Write a pictorial diagram of Angular architecture?
The main building blocks of an Angular application are shown in the diagram below:-
Angular is a completely revived component-based framework in which an application is a tree of individual components.
Here are some of the major differences in tabular format:-
| AngularJS | Angular |
|---|---|
| It is based on MVC architecture | This is based on Service/Controller |
| It uses JavaScript to build the application | Uses TypeScript to build the application |
| Based on controllers concept | This is a component based UI approach |
| No support for mobile platforms | Fully supports mobile platforms |
| Difficult to build SEO friendly application | Ease to build SEO friendly applications |
npm install -g typescript
function greeter(person: string) {
return "Hello, " + person;
}
let user = "Sudheer";
document.body.innerHTML = greeter(user);
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>
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';
}
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 a directive we use @Directive meta-data annotation |
| Components are typically used to create UI widgets | Directives are used to add behavior to an existing DOM element |
| Component is used to break down the application into smaller components | Directive is used 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 |
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';
}
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 { }
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,
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">
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>
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">
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!');
}
}
import { Component, Input } from '@angular/core';
@Component({
selector: 'my-component',
template: '<div>Property decorator</div>'
})
export class MyComponent {
@Input()
title: string;
}
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
}
}
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
}
}
npm install @angular/cli@latest
Creating New Project: ng new
Generating Components, Directives & Services: ng generate/g
Running the Project: ng serve
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.
export class App implements OnInit{
constructor(private myService: MyService){
//called first time before the ngOnInit()
}
ngOnInit(){
//called after the constructor and called after the first ngOnChanges()
//e.g. http call...
}
}
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.
Injectors in Angular have rules that can be leveraged to achieve the desired visibility of injectables in your applications. By understanding these rules, you can determine in which NgModule, Component, or Directive you should declare a provider.
When angular starts, it creates a root injector where the services will be registered, these are provided via injectable annotation. All services provided in the ng-model property are called providers (if those modules are not lazy-loaded).
Angular recursively goes through all models which are being used in the application and creates instances for provided services in the root injector. If you provide some service in an eagerly-loaded model, the service will be added to the root injector, which makes it available across the whole application.
During application bootstrapping angular creates a few more injectors, above the root injector goes the platform injector, this one is created by the platform browser dynamic function inside the main.ts file, and it provides some platform-specific features like DomSanitizer.
At the very top, the next parent injector in the hierarchy is the NullInjector().The responsibility of this injector is to throw the error if something tries to find dependencies there, unless you've used @Optional() because ultimately, everything ends at the NullInjector() and it returns an error or, in the case of @Optional(), null.
Angular creates ElementInjector hierarchies implicitly for each DOM element. ElementInjector injector is being created for any tag that matches the angular component, or any tag on which directive is applied, and you can configure it in component and directive annotations inside the provider's property, thus, it creates its own hierarchy likewise the upper one.
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: Observable<string>;
constructor() {
this.time = new Observable((observer) => {
setInterval(() => {
observer.next(new Date().toString());
}, 2000);
});
}
}
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
*ngFor directive?*ngFor directive in the template to display each item in the list. For example, here we can iterate over a list of users:<li *ngFor="let user of users">
{{ user }}
</li>
*ngFor double-quoted instruction is a template input variable.*ngIf directive?*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>
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>';
}
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.
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>
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>
Binding types can be grouped into three categories distinguished by the direction of data flow. They are listed as below,
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 |
Pipes are simple functions that use template expressions to accept data as input and transform it into 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
}
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.
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);
}
@Pipe decorator, which you import from the core Angular library
For example, @Pipe({name: 'myCustomPipe'})
PipeTransform would be as below,interface PipeTransform {
transform(value: any, ...args: any[]): any
}
@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}}`
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;
}
}
template: `
<h2>Find the size of a file</h2>
<p>Size: {{288966 | customFileSizePipe: 'GB'}}</p>
`
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:
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 { }
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 available 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,
HttpClient with an example?HttpClient.HttpClient into root module:import { HttpClientModule } from '@angular/common/http';
@NgModule({
imports: [
BrowserModule,
// import HttpClientModule after BrowserModule.
HttpClientModule,
],
......
})
export class AppModule {}
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);
}
}
userprofile.component.ts), which injects 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.
The response body doesn't or may not return full response data because sometimes servers also return special headers or status code, which are important for the application workflow. In order to get the 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.
If the request fails on the server or fails 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.
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';
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
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);
});
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.
Below are the list of differences between promise and observable:
| Observable | Promise |
|---|---|
| Declarative: Computation does not start until subscription, so they can run whenever you need the result | Executes immediately on creation |
| Provides multiple values over time | Provides only one |
| Subscribe method is used for error handling that facilitates centralized and predictable error handling | Push errors to the child promises |
| Provides chaining and subscription to handle complex applications | Uses only .then() clause |
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
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)}
});
The subscribe() method can accept callback function definitions in line, for next, error, and complete handlers. It is known as shorthand 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')
);
The RxJS library also provides below utility functions for creating and working with observables.
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'); }
});
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));
import { interval } from 'rxjs'; // interval function
const secondsCounter = interval(1000); // Created from Counter value
secondsCounter.subscribe(n =>
console.log(`Counter value: ${n}`));
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}`);
});
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.non-Angular environments.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 |
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.Below are the steps in an order about custom elements functionality,
createCustomElement() function to convert a component into a class that can be registered with the browser as a custom element.
Transforming components to custom elements involves two major steps,
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.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,
my-input-prop.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.
@Component(...)
class MyContainer {
@Input() message: string;
}
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`.
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.Let's take simple highlighter behavior as a example directive for DOM element. You can create and apply the attribute directive using below step:
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';
}
}
)
<p appHighlight>Highlight me!</p>
ng serve
The routing application should add
<base href="/">
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';
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 -->
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>
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 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>
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);
//...
}
}
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,
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));
}
}
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 { }
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 }
Ahead-of-Time (AOT) is a type of compilation that compiles your app at build time. This is the default starting in Angular 9. When you run the ng build (build only) or ng serve (build and serve locally) CLI commands, the type of compilation (JIT or AOT) depends on the value of the aot property in your build configuration specified in angular.json. By default, aot is set to true.
ng build
ng serve
Below are the list of AOT benefits,
tsconfig.json fileNo, 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.
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.
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 {}
Below are some of the errors encountered in metadata,
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
// ERROR
let username: string; // neither exported nor initialized
@Component({
selector: 'my-component',
template: ... ,
providers: [
{ provide: User, useValue: username }
]
})
export class MyComponent {}
export let username: string; // exported
(or)
let username = 'John'; // initialized
providers: [
{ provide: MyStrategy, useFactory: function() { ... } },
{ provide: OtherStrategy, useFactory: () => { ... } }
]
export function myStrategy() { ... }
export function otherStrategy() { ... }
... // metadata
providers: [
{ provide: MyStrategy, useFactory: myStrategy },
{ provide: OtherStrategy, useFactory: otherStrategy },
import { user } from './user';
// destructured assignment to name and age
const {name, age} = user;
... //metadata
providers: [
{provide: Name, useValue: name},
{provide: Age, useValue: age},
]
import { user } from './user';
... //metadata
providers: [
{provide: Name, useValue: user.name},
{provide: Age, useValue: user.age},
]
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,
...
}
}
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,
...
}
}
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'?
template:
'{{ $any(user).contacts.email }}'
template:
'{{ $any(this).contacts.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;
}
}
@Component({
selector: 'my-component',
template: '<span *ngIf="user"> {{user.contact.email}} </span>'
})
class MyComponent {
user?: User;
}
The dependencies section of package.json with in an angular application can be divided as follow,
ng new codelyzer
ng lint
You need to follow below steps to implement animation in your angular project,
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 { }
import {
trigger,
state,
style,
animate,
transition,
// ...
} from '@angular/animations';
@Component({
selector: 'app-root',
templateUrl: 'app.component.html',
styleUrls: ['app.component.css'],
animations: [
// animation triggers go here
]
})
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'
})),
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.
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() {
}
}
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')
]),
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>");
}
Below are the list of design goals of Angular's service workers,
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. |
Angular Ivy is a new rendering engine for Angular. You can choose to opt in a preview version of Ivy from Angular version 8.
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
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
}
}
You can expect below features with Ivy preview,
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,
}
}
}
}
}
}
tsconfig.json file, finds all the templates you have in your application, and then provides all the language services.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.
Basically there are 3 main features provided by Angular Language Service,
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.
Error checking: It can also warn you of mistakes in your code.
Navigation: Navigation allows you to hover a component, directive, module and then click and press F12 to go directly to its definition.
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,
addEventListener('message', ({ data }) => {
const response = `worker response to ${data}`;
postMessage(response);
});
app.component.ts file updated with web worker fileif (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.
You need to remember two important things when using Web Workers in Angular projects,
@angular/platform-webworker is not yet supported in Angular CLI.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.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.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.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]
Angular uses capitalization conventions to distinguish the names of various types. Angular follows the list of the below case types.
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,
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();
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
],
Below classes shouldn't be declared,
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);
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.
(): Used for Output and DOM events.[]: Used for Input and specific DOM element attributes.*: Structural directives(*ngFor or *ngIf) will affect/change the DOM structure.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);
Below are the list of key advantages of Bazel tool,
ng add @angular/bazel
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.
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.
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,
platform-browser package.platform-server package for providing web server implementation.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);
}
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
}
});
}
}
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 })
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.
Note: This strategy is used to support multiple browsers but it only load the code that the browser needs.
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)};
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'
}
];
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();
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
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
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 {}
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.
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".
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,
bash npm install --save web-animations-js
and create import statement in polyfill file.
javascript import 'web-animations-js'; 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,
There are different versions of Angular framework. Let's see the features of all the various versions,
Below are the list of security principles in angular,
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.
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
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:
...
Angular supports most recent browsers which includes both desktop and mobile browsers.
| Browser | Version |
|---|---|
| Chrome | latest |
| Firefox | latest |
| Edge | 2 most recent major versions |
| IE | 11, 10, 9 (Compatibility mode is not supported) |
| Safari | 2 most recent major versions |
| IE Mobile | 11 |
| iOS | 2 most recent major versions |
| Android | 7.0, 6.0, 5.0, 5.1, 4.4 |
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.
In schematics world, it's a function that operates on a file tree to create, delete, or modify files in a specific manner.
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
Below are the best practices of security in angular,
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.
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.
Angular defines the following security contexts for sanitization,
<a href>.<script src>.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.
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.
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.
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,
Inject DomSanitizer: You can inject DomSanitizer in component as parameter in constructor
Mark the trusted value by calling some of the below methods
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);
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.
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.
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.
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.
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 {}
The HTTP Interceptors can be used for different variety of tasks,
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.
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 (event 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 {}
Angular simplifies the below areas of internationalization,
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');
The i18n template translation process has four phases:
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>
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.
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>
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.
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.
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>
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>
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
<ng-container i18n>I'm not using any DOM element for translation</ng-container>
Remember that <ng-container> is transformed into an html comment
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="
Pluralization has below categories depending on the language.
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>
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,
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: [
// ...
]
});
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",
}
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.
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.
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';
}
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.
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
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.
You can create your own schematic collections to integrate your library with the Angular CLI. These collections are classified as 3 main schematics,
ng add command.
For example, @angular/material schematic tells the add command to install and set up Angular Material and theming.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 .ng update command.
For example, @angular/material update schematic updates material and cdk dependencies using ng update @angular/material command.You can use jquery in Angular using 3 simple steps,
npm install --save jquery
"scripts": [
"node_modules/jquery/dist/jquery.min.js"
]
<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%'});
});
}
}
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 { }
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.
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
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>
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.
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'];
}
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>
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; }
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>
Yes, it is possible to do aliasing for inputs and outputs in two ways.
inputs: ['input1: buyItem'],
outputs: ['outputEvent1: completedEvent']
@Input('buyItem') input1: string;
@Output('completedEvent') outputEvent1 = new EventEmitter<string>();
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.
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 }
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
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
The Angular template expression language supports three special template expression 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).
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 -
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
})
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
}
}
Yes, the bootstrapped component needs to be an entry component. This is because the bootstrapping process is an imperative process.
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.
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.
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.
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.
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.
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.
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.
The Angular core libraries and third-party libraries are available as NgModules.
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.
In the CLI generated feature module, there are two JavaScript import statements at the top of the file
@NgModule decoratorngIf and ngFor.Below are the main differences between Angular NgModule and javascript module,
| NgModule | JavaScript module |
|---|---|
| NgModule bounds declarable classes only | There is no restriction classes |
| List the module's classes in declarations array only | Can define all member classes in one giant file |
| It only export the declarable classes it owns or imports from other modules | It can export any classes |
| Extend the entire application with services by adding providers to provides array | Can't extend the application with services |
There are two common possible errors with declarations array,
Below are the steps to be followed to use declaration elements.
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.
Below are the five categories of feature modules,
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 {
}
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.
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.
import { Injectable } from '@angular/core';
import { SomeModule } from './some.module';
@Injectable({
providedIn: SomeModule,
})
export class SomeService {
}
import { NgModule } from '@angular/core';
import { SomeService } from './some.service';
@NgModule({
providers: [SomeService],
})
export class SomeModule {
}
There are two possible ways to provide a singleton service.
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 {
}
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],
...
})
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.
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.
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 { }
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.
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
}
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.
The below class types shouldn't be added to declarations
Angular provides a service called NgZone which creates a zone named angular to automatically trigger change detection when the following conditions are satisfied.
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,
/***************************************************************************************************
* Zone JS is required by default for Angular itself.
*/
// import 'zone.js/dist/zone'; // Included with Angular CLI.
platformBrowserDynamic().bootstrapModule(AppModule, {ngZone: 'noop'})
.catch(err => console.error(err));
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.
The change detection works in the following scenarios where the data changes needs to update the application HTML.
ApplicationRef.tick() to call change detection and View Rendering.@Component({
selector: 'app-event-listener',
template: `
<button (click)="onClick()">Click</button>
{{message}}`
})
export class EventListenerComponent {
message = '';
onClick() {
this.message = 'data updated';
}
}
data = 'default value';
constructor(private httpClient: HttpClient) {}
ngOnInit() {
this.httpClient.get(this.serverUrl).subscribe(response => {
this.data = response.data; // change detection will happen automatically
});
}
data = 'default value';
ngOnInit() {
setTimeout(() => {
this.data = 'data updated'; // Change detection will happen automatically
});
}
data = 'initial value';
ngOnInit() {
Promise.resolve(1).then(v => {
this.data = v; // Change detection will happen automatically
});
}
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.
There are four lifecycle hooks for asynchronous operations from zone.js.
onScheduleTask: function(delegate, curr, target, task) {
console.log('new task is scheduled:', task.type, task.source);
return delegate.scheduleTask(target, task);
}
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);
}
onHasTask: function(delegate, curr, target, hasTaskState) {
console.log('task state changed in the zone:', hasTaskState);
return delegate.hasTask(target, hasTaskState);
}
onInvoke: function(delegate, curr, target, callback, applyThis, applyArgs) {
console.log('the callback will be invoked:', callback);
return delegate.invoke(target, callback, applyThis, applyArgs);
}
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
});
});
}
}
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.
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;
}
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,
@Injectable() decorator for the service itself@NgModule() decorator for an NgModule@Component() decorator for a componentNo. 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,
export class AppService {
constructor() {
console.log('A new app service');
}
}
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); } }
⬆ Back to Top
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');
}
}
There are two types of injector hierarchies in Angular
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,
import { ReactiveFormsModule } from '@angular/forms';
@NgModule({
imports: [
// other imports ...
ReactiveFormsModule
],
})
export class AppModule { }
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('');
}
<label>
User name:
<input type="text" [formControl]="userName">
</label>
Finally, the component with reactive form control appears as below,
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('');
}
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. ⬆ Back to Top
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,
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 { }
<input type="text" class="form-control" id="name"
required
[(ngModel)]="model.name" name="name">
<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>
<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.
<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>
Below are the main differences between reactive forms and template driven forms
| Feature | Reactive | Template-Driven |
|---|---|---|
| Form model setup | Created(FormControl instance) in component explicitly | Created by directives |
| Data updates | Synchronous | Asynchronous |
| Form custom validation | Defined as Functions | Defined as Directives |
| Testing | No interaction with change detection cycle | Need knowledge of the change detection process |
| Mutability | Immutable(by always returning new value for FormControl instance) | Mutable(Property always modified to new value) |
| Scalability | More scalable using low-level APIs | Less scalable using due to abstraction on APIs |
Reactive forms provide two ways of grouping multiple related controls.
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>
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>
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.
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) { }
}
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>
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 state | If true | If false |
|---|---|---|
| Visited | ng-touched | ng-untouched |
| Value has changed | ng-dirty | ng-pristine |
| Value is valid | ng-valid | ng-invalid |
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.
In reactive forms, the validators can be either synchronous or asynchronous functions,
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*]
});
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.
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,
ngModelOptions directive<input [(ngModel)]="name" [ngModelOptions]="{updateOn: 'blur'}">
name = new FormControl('', {updateOn: 'blur'});
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>
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;
}
In Angular, there is an url property of router package to get the current route. You need to follow the below few steps,
import { Router } from '@angular/router';
constructor(private router: Router ) {
}
console.log(this.router.url); // /routename
A component harness is a testing API around an Angular directive or component to make tests simpler by hiding implementation details from test suites. This can be shared between unit tests, integration tests, and end-to-end tests. The idea for component harnesses comes from the PageObject pattern commonly used for integration testing.
During compile time, Angular CLI will download and inline the fonts that your application is using. This performance update speed up the first contentful paint(FCP) and this feature is enabled by default in apps built with version 11.
Content projection is a pattern in which you insert, or project, the content you want to use inside another component.
The ng-content is used to insert the content dynamically inside the component that helps to increase component reusability.
A standalone component is a type of component which is not part of any Angular module. It provides a simplified way to build Angular applications.
Generate standalone component using CLI command as shown below
ng generate component component-name --standalone
On running the command standalone component is created. Here is the list of file created.
component-name.component.tscomponent-name.component.csscomponent-name.component.speccomponent-name.component.html Next need to update app.module.ts as shown below.
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { ComponentNameComponent } from './component-name/component-name.component';
@NgModule({
imports: [
BrowserModule,
ComponentNameComponent
],
declarations: [AppComponent],
bootstrap: [AppComponent],
})
export class AppModule {}
To make existing component to standalone, then add standalone: true in component-name.component.ts
as shown below
import { CommonModule } from '@angular/common';
import { Component, OnInit } from '@angular/core';
@Component({
standalone: true,
imports: [CommonModule],
selector: 'app-standalone-component',
templateUrl: './standalone-component.component.html',
styleUrls: ['./standalone-component.component.css'],
})
export class ComponentNameComponent implements OnInit {
constructor() {}
ngOnInit() {}
}
Next need to update app.module.ts as shown below.
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { ComponentNameComponent } from './component-name/component-name.component';
@NgModule({
imports: [
BrowserModule,
ComponentNameComponent
],
declarations: [AppComponent],
bootstrap: [AppComponent],
})
export class AppModule {}
Hydration is the process that restores the server side rendered application on the client. This includes things like reusing the server rendered DOM structures, persisting the application state, transferring application data that was retrieved already by the server, and other processes.
To enable hydration, we have to enable server side rendering or Angular Universal. Once enabled, we can add the following piece of code in the root component.
import {
bootstrapApplication,
provideClientHydration,
} from '@angular/platform-browser';
bootstrapApplication(RootCmp, {
providers: [provideClientHydration()]
});
Alternatively we can add providers: [provideClientHydration()] in the App Module
import {provideClientHydration} from '@angular/platform-browser';
import {NgModule} from '@angular/core';
@NgModule({
declarations: [RootCmp],
exports: [RootCmp],
bootstrap: [RootCmp],
providers: [provideClientHydration()],
})
export class AppModule {}