What are Angular directives?
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Overview
Directives are HTML attributes. They tell Angular to alter the style of the Dom elements. AngularJS helps us extend HTML with new attributes.
There are three types of directives:
- Components directives
- Structural directives
- Attribute directives
Component directives
Component directives detail how the component should be processed, instantiated, and used at runtime, forming the main class.
import {Component} from "@angular/core";//decorator@Component({selector: 'my-App',template: '<h1>{{name}}</h1>'})export class ExampleComponent{name: string = "Angular 2"}
First, we import a component from "@angular/core".
Then, we decorate our ExmapleComponent class with the @component decorator.
Structural directives
Structural directives add or remove the elements from the DOM. Therefore, they are responsible for changing the structure of the DOM.
Structural directives include a * sign before them, for example, *NgSwitch and *ngFor.
<div *ngFor="let book of books">{{book.name}}</div><div template="ngFor let book of books">{{book.name}}</div><ng-template ngFor let-book [ngFor]="book">{{book.name}}</ng-template>
In the code above, we use the let keyword to declare an input variable referenced within the template.
book is the input variable in this example.
The let book is translated into variables named let-book by the parser.
In line 2, we can see that the NgFor directive sets and resets the properties of its context as it loops through the list.
Attribute directives
We use attribute directives to show or hide elements, apply a conditional style to them, or dynamically change a component’s behavior according to a changing property.
They manipulate the DOM by changing its behavior and appearance.
<p [ngStyle]="{'background': isRed ? 'red' : 'green'}"> This isan Attribute Directive</p>
In the code above, we add a red background if the value of the isRed variable is true. If the value of the isRed variable is false, the background of the above elements will be green.
Code
We can find a working example containing a few concepts discussed in this shot:
import { TestBed } from '@angular/core/testing';
import { AppComponent } from './app.component';
describe('AppComponent', () => {
beforeEach(async () => {
await TestBed.configureTestingModule({
declarations: [
AppComponent
],
}).compileComponents();
});
it('should create the app', () => {
const fixture = TestBed.createComponent(AppComponent);
const app = fixture.componentInstance;
expect(app).toBeTruthy();
});
it(`should have as title 'angular-test'`, () => {
const fixture = TestBed.createComponent(AppComponent);
const app = fixture.componentInstance;
expect(app.title).toEqual('angular-test');
});
it('should render title', () => {
const fixture = TestBed.createComponent(AppComponent);
fixture.detectChanges();
const compiled = fixture.nativeElement as HTMLElement;
expect(compiled.querySelector('.content span')?.textContent).toContain('angular-test app is running!');
});
});
Explanation
- Lines 4–15: We create a component with a simple HTML template where names of books are printed, and the books that have the
isRedproperty to betrueare red. Others are green.
