Angular Interview Questions Part 5

What is the purpose of ng For track By?

The main purpose of using *ng For with track By option is performance optimization. Normally if you use Ng For 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 track By 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 track By to the track By Todos() method

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

and define the track By Todos method,

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

What is the purpose of ng Switch directive?

Ng Switch 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 Ng SwitchNg Switch Case and Ng Switch Default directives.

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

<div [ngSwitch]="">
  <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>

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 property Name:alias.
inputs: ['input1: buyItem'],
outputs: ['outputEvent1: completedEvent']

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>();

Angular Interview Question

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 Test Bed API changes in Angular9?

Angular 9 provides type safe changes in Test Bed API changes by replacing the old get function with the new inject method. Because Test Bed.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

Advance Angular Interview Question

Is mandatory to pass static flag for View Child?

In Angular 8, the static flag is required for View Child. 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).

Angular Interview Question

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 Component Factory Resolver.

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 App Component in your root module using bootstrap property as below.

  declarations: [
  imports: [
  providers: [],
  bootstrap: [AppComponent] // bootstrapped entry component need to be declared here

How do you manually bootstrap an application?

You can use ng Do Bootstrap hook for a manual bootstrapping of the application instead of using bootstrap array in @NgModule annotation. This hook is part of Do Bootstep 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

Advance Angular Interview Question

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 entry Components.

Why is not necessary to use entry Components array every time?

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

Angular Interview Question

Do I still need to use entry Components array in Angular9?

No. In previous angular releases, the entry Components array of ng Module 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.

Advance Angular Interview Question

What is the role of ng Module 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 Ng Modules?

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

  1. Angular libraries such as Forms Module, HttpClientModule, and Router Module are Ng Modules.
  2. Many third-party libraries such as Material Design, Ionic, and AngularFire2 are Ng Modules.

Angular Interview Question

What are feature modules?

Feature modules are Ng Modules, 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';

  imports: [
  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. Ng Module: In Order to use the @NgModule decorator
  2. Common Module: It provides many common directives such as ngIf and ngFor.

What are the differences between ng module and JavaScript module?

Below are the main differences between Angular Ng Module 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

Advance Angular Interview Question

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 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)

Angular Interview Question

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';

  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';

  providedIn: SomeModule,
export class SomeService {

Declare provider for the service in module:

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

import { SomeService } from './some.service';

  providers: [SomeService],
export class SomeModule {

Advance Angular Interview Question

How do you provide a singleton service?

There are two possible ways to provide a singleton service.

  1. Set the provide din 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';

  providedIn: 'root',
export class MyService {

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.

  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 provide dIn syntax instead of registering the service in the module.
  2. Separate your services into their own module.
  3. Define for Root() and for Child() methods in the module.

How does for Root method helpful to avoid duplicate router instances?

If the Router Module module didn’t have for Root() 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 Router Module.forRoot(...) and gets a Router, and all feature modules import Router Module.forChild(...) which does not instantiate another Router.

Angular Interview Question

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 Common Module, Forms Module 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';

 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 get Locale Direction 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

Advance Angular Interview Question

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 Ng Zone?

Angular provides a service called Ng Zone 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.

Angular Interview Question

What is Noop Zone?

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.

Bootstrap Angular with noop zone in src/main.ts

platformBrowserDynamic().bootstrapModule(AppModule, {ngZone: 'noop'})
  .catch(err => console.error(err));

How do you create display Block 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 display Block 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.
  selector: 'app-event-listener',
  template: `
    <button (click)="onClick()">Click</button>
export class EventListenerComponent {
  message = '';

  onClick() {
    this.message = 'data updated';

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 => { =; // change detection will happen automatically

Macro tasks set Timeout() or set Interval(): You can update the data in the callback function of set Timeout or set Interval

data = 'default value';

  ngOnInit() {
    setTimeout(() => { = 'data updated'; // Change detection will happen automatically

Micro tasks Promises: You can update the data in the callback function of promise

data = 'initial value';

  ngOnInit() {
    Promise.resolve(1).then(v => { = v; // Change detection will happen automatically

Async operations like Web sockets and Canvas: The data can be updated asynchronously using Web Socket.onmessage() and Canvas.toBlob().

Advance Angular Interview Question

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 set Timeout callback function, => {
  // outside zone
  setTimeout(function() {
    // the same outside zone exist here

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. on Schedule Task: This hook triggers when a new asynchronous task is scheduled. For example, when you call set Timeout()
onScheduleTask: function(delegate, curr, target, task) {
    console.log('new task is scheduled:', task.type, task.source);
    return delegate.scheduleTask(target, task);

on Invoke Task: This hook triggers when an asynchronous task is about to execute. For example, when the callback of set Timeout() 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);

on Has Task: 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);

on Invoke: 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 Ng Zone 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 to make the asynchronous operation in the angular zone => {
      someNewAsyncAPI(() => {
        // update the data of the component

Whereas run Out side Angular() 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

Angular Interview Question

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 reques tAnimation Frame() monkey patch to prevent change detection for no data update as one setting and prevent DOM events(a mouse move 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.

  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', [
      transition('down => up', [
  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

Advance Angular Interview Question

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 App Service in a root component,

  1. The below App Service can be injected in App Component without any problems. This is because there are no dependency services inside App Service.
export class App Service {
  constructor() {
    console.log('A new app service');

The below App Service with dummy decorator and HTTP Service can be injected in App Component without any problems. This is because meta information is generated with dummy decorator.

function SomeDummyDecorator() {
  return (constructor: Function) => console.log(constructor);

export class AppService {
  constructor(http: HttpService) {

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');
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