詳細ガイド
テンプレート構文

Local template variables

Angular's @let syntax allows you to define a local variable and re-use it across the template.

IMPORTANT: the @let syntax is currently in Developer Preview.

Syntax

@let declarations are similar to JavaScript's let and their values can be any valid Angular expression. The expressions will be re-evaluated any time the template is executed.

      
@let name = user.name;
@let greeting = 'Hello, ' + name;
@let data = data$ | async;
@let pi = 3.1459;
@let coordinates = {x: 50, y: 100};
@let longExpression = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit ' +
                      'sed do eiusmod tempor incididunt ut labore et dolore magna ' +
                      'Ut enim ad minim veniam...';

Referencing the value of @let

Once you've declared the @let, you can reuse it anywhere in the template:

      
@let user = user$ | async;
@if (user) {
  <h1>Hello, {{user.name}}</h1>
  <user-avatar [photo]="user.photo"/>
  <ul>
    @for (snack of user.favoriteSnacks; track snack.id) {
      <li>{{snack.name}}</li>
    }
  </ul>
  <button (click)="update(user)">Update profile</button>
}

Assignability

A key difference between @let and JavaScript's let is that @let cannot be re-assigned within the template, however its value will be recomputed when Angular runs change detection.

      
@let value = 1;
<!-- Invalid -->
<button (click)="value = value + 1">Increment the value</button>

Scope

@let declarations are scoped to the current view and its descendants. Since they are not hoisted, they cannot be accessed by parent views or siblings:

      
@let topLevel = value;
<div>
  @let insideDiv = value;
</div>
{{topLevel}} <!-- Valid -->
{{insideDiv}} <!-- Valid -->
@if (condition) {
  {{topLevel + insideDiv}} <!-- Valid -->
  @let nested = value;
  @if (condition) {
    {{topLevel + insideDiv + nested}} <!-- Valid -->
  }
}
<div *ngIf="condition">
  {{topLevel + insideDiv}} <!-- Valid -->
  @let nestedNgIf = value;
  <div *ngIf="condition">
     {{topLevel + insideDiv + nestedNgIf}} <!-- Valid -->
  </div>
</div>
{{nested}} <!-- Error, not hoisted from @if -->
{{nestedNgIf}} <!-- Error, not hoisted from *ngIf -->

Syntax definition

The @let syntax is formally defined as:

  • The @let keyword.
  • Followed by one or more whitespaces, not including new lines.
  • Followed by a valid JavaScript name and zero or more whitespaces.
  • Followed by the = symbol and zero or more whitespaces.
  • Followed by an Angular expression which can be multi-line.
  • Terminated by the ; symbol.