Angular interview questions and answers

Ques 1. What is Angular Framework?

Angular is a TypeScript-based open-source front-end platform that makes it easy to build web, mobile and desktop applications. The major features of this framework include declarative templates, dependency injection, end to end tooling which ease application development.

Ques 2. 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. For tabular view check here:

http://withoutbook.com/DifferenceBetweenSubjects.php?subId1=79&subId2=115&d=Difference%20between%20AngularJS%20and%20Angular

Ques 3. What is TypeScript?

TypeScript is a strongly typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await and many other features, and compiles to plain JavaScript. Angular is written entirely in TypeScript as a primary language. You can install TypeScript globally as:

npm install -g typescript

Let's see a simple example of TypeScript usage:-

function greeter(person: string) {

  return "Hello, " + person;

}

let user = "Arindam";

document.body.innerHTML = greeter(user);

The greeter method allows only string type as argument.

Ques 4. What are the key components of Angular?

Angular has the key components below,

1. 1. Component: These are the basic building blocks of an Angular application to control HTML views.
   2. Modules: An Angular module is a set of angular basic building blocks like components, 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: These represent the views of an Angular application.
   4. Services: Are 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.

Ques 5. 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>

Ques 6. 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';

}

Ques 7. What are the differences between Component and Directive?

In short, A component(@component) is a directive-with-a-template.

Some of the major differences are mentioned in a tabular form

Ques 8. 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,

Using separate template file such as app.component.html

Ques 9. 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.

Ques 10. 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">

Ques 11. 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

     }

   }

Ques 12. 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,

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

Ques 13. What is the difference between constructor and ngOnInit in Angular?

The Constructor is a default method of the class that is executed when the class is instantiated and ensures proper initialisation of fields in the class and its subclasses. Angular, or better Dependency Injector (DI), analyses the constructor parameters and when it creates a new instance by calling new MyClass() it tries to find providers that match the types of the constructor parameters, resolves them and passes them to the constructor.
ngOnInit is a life cycle hook called by Angular to indicate that Angular is done creating the component.
Mostly we use ngOnInit for all the initialization/declaration and avoid stuff to work in the constructor. The constructor should only be used to initialize class members but shouldn't do actual "work". So you should use constructor() to setup Dependency Injection and not much else. ngOnInit() is better place to "start" - it's where/when components' bindings are resolved.

Ques 14. 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,

The above service uses Http service as a dependency.

Ques 15. 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,

The user variable in the ngFor double-quoted instruction is a template input variable

Ques 16. What is the purpose of ngIf directive in Angular?

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,

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.

Ques 17. What are pipes in Angular?

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.

Ques 18. What are lifecycle hooks available in Angular? What is the lifecycle of Angular?

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.

Ques 19. 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.

Ques 20. 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.

Ques 21. 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,

Ques 22. 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,

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.

Ques 23. What is a parameterized pipe in Angular?

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):

Ques 24. How do you chain pipes in Angular?

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

Ques 25. What are observables in Angular?

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.

Ques 26. What is HttpClient and its benefits in Angular?

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,

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

Ques 27. Explain on how to use HttpClient with an example in Angular?

Below are the steps need to be followed for the usage of HttpClient.
Import HttpClient into root module:

import { HttpClientModule } from '@angular/common/http';
@NgModule({
  imports: [
    BrowserModule,
    // import HttpClientModule after BrowserModule.
    HttpClientModule,
  ],
  ......
  })
 export class AppModule {}

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);
  }
}

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']
    });
}

Ques 28. 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.

Ques 29. 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.

Ques 30. 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,

Ques 31. What are template expressions in Angular?

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}},

The below javascript expressions are prohibited in template expression

1. assignments (=, +=, -=, ...)
2. new
3. chaining expressions with ; or ,
4. increment and decrement operators (++ and --)

Ques 32. 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

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

Ques 33. How do you categorize data binding types in Angular?

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,

Ques 34. What is a custom pipe in Angular?

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}}`

Ques 35. Give an example of custom pipe in Angular?

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,

Now you can use the above pipe in template expression as below,

Ques 36. What is the difference between pure and impure pipe in Angular?

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.

Ques 37. What is RxJS in Angular?

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,

Ques 38. What is subscribing in Angular?

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

Ques 39. What is an observable in Angular?

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);
});

Ques 40. What is an observer in Angular?

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.

Ques 41. What is the difference between promise and observable?

Ques 42. What is multicasting in Angular?

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

Ques 43. How do you perform error handling in observables in Angular?

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)}
});

Ques 44. 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

Ques 45. What are observable creation functions in Angular?

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:

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'); }
});

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));

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}`));

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}`);
  });

Ques 46. 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.

Ques 47. 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.

Ques 48. What are custom elements in Angular?

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.

Ques 49. Do I need to bootstrap custom elements in Angular?

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.

Ques 50. 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".