Directives are classes that add behavior or modify elements in the DOM. Angular has three kinds: components (directives with a template), structural directives (change DOM layout), and attribute directives (change appearance/behavior).
They add or remove elements, and are prefixed with :
*<p *ngIf="isLoggedIn">Welcome back!</p> <!-- add/remove based on condition -->
<li *ngFor="let item of items; let i = index"> <!-- repeat an element per item -->
{{ i }}: {{ item.name }}
</li>
<div [ngSwitch]="status">
<p *ngSwitchCase="'active'">Active</p>
<p *ngSwitchDefault>Unknown</p>
</div>
(Modern Angular also offers the newer @if / @for control-flow syntax that replaces *ngIf/*ngFor.)
They modify an existing element without adding/removing it:
<div [ngClass]="{ active: isActive, error: hasError }">...</div> <!-- toggle classes -->
<div [ngStyle]="{ color: textColor }">...</div> <!-- dynamic styles -->
<input [(ngModel)]="name" /> <!-- two-way binding -->
import { Directive, ElementRef, HostListener } from "@angular/core";
@Directive({ selector: "[appHighlight]" }) // used as <p appHighlight>
export class HighlightDirective {
constructor(private el: ElementRef) {}
@HostListener("mouseenter") onEnter() {
this.el.nativeElement.style.background = "yellow"; // change the host element
}
@HostListener("mouseleave") onLeave() {
this.el.nativeElement.style.background = "";
}
}
A custom directive gets a reference to its host element and can react to events — here, highlighting on hover. Used as <p appHighlight>Text</p>.
<p *ngIf="x">...</p>
<!-- desugars to: -->
<ng-template [ngIf]="x"><p>...</p></ng-template>
The * is shorthand that wraps the element in an <ng-template> — which is why structural directives can add/remove the element entirely.
Directives are how Angular manipulates the DOM declaratively.
The built-in structural (*ngIf/*ngFor/@if/@for) and attribute (ngClass/ngStyle/ngModel) directives cover most needs, and custom directives let you encapsulate reusable DOM behavior (highlight, tooltip, click-outside).
Understanding the three categories — and that components are themselves directives with templates — is core to the Angular template model.
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