you get the prototype of an object
    You can use the Object.getPrototypeOf(obj) method to return the prototype of the specified object. i.e. The value of the internal prototype property. If there are no inherited properties then null value is returned.
    const newPrototype = {};
    const newObject = Object.create(newPrototype);

console.log(Object.getPrototypeOf(newObject) === newPrototype); // true

  1. What happens If I pass string type for getPrototype method
    In ES5, it will throw a TypeError exception if the obj parameter isn’t an object. Whereas in ES2015, the parameter will be coerced to an Object.
    // ES5
    Object.getPrototypeOf(‘James’); // TypeError: “James” is not an object
    // ES2015
    Object.getPrototypeOf(‘James’); // String.prototype
  2. How do you set prototype of one object to another
    You can use the Object.setPrototypeOf() method that sets the prototype (i.e., the internal Prototype property) of a specified object to another object or null. For example, if you want to set prototype of a square object to rectangle object would be as follows,
    Object.setPrototypeOf(Square.prototype, Rectangle.prototype);
    Object.setPrototypeOf({}, null);
  3. How do you check whether an object can be extendable or not
    The Object.isExtensible() method is used to determine if an object is extendable or not. i.e, Whether it can have new properties added to it or not.
    const newObject = {};
    console.log(Object.isExtensible(newObject)); //true
    Note: By default, all the objects are extendable. i.e, The new properties can be added or modified.
  4. How do you prevent an object to extend
    The Object.preventExtensions() method is used to prevent new properties from ever being added to an object. In other words, it prevents future extensions to the object. Let’s see the usage of this property,
    const newObject = {};
    Object.preventExtensions(newObject); // NOT extendable

try {
Object.defineProperty(newObject, ‘newProperty’, { // Adding new property
value: 100
});
} catch (e) {
console.log(e); // TypeError: Cannot define property newProperty, object is not extensible
}

  1. What are the different ways to make an object non-extensible
    You can mark an object non-extensible in 3 ways,
    i. Object.preventExtensions
    ii. Object.seal
    iii. Object.freeze
    var newObject = {};

Object.preventExtensions(newObject); // Prevent objects are non-extensible
Object.isExtensible(newObject); // false

var sealedObject = Object.seal({}); // Sealed objects are non-extensible
Object.isExtensible(sealedObject); // false

var frozenObject = Object.freeze({}); // Frozen objects are non-extensible
Object.isExtensible(frozenObject); // false

  1. How do you define multiple properties on an object
    The Object.defineProperties() method is used to define new or modify existing properties directly on an object and returning the object. Let’s define multiple properties on an empty object,
    const newObject = {};

Object.defineProperties(newObject, {
newProperty1: {
value: ‘John’,
writable: true
},
newProperty2: {}
});

  1. What is MEAN in javascript
    The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open-source JavaScript software tech stack available for building dynamic web apps where you can write both the server-side and client-side halves of the web project entirely in JavaScript.
  2. What Is Obfuscation in javascript
    Obfuscation is the deliberate act of creating obfuscated javascript code(i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it. Let’s see the below function before Obfuscation,
    function greeting() {
    console.log(‘Hello, welcome to JS world’);
    }
    And after the code Obfuscation, it would be appeared as below,
    eval(function(p,a,c,k,e,d){e=function(c){return c};if(!”.replace(/^/,String)){while(c–){d[c]=k[c]||c}k=[function(e){return d[e]}];e=function(){return’\w+’};c=1};while(c–){if(k[c]){p=p.replace(new RegExp(‘\b’+e(c)+’\b’,’g’),k[c])}}return p}(‘2 1(){0.3(\’4, 7 6 5 8\’)}’,9,9,’console|greeting|function|log|Hello|JS|to|welcome|world’.split(‘|’),0,{}))
  3. Why do you need Obfuscation
    Below are the few reasons for Obfuscation,
    i. The Code size will be reduced. So data transfers between server and client will be fast.
    ii. It hides the business logic from outside world and protects the code from others
    iii. Reverse engineering is highly difficult
    iv. The download time will be reduced
  4. What is Minification
    Minification is the process of removing all unnecessary characters(empty spaces are removed) and variables will be renamed without changing it’s functionality. It is also a type of obfuscation .
  5. What are the advantages of minification
    Normally it is recommended to use minification for heavy traffic and intensive requirements of resources. It reduces file sizes with below benefits,
    i. Decreases loading times of a web page
    ii. Saves bandwidth usages
  6. What are the differences between Obfuscation and Encryption
    Below are the main differences between Obfuscation and Encryption,
    Feature Obfuscation Encryption
    Definition Changing the form of any data in any other form Changing the form of information to an unreadable format by using a key
    A key to decode It can be decoded without any key It is required
    Target data format It will be converted to a complex form Converted into an unreadable format
  7. What are the common tools used for minification
    There are many online/offline tools to minify the javascript files,
    i. Google’s Closure Compiler
    ii. UglifyJS2
    iii. jsmin
    iv. javascript-minifier.com/
    v. prettydiff.com
  8. How do you perform form validation using javascript
    JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted. Lets’ perform user login in an html form,

User name:

And the validation on user login is below,
function validateForm() {
var x = document.forms[“myForm”][“uname”].value;
if (x == “”) {
alert(“The username shouldn’t be empty”);
return false;
}
}

  1. How do you perform form validation without javascript
    You can perform HTML form validation automatically without using javascript. The validation enabled by applying the required attribute to prevent form submission when the input is empty.

Note: Automatic form validation does not work in Internet Explorer 9 or earlier.

  1. What are the DOM methods available for constraint validation
    The below DOM methods are available for constraint validation on an invalid input,
    i. checkValidity(): It returns true if an input element contains valid data.
    ii. setCustomValidity(): It is used to set the validationMessage property of an input element. Let’s take an user login form with DOM validations
    function myFunction() {
    var userName = document.getElementById(“uname”);
    if (!userName.checkValidity()) {
    document.getElementById(“message”).innerHTML = userName.validationMessage;
    } else {
    document.getElementById(“message”).innerHTML = “Entered a valid username”;
    }
    }
  2. What are the available constraint validation DOM properties
    Below are the list of some of the constraint validation DOM properties available,
    i. validity: It provides a list of boolean properties related to the validity of an input element.
    ii. validationMessage: It displays the message when the validity is false.
    iii. willValidate: It indicates if an input element will be validated or not.
  3. What are the list of validity properties
    The validity property of an input element provides a set of properties related to the validity of data.
    i. customError: It returns true, if a custom validity message is set.
    ii. patternMismatch: It returns true, if an element’s value does not match its pattern attribute.
    iii. rangeOverflow: It returns true, if an element’s value is greater than its max attribute.
    iv. rangeUnderflow: It returns true, if an element’s value is less than its min attribute.
    v. stepMismatch: It returns true, if an element’s value is invalid according to step attribute.
    vi. tooLong: It returns true, if an element’s value exceeds its maxLength attribute.
    vii. typeMismatch: It returns true, if an element’s value is invalid according to type attribute.
    viii. valueMissing: It returns true, if an element with a required attribute has no value.
    ix. valid: It returns true, if an element’s value is valid.
  4. Give an example usage of rangeOverflow property
    If an element’s value is greater than its max attribute then rangeOverflow property returns true. For example, the below form submission throws an error if the value is more than 100,

    OK
    function myOverflowFunction() {
    if (document.getElementById(“age”).validity.rangeOverflow) {
    alert(“The mentioned age is not allowed”);
    }
    }
  5. Is enums feature available in javascript
    No, javascript does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object,
    var DaysEnum = Object.freeze({“monday”:1, “tuesday”:2, “wednesday”:3, …})
  6. What is an enum
    An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but typescript provides built-in enum support.
    enum Color {
    RED, GREEN, BLUE
    }
  7. How do you list all properties of an object
    You can use the Object.getOwnPropertyNames() method which returns an array of all properties found directly in a given object. Let’s the usage of it in an example,
    const newObject = {
    a: 1,
    b: 2,
    c: 3
    };

console.log(Object.getOwnPropertyNames(newObject)); [“a”, “b”, “c”]

  1. How do you get property descriptors of an object
    You can use the Object.getOwnPropertyDescriptors() method which returns all own property descriptors of a given object. The example usage of this method is below,
    const newObject = {
    a: 1,
    b: 2,
    c: 3
    };
    const descriptorsObject = Object.getOwnPropertyDescriptors(newObject);
    console.log(descriptorsObject.a.writable); //true
    console.log(descriptorsObject.a.configurable); //true
    console.log(descriptorsObject.a.enumerable); //true
    console.log(descriptorsObject.a.value); // 1
  2. What are the attributes provided by a property descriptor
    A property descriptor is a record which has the following attributes
    i. value: The value associated with the property
    ii. writable: Determines whether the value associated with the property can be changed or not
    iii. configurable: Returns true if the type of this property descriptor can be changed and if the property can be deleted from the corresponding object.
    iv. enumerable: Determines whether the property appears during enumeration of the properties on the corresponding object or not.
    v. set: A function which serves as a setter for the property
    vi. get: A function which serves as a getter for the property
  3. How do you extend classes
    The extends keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,
    class ChildClass extends ParentClass { … }
    Let’s take an example of Square subclass from Polygon parent class,
    class Square extends Rectangle {
    constructor(length) {
    super(length, length);
    this.name = ‘Square’;
    } get area() {
    return this.width * this.height;
    } set area(value) {
    this.area = value;
    }
    }
  4. How do I modify the url without reloading the page
    The window.location.url property will be helpful to modify the url but it reloads the page. HTML5 introduced the history.pushState() and history.replaceState() methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below,
    window.history.pushState(‘page2’, ‘Title’, ‘/page2.html’);
  5. How do you check whether an array includes a particular value or not
    The Array#includes() method is used to determine whether an array includes a particular value among its entries by returning either true or false. Let’s see an example to find an element(numeric and string) within an array.
    var numericArray = [1, 2, 3, 4];
    console.log(numericArray.includes(3)); // true

var stringArray = [‘green’, ‘yellow’, ‘blue’];
console.log(stringArray.includes(‘blue’)); //true

  1. How do you compare scalar arrays
    You can use length and every method of arrays to compare two scalar(compared directly using ===) arrays. The combination of these expressions can give the expected result,
    const arrayFirst = [1,2,3,4,5];
    const arraySecond = [1,2,3,4,5];
    console.log(arrayFirst.length === arraySecond.length && arrayFirst.every((value, index) => value === arraySecond[index])); // true
    If you would like to compare arrays irrespective of order then you should sort them before,
    const arrayFirst = [2,3,1,4,5];
    const arraySecond = [1,2,3,4,5];
    console.log(arrayFirst.length === arraySecond.length && arrayFirst.sort().every((value, index) => value === arraySecond[index])); //true
  2. How to get the value from get parameters
    The new URL() object accepts the url string and searchParams property of this object can be used to access the get parameters. Remember that you may need to use polyfill or window.location to access the URL in older browsers(including IE).
    let urlString = “http://www.some-domain.com/about.html?x=1&y=2&z=3”; //window.location.href
    let url = new URL(urlString);
    let parameterZ = url.searchParams.get(“z”);
    console.log(parameterZ); // 3
  3. How do you print numbers with commas as thousand separators
    You can use the Number.prototype.toLocaleString() method which returns a string with a language-sensitive representation such as thousand separator,currency etc of this number.
    function convertToThousandFormat(x){
    return x.toLocaleString(); // 12,345.679
    }

console.log(convertToThousandFormat(12345.6789));

  1. What is the difference between java and javascript
    Both are totally unrelated programming languages and no relation between them. Java is statically typed, compiled, runs on its own VM. Whereas Javascript is dynamically typed, interpreted, and runs in a browser and nodejs environments. Let’s see the major differences in a tabular format,
    Feature Java JavaScript
    Typed It’s a strongly typed language It’s a dynamic typed language
    Paradigm Object oriented programming Prototype based programming
    Scoping Block scoped Function-scoped
    Concurrency Thread based event based
    Memory Uses more memory Uses less memory. Hence it will be used for web pages
  2. Is javascript supports namespace
    JavaScript doesn’t support namespace by default. So if you create any element(function, method, object, variable) then it becomes global and pollutes the global namespace. Let’s take an example of defining two functions without any namespace,
    function func1() {
    console.log(“This is a first definition”);

}
function func1() {
console.log(“This is a second definition”);
}
func1(); // This is a second definition
It always calls the second function definition. In this case, namespace will solve the name collision problem.

  1. How do you declare namespace
    Even though JavaScript lacks namespaces, we can use Objects , IIFE to create namespaces.
    i. Using Object Literal Notation: Let’s wrap variables and functions inside an Object literal which acts as a namespace. After that you can access them using object notation
    var namespaceOne = {
    function func1() {
    console.log(“This is a first definition”);
    }
    }
    var namespaceTwo = {
    function func1() {
    console.log(“This is a second definition”);
    }
    }
    namespaceOne.func1(); // This is a first definition
    namespaceTwo.func1(); // This is a second definition
    ii. Using IIFE (Immediately invoked function expression): The outer pair of parentheses of IIFE creates a local scope for all the code inside of it and makes the anonymous function a function expression. Due to that, you can create the same function in two different function expressions to act as a namespace.
    (function() {
    function fun1(){
    console.log(“This is a first definition”);
    } fun1();
    }());

(function() {
function fun1(){
console.log(“This is a second definition”);
} fun1();
}());
iii. Using a block and a let/const declaration: In ECMAScript 6, you can simply use a block and a let declaration to restrict the scope of a variable to a block.
{
let myFunction= function fun1(){
console.log(“This is a first definition”);
}
myFunction();
}
//myFunction(): ReferenceError: myFunction is not defined.

{
let myFunction= function fun1(){
console.log(“This is a second definition”);
}
myFunction();
}
//myFunction(): ReferenceError: myFunction is not defined.

  1. How do you invoke javascript code in an iframe from parent page
    Initially iFrame needs to be accessed using either document.getElementBy or window.frames. After that contentWindow property of iFrame gives the access for targetFunction
    document.getElementById(‘targetFrame’).contentWindow.targetFunction();
    window.frames[0].frameElement.contentWindow.targetFunction(); // Accessing iframe this way may not work in latest versions chrome and firefox
  2. How do get the timezone offset from date
    You can use the getTimezoneOffset method of the date object. This method returns the time zone difference, in minutes, from current locale (host system settings) to UTC
    var offset = new Date().getTimezoneOffset();
    console.log(offset); // -480
  3. How do you load CSS and JS files dynamically
    You can create both link and script elements in the DOM and append them as child to head tag. Let’s create a function to add script and style resources as below,
    function loadAssets(filename, filetype) {
    if (filetype == “css”) { // External CSS file
    var fileReference = document.createElement(“link”)
    fileReference.setAttribute(“rel”, “stylesheet”);
    fileReference.setAttribute(“type”, “text/css”);
    fileReference.setAttribute(“href”, filename);
    } else if (filetype == “js”) { // External JavaScript file
    var fileReference = document.createElement(‘script’);
    fileReference.setAttribute(“type”, “text/javascript”);
    fileReference.setAttribute(“src”, filename);
    }
    if (typeof fileReference != “undefined”)
    document.getElementsByTagName(“head”)[0].appendChild(fileReference)
    }
  4. What are the different methods to find HTML elements in DOM
    If you want to access any element in an HTML page, you need to start with accessing the document object. Later you can use any of the below methods to find the HTML element,
    i. document.getElementById(id): It finds an element by Id
    ii. document.getElementsByTagName(name): It finds an element by tag name
    iii. document.getElementsByClassName(name): It finds an element by class name
  5. What is jQuery
    jQuery is a popular cross-browser JavaScript library that provides Document Object Model (DOM) traversal, event handling, animations and AJAX interactions by minimizing the discrepancies across browsers. It is widely famous with its philosophy of “Write less, do more”. For example, you can display welcome message on the page load using jQuery as below,
    $(document).ready(function(){ // It selects the document and apply the function on page load
    alert(‘Welcome to jQuery world’);
    });
    Note: You can download it from jquery’s official site or install it from CDNs, like google.
  6. What is V8 JavaScript engine
    V8 is an open source high-performance JavaScript engine used by the Google Chrome browser, written in C++. It is also being used in the node.js project. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors. Note: It can run standalone, or can be embedded into any C++ application.
  7. Why do we call javascript as dynamic language
    JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.
    let age = 50; // age is a number now
    age = ‘old’; // age is a string now
    age = true; // age is a boolean
  8. What is a void operator
    The void operator evaluates the given expression and then returns undefined(i.e, without returning value). The syntax would be as below,
    void (expression)
    void expression
    Let’s display a message without any redirections or reloads
    Click here to see a message
    Note: This operator is often used to obtain the undefined primitive value, using “void(0)”.
  9. How to set the cursor to wait
    The cursor can be set to wait in JavaScript by using the property “cursor”. Let’s perform this behavior on page load using the below function.
    function myFunction() {
    window.document.body.style.cursor = “wait”;
    }
    and this function invoked on page load
  10. How do you create an infinite loop
    You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools,
    for (;;) {}
    while(true) {
    }
  11. Why do you need to avoid with statement
    JavaScript’s with statement was intended to provide a shorthand for writing recurring accesses to objects. So it can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. Let’s take an example where it is used to avoid redundancy when accessing an object several times.
    a.b.c.greeting = ‘welcome’;
    a.b.c.age = 32;
    Using with it turns this into:
    with(a.b.c) {
    greeting = “welcome”;
    age = 32;
    }
    But this with statement creates performance problems since one cannot predict whether an argument will refer to a real variable or to a property inside the with argument.
  12. What is the output of below for loops
  13. for (var i = 0; i < 4; i++) { // global scope
  14. setTimeout(() => console.log(i));
  15. }
    1. for (let i = 0; i < 4; i++) { // block scope
  16. setTimeout(() => console.log(i));
    }
    The output of the above for loops is 4 4 4 4 and 0 1 2 3 Explanation: Due to the event queue/loop of javascript, the setTimeout callback function is called after the loop has been executed. Since the variable i is declared with the var keyword it became a global variable and the value was equal to 4 using iteration when the time setTimeout function is invoked. Hence, the output of the first loop is 4 4 4 4. Whereas in the second loop, the variable i is declared as the let keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is 0 1 2 3.
  17. List down some of the features of ES6
    Below are the list of some new features of ES6,
    i. Support for constants or immutable variables
    ii. Block-scope support for variables, constants and functions
    iii. Arrow functions
    iv. Default parameters
    v. Rest and Spread Parameters
    vi. Template Literals
    vii. Multi-line Strings
    viii. Destructuring Assignment
    ix. Enhanced Object Literals
    x. Promises
    xi. Classes
    xii. Modules
  18. What is ES6
    ES6 is the sixth edition of the javascript language and it was released in June 2015. It was initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015. Almost all the modern browsers support ES6 but for the old browsers there are many transpilers, like Babel.js etc.
  19. Can I redeclare let and const variables
    No, you cannot redeclare let and const variables. If you do, it throws below error
    Uncaught SyntaxError: Identifier ‘someVariable’ has already been declared
    Explanation: The variable declaration with var keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let’s take an example of re-declaring variables in the same scope for both var and let/const variables.
    var name = ‘John’;
    function myFunc() {
    var name = ‘Nick’;
    var name = ‘Abraham’; // Re-assigned in the same function block
    alert(name); // Abraham
    }
    myFunc();
    alert(name); // John
    The block-scoped multi-declaration throws syntax error,
    let name = ‘John’;
    function myFunc() {
    let name = ‘Nick’;
    let name = ‘Abraham’; // Uncaught SyntaxError: Identifier ‘name’ has already been declared
    alert(name);
    }

myFunc();
alert(name);

  1. Is const variable makes the value immutable
    No, the const variable doesn’t make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can’t assign another value later)
    const userList = [];
    userList.push(‘John’); // Can mutate even though it can’t re-assign
    console.log(userList); // [‘John’]
  2. What are default parameters
    In E5, we need to depend on logical OR operators to handle default values of function parameters. Whereas in ES6, Default function parameters feature allows parameters to be initialized with default values if no value or undefined is passed. Let’s compare the behavior with an examples,
    //ES5
    var calculateArea = function(height, width) {
    height = height || 50;
    width = width || 60; return width * height;
    }
    console.log(calculateArea()); //300
    The default parameters makes the initialization more simpler,
    //ES6
    var calculateArea = function(height = 50, width = 60) {
    return width * height;
    }

console.log(calculateArea()); //300

  1. What are template literals
    Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick () character instead of double or single quotes. In E6, this feature enables using dynamic expressions as below, var greeting =Welcome to JS World, Mr. ${firstName} ${lastName}. In ES5, you need break string like below, var greeting = 'Welcome to JS World, Mr. ' + firstName + ' ' + lastName.
    Note: You can use multi-line strings and string interpolation features with template literals.
  2. How do you write multi-line strings in template literals
    In ES5, you would have to use newline escape characters(‘\n’) and concatenation symbols(+) in order to get multi-line strings.
    console.log(‘This is string sentence 1\n’ +
    ‘This is string sentence 2’);
    Whereas in ES6, You don’t need to mention any newline sequence character,
    console.log(This is string sentence 'This is string sentence 2);
  3. What are nesting templates
    The nesting template is a feature supported within template literals syntax to allow inner backticks inside a placeholder ${ } within the template. For example, the below nesting template is used to display the icons based on user permissions whereas outer template checks for platform type,
    const iconStyles = icon ${ isMobilePlatform() ? '' : icon-${user.isAuthorized ? ‘submit’ : ‘disabled’}};
    You can write the above use case without nesting template features as well. However, the nesting template feature is more compact and readable.
    //Without nesting templates
    const iconStyles = icon ${ isMobilePlatform() ? '' : (user.isAuthorized ? 'icon-submit' : 'icon-disabled'};
  4. What are tagged templates
    Tagged templates are the advanced form of templates in which tags allow you to parse template literals with a function. The tag function accepts the first parameter as an array of strings and remaining parameters as expressions. This function can also return manipulated strings based on parameters. Let’s see the usage of this tagged template behavior of an IT professional skill set in an organization,
    var user1 = ‘John’;
    var skill1 = ‘JavaScript’;
    var experience1 = 15;

var user2 = ‘Kane’;
var skill2 = ‘JavaScript’;
var experience2 = 5;

function myInfoTag(strings, userExp, experienceExp, skillExp) {
var str0 = strings[0]; // “Mr/Ms. “
var str1 = strings[1]; // ” is a/an “
var str2 = strings[2]; // “in”

var expertiseStr;
if (experienceExp > 10){
expertiseStr = ‘expert developer’;
} else if(skillExp > 5 && skillExp <= 10) {
expertiseStr = ‘senior developer’;
} else {
expertiseStr = ‘junior developer’;
}

return ${str0}${userExp}${str1}${expertiseStr}${str2}${skillExp};
}

var output1 = myInfoTagMr/Ms. ${ user1 } is a/an ${ experience1 } in ${skill1};
var output2 = myInfoTagMr/Ms. ${ user2 } is a/an ${ experience2 } in ${skill2};

console.log(output1);// Mr/Ms. John is a/an expert developer in JavaScript
console.log(output2);// Mr/Ms. Kane is a/an junior developer in JavaScript

  1. What are raw strings
    ES6 provides a raw strings feature using the String.raw() method which is used to get the raw string form of template strings. This feature allows you to access the raw strings as they were entered, without processing escape sequences. For example, the usage would be as below,
    var calculationString = String.raw The sum of numbers is \n${1+2+3+4}!;
    console.log(calculationString); // The sum of numbers is 10
    If you don’t use raw strings, the newline character sequence will be processed by displaying the output in multiple lines
    var calculationString = The sum of numbers is \n${1+2+3+4}!;
    console.log(calculationString);
    // The sum of numbers is
    // 10
    Also, the raw property is available on the first argument to the tag function
    function tag(strings) {
    console.log(strings.raw[0]);
    }
  2. What is destructuring assignment
    The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let’s get the month values from an array using destructuring assignment
    var [one, two, three] = [‘JAN’, ‘FEB’, ‘MARCH’];

console.log(one); // “JAN”
console.log(two); // “FEB”
console.log(three); // “MARCH”
and you can get user properties of an object using destructuring assignment,
var {name, age} = {name: ‘John’, age: 32};

console.log(name); // John
console.log(age); // 32

  1. What are default values in destructuring assignment
    A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let’s take an example for both arrays and object use cases,
    Arrays destructuring:
    var x, y, z;

[x=2, y=4, z=6] = [10];
console.log(x); // 10
console.log(y); // 4
console.log(z); // 6
Objects destructuring:
var {x=2, y=4, z=6} = {x: 10};

console.log(x); // 10
console.log(y); // 4
console.log(z); // 6

  1. How do you swap variables in destructuring assignment
    If you don’t use destructuring assignment, swapping two values requires a temporary variable. Whereas using a destructuring feature, two variable values can be swapped in one destructuring expression. Let’s swap two number variables in array destructuring assignment,
    var x = 10, y = 20;

[x, y] = [y, x];
console.log(x); // 20
console.log(y); // 10

  1. What are enhanced object literals
    Object literals make it easy to quickly create objects with properties inside the curly braces. For example, it provides shorter syntax for common object property definition as below.
    //ES6
    var x = 10, y = 20
    obj = { x, y }
    console.log(obj); // {x: 10, y:20}
    //ES5
    var x = 10, y = 20
    obj = { x : x, y : y}
    console.log(obj); // {x: 10, y:20}
  2. What are dynamic imports
    The dynamic imports using import() function syntax allows us to load modules on demand by using promises or the async/await syntax. Currently this feature is in stage4 proposal. The main advantage of dynamic imports is reduction of our bundle’s sizes, the size/payload response of our requests and overall improvements in the user experience. The syntax of dynamic imports would be as below,
    import(‘./Module’).then(Module => Module.method());
  3. What are the use cases for dynamic imports
    Below are some of the use cases of using dynamic imports over static imports,
    i. Import a module on-demand or conditionally. For example, if you want to load a polyfill on legacy browser
    if (isLegacyBrowser()) {
    import(···)
    .then(···);
    }
    ii. Compute the module specifier at runtime. For example, you can use it for internationalization.
    import(messages_${getLocale()}.js).then(···);
    iii. Import a module from within a regular script instead a module.
  4. What are typed arrays
    Typed arrays are array-like objects from ECMAScript 6 API for handling binary data. JavaScript provides 8 Typed array types,
    i. Int8Array: An array of 8-bit signed integers
    ii. Int16Array: An array of 16-bit signed integers
    iii. Int32Array: An array of 32-bit signed integers
    iv. Uint8Array: An array of 8-bit unsigned integers
    v. Uint16Array: An array of 16-bit unsigned integers
    vi. Uint32Array: An array of 32-bit unsigned integers
    vii. Float32Array: An array of 32-bit floating point numbers
    viii. Float64Array: An array of 64-bit floating point numbers
    For example, you can create an array of 8-bit signed integers as below
    const a = new Int8Array();
    // You can pre-allocate n bytes
    const bytes = 1024
    const a = new Int8Array(bytes)
  5. What are the advantages of module loaders
    The module loaders provides the below features,
    i. Dynamic loading
    ii. State isolation
    iii. Global namespace isolation
    iv. Compilation hooks
    v. Nested virtualization
  6. What is collation
    Collation is used for sorting a set of strings and searching within a set of strings. It is parameterized by locale and aware of Unicode. Let’s take comparison and sorting features,
    i. Comparison:
    var list = [ “ä”, “a”, “z” ]; // In German, “ä” sorts with “a” Whereas in Swedish, “ä” sorts after “z”
    var l10nDE = new Intl.Collator(“de”);
    var l10nSV = new Intl.Collator(“sv”);
    console.log(l10nDE.compare(“ä”, “z”) === -1); // true
    console.log(l10nSV.compare(“ä”, “z”) === +1); // true
    ii. Sorting:
    var list = [ “ä”, “a”, “z” ]; // In German, “ä” sorts with “a” Whereas in Swedish, “ä” sorts after “z”
    var l10nDE = new Intl.Collator(“de”);
    var l10nSV = new Intl.Collator(“sv”);
    console.log(list.sort(l10nDE.compare)) // [ “a”, “ä”, “z” ]
    console.log(list.sort(l10nSV.compare)) // [ “a”, “z”, “ä” ]
  7. What is for…of statement
    The for…of statement creates a loop iterating over iterable objects or elements such as built-in String, Array, Array-like objects (like arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. The basic usage of for…of statement on arrays would be as below,
    let arrayIterable = [10, 20, 30, 40, 50];

for (let value of arrayIterable) {
value ++;
console.log(value); // 11 21 31 41 51
}

  1. What is the output of below spread operator array
    […’John Resig’]
    The output of the array is [‘J’, ‘o’, ‘h’, ‘n’, ”, ‘R’, ‘e’, ‘s’, ‘i’, ‘g’] Explanation: The string is an iterable type and the spread operator within an array maps every character of an iterable to one element. Hence, each character of a string becomes an element within an Array.
  2. Is PostMessage secure
    Yes, postMessages can be considered very secure as long as the programmer/developer is careful about checking the origin and source of an arriving message. But if you try to send/receive a message without verifying its source will create cross-site scripting attacks.
  3. What are the problems with postmessage target origin as wildcard
    The second argument of postMessage method specifies which origin is allowed to receive the message. If you use the wildcard “” as an argument then any origin is allowed to receive the message. In this case, there is no way for the sender window to know if the target window is at the target origin when sending the message. If the target window has been navigated to another origin, the other origin would receive the data. Hence, this may lead to XSS vulnerabilities. targetWindow.postMessage(message, ‘‘);
  4. How do you avoid receiving postMessages from attackers
    Since the listener listens for any message, an attacker can trick the application by sending a message from the attacker’s origin, which gives an impression that the receiver received the message from the actual sender’s window. You can avoid this issue by validating the origin of the message on the receiver’s end using the “message.origin” attribute. For examples, let’s check the sender’s origin http://www.some-sender.com on receiver side www.some-receiver.com,
    //Listener on http://www.some-receiver.com/
    window.addEventListener(“message”, function(message){
    if(/^http://www.some-sender.com$/.test(message.origin)){
    console.log(‘You received the data from valid sender’, message.data);
    }
    });
  5. Can I avoid using postMessages completely
    You cannot avoid using postMessages completely(or 100%). Even though your application doesn’t use postMessage considering the risks, a lot of third party scripts use postMessage to communicate with the third party service. So your application might be using postMessage without your knowledge.
  6. Is postMessages synchronous
    The postMessages are synchronous in IE8 browser but they are asynchronous in IE9 and all other modern browsers (i.e, IE9+, Firefox, Chrome, Safari).Due to this asynchronous behaviour, we use a callback mechanism when the postMessage is returned.
  7. What paradigm is Javascript
    JavaScript is a multi-paradigm language, supporting imperative/procedural programming, Object-Oriented Programming and functional programming. JavaScript supports Object-Oriented Programming with prototypical inheritance.
  8. What is the difference between internal and external javascript
    Internal JavaScript: It is the source code within the script tag. External JavaScript: The source code is stored in an external file(stored with .js extension) and referred with in the tag.
  9. Is JavaScript faster than server side script
    Yes, JavaScript is faster than server side script. Because JavaScript is a client-side script it does not require any web server’s help for its computation or calculation. So JavaScript is always faster than any server-side script like ASP, PHP, etc.
  10. How do you get the status of a checkbox
    You can apply the checked property on the selected checkbox in the DOM. If the value is True means the checkbox is checked otherwise it is unchecked. For example, the below HTML checkbox element can be access using javascript as below,
    Agree the conditions

    console.log(document.getElementById(‘checkboxname’).checked); // true or false
  11. What is the purpose of double tilde operator
    The double tilde operator(~~) is known as double NOT bitwise operator. This operator is going to be a quicker substitute for Math.floor().
  12. How do you convert character to ASCII code
    You can use the String.prototype.charCodeAt() method to convert string characters to ASCII numbers. For example, let’s find ASCII code for the first letter of ‘ABC’ string,
    “ABC”.charCodeAt(0) // returns 65
    Whereas String.fromCharCode() method converts numbers to equal ASCII characters.
    String.fromCharCode(65,66,67); // returns ‘ABC’
  13. What is ArrayBuffer
    An ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. You can create it as below,
    let buffer = new ArrayBuffer(16); // create a buffer of length 16
    alert(buffer.byteLength); // 16
    To manipulate an ArrayBuffer, we need to use a “view” object.
    //Create a DataView referring to the buffer
    let view = new DataView(buffer);
  14. What is the output of below string expression
    console.log(“Welcome to JS world”[0])
    The output of the above expression is “W”. Explanation: The bracket notation with specific index on a string returns the character at a specific location. Hence, it returns the character “W” of the string. Since this is not supported in IE7 and below versions, you may need to use the .charAt() method to get the desired result.
  15. What is the purpose of Error object
    The Error constructor creates an error object and the instances of error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions. The syntax of error object would be as below,
    new Error([message[, fileName[, lineNumber]]])
    You can throw user defined exceptions or errors using Error object in try…catch block as below,
    try {
    if(withdraw > balance)
    throw new Error(“Oops! You don’t have enough balance”);
    } catch (e) {
    console.log(e.name + ‘: ‘ + e.message);
    }
  16. What is the purpose of EvalError object
    The EvalError object indicates an error regarding the global eval() function. Even though this exception is not thrown by JavaScript anymore, the EvalError object remains for compatibility. The syntax of this expression would be as below,
    new EvalError([message[, fileName[, lineNumber]]])
    You can throw EvalError with in try…catch block as below,
    try {
    throw new EvalError(‘Eval function error’, ‘someFile.js’, 100);
    } catch (e) {
    console.log(e.message, e.name, e.fileName); // “Eval function error”, “EvalError”, “someFile.js”
  17. What are the list of cases error thrown from non-strict mode to strict mode
    When you apply ‘use strict’; syntax, some of the below cases will throw a SyntaxError before executing the script
    i. When you use Octal syntax
    var n = 022;
    ii. Using with statement
    iii. When you use delete operator on a variable name
    iv. Using eval or arguments as variable or function argument name
    v. When you use newly reserved keywords
    vi. When you declare a function in a block
    if (someCondition) { function f() {} }
    Hence, the errors from above cases are helpful to avoid errors in development/production environments.
  18. Is all objects have prototypes
    No. All objects have prototypes except for the base object which is created by the user, or an object that is created using the new keyword.
  19. What is the difference between a parameter and an argument
    Parameter is the variable name of a function definition whereas an argument represents the value given to a function when it is invoked. Let’s explain this with a simple function
    function myFunction(parameter1, parameter2, parameter3) {
    console.log(arguments[0]) // “argument1”
    console.log(arguments[1]) // “argument2”
    console.log(arguments[2]) // “argument3”
    }
    myFunction(“argument1”, “argument2”, “argument3”)
  20. What is the purpose of some method in arrays
    The some() method is used to test whether at least one element in the array passes the test implemented by the provided function. The method returns a boolean value. Let’s take an example to test for any odd elements,
    var array = [1, 2, 3, 4, 5, 6 ,7, 8, 9, 10];

var odd = element ==> element % 2 !== 0;

console.log(array.some(odd)); // true (the odd element exists)

  1. How do you combine two or more arrays
    The concat() method is used to join two or more arrays by returning a new array containing all the elements. The syntax would be as below,
    array1.concat(array2, array3, …, arrayX)
    Let’s take an example of array’s concatenation with veggies and fruits arrays,
    var veggies = [“Tomato”, “Carrot”, “Cabbage”];
    var fruits = [“Apple”, “Orange”, “Pears”];
    var veggiesAndFruits = veggies.concat(fruits);
    console.log(veggiesAndFruits); // Tomato, Carrot, Cabbage, Apple, Orange, Pears
  2. What is the difference between Shallow and Deep copy
    There are two ways to copy an object,
    Shallow Copy: Shallow copy is a bitwise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.
    Example
    var empDetails = {
    name: “John”, age: 25, expertise: “Software Developer”
    }
    to create a duplicate
    var empDetailsShallowCopy = empDetails //Shallow copying!
    if we change some property value in the duplicate one like this:
    empDetailsShallowCopy.name = “Johnson”
    The above statement will also change the name of empDetails, since we have a shallow copy. That means we’re losing the original data as well.
    Deep copy: A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.
    Example
    var empDetails = {
    name: “John”, age: 25, expertise: “Software Developer”
    }
    Create a deep copy by using the properties from the original object into new variable
    var empDetailsDeepCopy = {
    name: empDetails.name,
    age: empDetails.age,
    expertise: empDetails.expertise
    }
    Now if you change empDetailsDeepCopy.name, it will only affect empDetailsDeepCopy & not empDetails
  3. How do you create specific number of copies of a string
    The repeat() method is used to construct and return a new string which contains the specified number of copies of the string on which it was called, concatenated together. Remember that this method has been added to the ECMAScript 2015 specification. Let’s take an example of Hello string to repeat it 4 times,
    ‘Hello’.repeat(4); // ‘HelloHelloHelloHello’
  4. How do you return all matching strings against a regular expression
    The matchAll() method can be used to return an iterator of all results matching a string against a regular expression. For example, the below example returns an array of matching string results against a regular expression,
    let regexp = /Hello(\d?))/g;
    let greeting = ‘Hello1Hello2Hello3’;

let greetingList = […greeting.matchAll(regexp)];

console.log(greetingList[0]); //Hello1
console.log(greetingList[1]); //Hello2
console.log(greetingList[2]); //Hello3

  1. How do you trim a string at the beginning or ending
    The trim method of string prototype is used to trim on both sides of a string. But if you want to trim especially at the beginning or ending of the string then you can use trimStart/trimLeft and trimEnd/trimRight methods. Let’s see an example of these methods on a greeting message,
    var greeting = ‘ Hello, Goodmorning! ‘;

console.log(greeting); // ” Hello, Goodmorning! “
console.log(greeting.trimStart()); // “Hello, Goodmorning! “
console.log(greeting.trimLeft()); // “Hello, Goodmorning! “

console.log(greeting.trimEnd()); // ” Hello, Goodmorning!”
console.log(greeting.trimRight()); // ” Hello, Goodmorning!”

  1. What is the output of below console statement with unary operator
    Let’s take console statement with unary operator as given below,
    console.log(+ ‘Hello’);
    The output of the above console log statement returns NaN. Because the element is prefixed by the unary operator and the JavaScript interpreter will try to convert that element into a number type. Since the conversion fails, the value of the statement results in NaN value.
  2. Does javascript uses mixins
  3. What is a thunk function
    A thunk is just a function which delays the evaluation of the value. It doesn’t take any arguments but gives the value whenever you invoke the thunk. i.e, It is used not to execute now but it will be sometime in the future. Let’s take a synchronous example,
    const add = (x,y) => x + y;

const thunk = () => add(2,3);

thunk() // 5

  1. What are asynchronous thunks
    The asynchronous thunks are useful to make network requests. Let’s see an example of network requests,
    function fetchData(fn){
    fetch(‘https://jsonplaceholder.typicode.com/todos/1’)
    .then(response => response.json())
    .then(json => fn(json))
    }

const asyncThunk = function (){
return fetchData(function getData(data){
console.log(data)
})
}

asyncThunk()
The getData function won’t be called immediately but it will be invoked only when the data is available from API endpoint. The setTimeout function is also used to make our code asynchronous. The best real time example is redux state management library which uses the asynchronous thunks to delay the actions to dispatch.

  1. What is the output of below function calls
    Code snippet:
    const circle = {
    radius: 20,
    diameter() {
    return this.radius * 2;
    },
    perimeter: () => 2 * Math.PI * this.radius
    };
    console.log(circle.diameter()); console.log(circle.perimeter());
    Output:
    The output is 40 and NaN. Remember that diameter is a regular function, whereas the value of perimeter is an arrow function. The this keyword of a regular function(i.e, diameter) refers to the surrounding scope which is a class(i.e, Shape object). Whereas this keyword of perimeter function refers to the surrounding scope which is a window object. Since there is no radius property on window objects it returns an undefined value and the multiple of number value returns NaN value.
  2. How to remove all line breaks from a string
    The easiest approach is using regular expressions to detect and replace newlines in the string. In this case, we use replace function along with string to replace with, which in our case is an empty string.
    function remove_linebreaks( var message ) {
    return message.replace( /[\r\n]+/gm, “” );
    }
    In the above expression, g and m are for global and multiline flags.
  3. What is the difference between reflow and repaint
    A repaint occurs when changes are made which affect the visibility of an element, but not its layout. Examples of this include outline, visibility, or background color. A reflow involves changes that affect the layout of a portion of the page (or the whole page). Resizing the browser window, changing the font, content changing (such as user typing text), using JavaScript methods involving computed styles, adding or removing elements from the DOM, and changing an element’s classes are a few of the things that can trigger reflow. Reflow of an element causes the subsequent reflow of all child and ancestor elements as well as any elements following it in the DOM.
  4. What happens with negating an array
    Negating an array with ! character will coerce the array into a boolean. Since Arrays are considered to be truthy So negating it will return false.
    console.log(![]); // false
  5. What happens if we add two arrays
    If you add two arrays together, it will convert them both to strings and concatenate them. For example, the result of adding arrays would be as below,
    console.log([‘a’] + [‘b’]); // “ab”
    console.log([] + []); // “”
    console.log(![] + []); // “false”, because ![] returns false.
  6. What is the output of prepend additive operator on falsy values
    If you prepend the additive(+) operator on falsy values(null, undefined, NaN, false, “”), the falsy value converts to a number value zero. Let’s display them on browser console as below,
    console.log(+null); // 0
    console.log(+undefined);// NaN
    console.log(+false); // 0
    console.log(+NaN); // NaN
    console.log(+””); // 0
  7. How do you create self string using special characters
    The self string can be formed with the combination of []()!+ characters. You need to remember the below conventions to achieve this pattern.
    i. Since Arrays are truthful values, negating the arrays will produce false: ![] === false
    ii. As per JavaScript coercion rules, the addition of arrays together will toString them: [] + [] === “”
    iii. Prepend an array with + operator will convert an array to false, the negation will make it true and finally converting the result will produce value ‘1’: +(!(+[])) === 1
    By applying the above rules, we can derive below conditions
    ![] + [] === “false”
    +!+[] === 1
    Now the character pattern would be created as below,
    s e l f
    ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ (![] + [])[3] + (![] + [])[4] + (![] + [])[2] + (![] + [])[0]
    ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^
    (![] + [])[+!+[]+!+[]+!+[]] +
    (![] + [])[+!+[]+!+[]+!+[]+!+[]] +
    (![] + [])[+!+[]+!+[]] +
    (![] + [])[+[]]
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    (![]+[])[+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]]+(![]+[])[+[]]
  8. How do you remove falsy values from an array
    You can apply the filter method on the array by passing Boolean as a parameter. This way it removes all falsy values(0, undefined, null, false and “”) from the array.
    const myArray = [false, null, 1,5, undefined]
    myArray.filter(Boolean); // [1, 5] // is same as myArray.filter(x => x);
  9. How do you get unique values of an array
    You can get unique values of an array with the combination of Set and rest expression/spread(…) syntax.
    console.log([…new Set([1, 2, 4, 4, 3])]); // [1, 2, 4, 3]
  10. What is destructuring aliases
    Sometimes you would like to have a destructured variable with a different name than the property name. In that case, you’ll use a : newName to specify a name for the variable. This process is called destructuring aliases.
    const obj = { x: 1 };
    // Grabs obj.x as as { otherName }
    const { x: otherName } = obj;
  11. How do you map the array values without using map method
    You can map the array values without using the map method by just using the from method of Array. Let’s map city names from Countries array,
    const countries = [
    { name: ‘India’, capital: ‘Delhi’ },
    { name: ‘US’, capital: ‘Washington’ },
    { name: ‘Russia’, capital: ‘Moscow’ },
    { name: ‘Singapore’, capital: ‘Singapore’ },
    { name: ‘China’, capital: ‘Beijing’ },
    { name: ‘France’, capital: ‘Paris’ },
    ];

const cityNames = Array.from(countries, ({ capital}) => capital);
console.log(cityNames); // [‘Delhi, ‘Washington’, ‘Moscow’, ‘Singapore’, ‘Beijing’, ‘Paris’]

  1. How do you empty an array
    You can empty an array quickly by setting the array length to zero.
    let cities = [‘Singapore’, ‘Delhi’, ‘London’];
    cities.length = 0; // cities becomes []
  2. How do you rounding numbers to certain decimals
    You can round numbers to a certain number of decimals using toFixed method from native javascript.
    let pie = 3.141592653;
    pie = pie.toFixed(3); // 3.142
  3. What is the easiest way to convert an array to an object
    You can convert an array to an object with the same data using spread(…) operator.
    var fruits = [“banana”, “apple”, “orange”, “watermelon”];
    var fruitsObject = {…fruits};
    console.log(fruitsObject); // {0: “banana”, 1: “apple”, 2: “orange”, 3: “watermelon”}
  4. How do you create an array with some data
    You can create an array with some data or an array with the same values using fill method.
    var newArray = new Array(5).fill(“0”);
    console.log(newArray); // [“0”, “0”, “0”, “0”, “0”]
  5. What are the placeholders from console object
    Below are the list of placeholders available from console object,
    i. %o — It takes an object,
    ii. %s — It takes a string,
    iii. %d — It is used for a decimal or integer These placeholders can be represented in the console.log as below
    const user = { “name”:”John”, “id”: 1, “city”: “Delhi”};
    console.log(“Hello %s, your details %o are available in the object form”, “John”, user); // Hello John, your details {name: “John”, id: 1, city: “Delhi”} are available in object
  6. Is it possible to add CSS to console messages
    Yes, you can apply CSS styles to console messages similar to html text on the web page.
    console.log(‘%c The text has blue color, with large font and red background’, ‘color: blue; font-size: x-large; background: red’);
    The text will be displayed as below,
    Note: All CSS styles can be applied to console messages.
  7. What is the purpose of dir method of console object
    The console.dir() is used to display an interactive list of the properties of the specified JavaScript object as JSON.
    const user = { “name”:”John”, “id”: 1, “city”: “Delhi”};
    console.dir(user);
    The user object displayed in JSON representation
  8. Is it possible to debug HTML elements in console
    Yes, it is possible to get and debug HTML elements in the console just like inspecting elements.
    const element = document.getElementsByTagName(“body”)[0];
    console.log(element);
    It prints the HTML element in the console
  9. How do you display data in a tabular format using console object
    The console.table() is used to display data in the console in a tabular format to visualize complex arrays or objects.
    const users = [{ “name”:”John”, “id”: 1, “city”: “Delhi”},
    { “name”:”Max”, “id”: 2, “city”: “London”},
    { “name”:”Rod”, “id”: 3, “city”: “Paris”}];
    console.table(users);
    The data visualized in a table format Not: Remember that console.table() is not supported in IE.
  10. How do you verify that an argument is a Number or not
    The combination of IsNaN and isFinite methods are used to confirm whether an argument is a number or not.
    function isNumber(n){
    return !isNaN(parseFloat(n)) && isFinite(n);
    }
  11. How do you create copy to clipboard button
    You need to select the content(using .select() method) of the input element and execute the copy command with execCommand (i.e, execCommand(‘copy’)). You can also execute other system commands like cut and paste.
    document.querySelector(“#copy-button”).onclick = function() {
    // Select the content
    document.querySelector(“#copy-input”).select();
    // Copy to the clipboard
    document.execCommand(‘copy’);
    };
  12. What is the shortcut to get timestamp
    You can use new Date().getTime() to get the current timestamp. There is an alternative shortcut to get the value.
    console.log(+new Date());
    console.log(Date.now());
  13. How do you flattening multi dimensional arrays
    Flattening bi-dimensional arrays is trivial with Spread operator.
    const biDimensionalArr = [11, [22, 33], [44, 55], [66, 77], 88, 99];
    const flattenArr = [].concat(…biDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
    But you can make it work with multi-dimensional arrays by recursive calls,
    function flattenMultiArray(arr) {
    const flattened = [].concat(…arr);
    return flattened.some(item => Array.isArray(item)) ? flattenMultiArray(flattened) : flattened;
    }
    const multiDimensionalArr = [11, [22, 33], [44, [55, 66, [77, [88]], 99]]];
    const flatArr = flattenMultiArray(multiDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
  14. What is the easiest multi condition checking
    You can use indexOf to compare input with multiple values instead of checking each value as one condition.
    // Verbose approach
    if (input === ‘first’ || input === 1 || input === ‘second’ || input === 2) {
    someFunction();
    }
    // Shortcut
    if ([‘first’, 1, ‘second’, 2].indexOf(input) !== -1) {
    someFunction();
    }
  15. How do you capture browser back button
    The window.onbeforeunload method is used to capture browser back button events. This is helpful to warn users about losing the current data.
    window.onbeforeunload = function() {
    alert(“You work will be lost”);
    };
  16. How do you disable right click in the web page
    The right click on the page can be disabled by returning false from the oncontextmenu attribute on the body element.
  17. What are wrapper objects
    Primitive Values like string,number and boolean don’t have properties and methods but they are temporarily converted or coerced to an object(Wrapper object) when you try to perform actions on them. For example, if you apply toUpperCase() method on a primitive string value, it does not throw an error but returns uppercase of the string.
    let name = “john”;

console.log(name.toUpperCase()); // Behind the scenes treated as console.log(new String(name).toUpperCase());
i.e, Every primitive except null and undefined have Wrapper Objects and the list of wrapper objects are String,Number,Boolean,Symbol and BigInt.

  1. What is AJAX
    AJAX stands for Asynchronous JavaScript and XML and it is a group of related technologies(HTML, CSS, JavaScript, XMLHttpRequest API etc) used to display data asynchronously. i.e. We can send data to the server and get data from the server without reloading the web page.
  2. What are the different ways to deal with Asynchronous Code
    Below are the list of different ways to deal with Asynchronous code.
    i. Callbacks
    ii. Promises
    iii. Async/await
    iv. Third-party libraries such as async.js,bluebird etc
  3. How to cancel a fetch request
    Until a few days back, One shortcoming of native promises is no direct way to cancel a fetch request. But the new AbortController from js specification allows you to use a signal to abort one or multiple fetch calls. The basic flow of cancelling a fetch request would be as below,
    i. Create an AbortController instance
    ii. Get the signal property of an instance and pass the signal as a fetch option for signal
    iii. Call the AbortController’s abort property to cancel all fetches that use that signal For example, let’s pass the same signal to multiple fetch calls will cancel all requests with that signal,
    const controller = new AbortController();
    const { signal } = controller;

fetch(“http://localhost:8000”, { signal }).then(response => {
console.log(Request 1 is complete!);
}).catch(e => {
if(e.name === “AbortError”) {
// We know it’s been canceled!
}
});

fetch(“http://localhost:8000”, { signal }).then(response => {
console.log(Request 2 is complete!);
}).catch(e => {
if(e.name === “AbortError”) {
// We know it’s been canceled!
}
});

// Wait 2 seconds to abort both requests
setTimeout(() => controller.abort(), 2000);

  1. What is web speech API
    Web speech API is used to enable modern browsers recognize and synthesize speech(i.e, voice data into web apps). This API has been introduced by W3C Community in the year 2012. It has two main parts,
    i. SpeechRecognition (Asynchronous Speech Recognition or Speech-to-Text): It provides the ability to recognize voice context from an audio input and respond accordingly. This is accessed by the SpeechRecognition interface. The below example shows on how to use this API to get text from speech,
    window.SpeechRecognition = window.webkitSpeechRecognition || window.SpeechRecognition; // webkitSpeechRecognition for Chrome and SpeechRecognition for FF
    const recognition = new window.SpeechRecognition();
    recognition.onresult = (event) => { // SpeechRecognitionEvent type
    const speechToText = event.results[0][0].transcript;
    console.log(speechToText);
    }
    recognition.start();
    In this API, browser is going to ask you for permission to use your microphone
    ii. SpeechSynthesis (Text-to-Speech): It provides the ability to recognize voice context from an audio input and respond. This is accessed by the SpeechSynthesis interface. For example, the below code is used to get voice/speech from text,
    if(‘speechSynthesis’ in window){
    var speech = new SpeechSynthesisUtterance(‘Hello World!’);
    speech.lang = ‘en-US’;
    window.speechSynthesis.speak(speech);
    }
    The above examples can be tested on chrome(33+) browser’s developer console. Note: This API is still a working draft and only available in Chrome and Firefox browsers(ofcourse Chrome only implemented the specification)
  2. What is minimum timeout throttling
    Both browser and NodeJS javascript environments throttles with a minimum delay that is greater than 0ms. That means even though setting a delay of 0ms will not happen instantaneously. Browsers: They have a minimum delay of 4ms. This throttle occurs when successive calls are triggered due to callback nesting(certain depth) or after a certain number of successive intervals. Note: The older browsers have a minimum delay of 10ms. Nodejs: They have a minimum delay of 1ms. This throttle happens when the delay is larger than 2147483647 or less than 1. The best example to explain this timeout throttling behavior is the order of below code snippet.
    function runMeFirst() {
    console.log(‘My script is initialized’);
    }
    setTimeout(runMeFirst, 0);
    console.log(‘Script loaded’);
    and the output would be in
    Script loaded
    My script is initialized
    If you don’t use setTimeout, the order of logs will be sequential.
    function runMeFirst() {
    console.log(‘My script is initialized’);
    }
    runMeFirst();
    console.log(‘Script loaded’);
    and the output is,
    My script is initialized
    Script loaded
  3. How do you implement zero timeout in modern browsers
    You can’t use setTimeout(fn, 0) to execute the code immediately due to minimum delay of greater than 0ms. But you can use window.postMessage() to achieve this behavior.
  4. What are tasks in event loop
    A task is any javascript code/program which is scheduled to be run by the standard mechanisms such as initially starting to run a program, run an event callback, or an interval or timeout being fired. All these tasks are scheduled on a task queue. Below are the list of use cases to add tasks to the task queue,
    i. When a new javascript program is executed directly from console or running by the

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BBA(CAM), MCA, Professional Coach

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