How to declare a variable in a template in Angular

Question

I have the following template :

<div>
  <span>{{aVariable}}</span>
</div>

and would like to end up with :

<div "let a = aVariable">
  <span>{{a}}</span>
</div>

Is there a way to do it ?

Answer 1

Update

We can just create directive like *ngIf and call it *ngVar

ng-var.directive.ts

@Directive({
    selector: '[ngVar]',
})
export class VarDirective {
  @Input()
  set ngVar(context: any) {
    this.context.$implicit = this.context.ngVar = context;
    this.updateView();
  }

  context: any = {};

  constructor(private vcRef: ViewContainerRef, private templateRef: TemplateRef<any>) {}

  updateView() {
    this.vcRef.clear();
    this.vcRef.createEmbeddedView(this.templateRef, this.context);
  }
}

with this *ngVar directive we can use the following

<div *ngVar="false as variable">
      <span>{{variable | json}}</span>
</div>

or

<div *ngVar="false; let variable">
    <span>{{variable | json}}</span>
</div>

or

<div *ngVar="45 as variable">
    <span>{{variable | json}}</span>
</div>

or

<div *ngVar="{ x: 4 } as variable">
    <span>{{variable | json}}</span>
</div>

Plunker Example Angular4 ngVar

See also

• Where does Angular 4 define "as local-var" behavior for *ngIf?

Original answer

Angular v4

1) div + ngIf + let

<div *ngIf="{ a: 1, b: 2 }; let variable">
  <span>{{variable.a}}</span>
  <span>{{variable.b}}</span>
</div>

2) div + ngIf + as

view

<div *ngIf="{ a: 1, b: 2, c: 3 + x } as variable">
  <span>{{variable.a}}</span>
  <span>{{variable.b}}</span>
  <span>{{variable.c}}</span>
</div>

component.ts

export class AppComponent {
  x = 5;
}

3) If you don't want to create wrapper like div you can use ng-container

view

<ng-container *ngIf="{ a: 1, b: 2, c: 3 + x } as variable">
  <span>{{variable.a}}</span>
  <span>{{variable.b}}</span>
  <span>{{variable.c}}</span>
</ng-container>

As @Keith mentioned in comments

this will work in most cases but it is not a general solution since it relies on variable being truthy

See update for another approach.