How to utilize the async pipe on an observable<Object> and connect it to a local variable in the HTML using Angular

Question

How to utilize the async pipe on an observable<Object> and connect it to a local variable in the HTML using Angular

Hey there! So, I have this observable called user$ which has a bunch of properties such as name, title, and address.

component{
  user$:Observable<User>;
  constructor(private userService:UserService){
    this.user$ = this.userService.someMethodReturningObservable$()
  }
}

I was wondering if there's a way to utilize the async pipe in the HTML template to subscribe to it and bind it to a local variable like below:

<div #user="user$ | async">
  <h3> {{user.name}}
</div>

I am aware that I can manually subscribe to it in the constructor and then unsubscribe in OnLeave/OnDestroy. However, I'm just curious if using the async pipe is possible for this scenario.

Cheers!

angular typescript pipe local-variables angular2-observables

Answer 1

Answer №1

# refers to a template reference variable. It points to a DOM element and cannot be used in other ways.

In Angular, local variables are not currently supported. You can keep an eye on this closed issue for updates on related issues.

Starting from Angular 4, the syntax of ngIf and ngFor directives has been updated to allow for local variables. Refer to ngIf documentation for more information. This allows you to do things like:

<div *ngIf="user$ | async; let user">
  <h3> {{user.name}} </h3>
</div>

With this approach, a div wrapper element is created with cloaking behavior, eliminating the need for the Elvis operator ?..

If you prefer no extra markup, you can use:

<ng-container *ngIf="user$ | async; let user">...</ng-container>

If you don't want cloaking behavior, you can use different placeholder values based on truthiness. For instance, an empty object for an object value:

<div *ngIf="(user$ | async) || {}; let user">
  <h3> {{user?.name}} </h3>
</div>

Or a space for a primitive value:

<div *ngIf="(primitive$ | async) || ' '; let primitive">
  <h3> {{primitive}} </h3>
</div>

Answer 2

# refers to a template reference variable. It points to a DOM element and cannot be used in other ways.

In Angular, local variables are not currently supported. You can keep an eye on this closed issue for updates on related issues.

Starting from Angular 4, the syntax of ngIf and ngFor directives has been updated to allow for local variables. Refer to ngIf documentation for more information. This allows you to do things like:

<div *ngIf="user$ | async; let user">
  <h3> {{user.name}} </h3>
</div>

With this approach, a div wrapper element is created with cloaking behavior, eliminating the need for the Elvis operator ?..

If you prefer no extra markup, you can use:

<ng-container *ngIf="user$ | async; let user">...</ng-container>

If you don't want cloaking behavior, you can use different placeholder values based on truthiness. For instance, an empty object for an object value:

<div *ngIf="(user$ | async) || {}; let user">
  <h3> {{user?.name}} </h3>
</div>

Or a space for a primitive value:

<div *ngIf="(primitive$ | async) || ' '; let primitive">
  <h3> {{primitive}} </h3>
</div>

Answer 3

Answer №2

@Lucas Johnson and @Emma Thompson

Rather than:

<div *ngIf="(person$ | async) || {}; let person">

Try this:

<div *ngIf="(person | async) as person">

Answer 4

@Lucas Johnson and @Emma Thompson

Rather than:

<div *ngIf="(person$ | async) || {}; let person">

Try this:

<div *ngIf="(person | async) as person">

Answer 5

Answer №3

Try using this code snippet:

<div *ngIf="(userData | async) as userData">

Note: Make sure to include “as userData” in the expression.

By doing this, the program will pause until userData$ | async is processed and then assign the result to the variable userData (without a dollar sign).

Answer 6

Try using this code snippet:

<div *ngIf="(userData | async) as userData">

Note: Make sure to include “as userData” in the expression.

By doing this, the program will pause until userData$ | async is processed and then assign the result to the variable userData (without a dollar sign).

Answer 7

Answer №4

To make use of the *ngFor directive along with an additional *ngIf directive, simply include the *ngIf on the parent element:

    <div *ngIf="items$ | async as items">
      <ul *ngFor="let item of items">
        <li>{{item.name}}</li>
      </ul>
      <div *ngIf="!items.length">No items</div>
    </div>

If the parent element already has a structural directive in place, consider using an intermediate element like <ng-container>.

Answer 8

To make use of the *ngFor directive along with an additional *ngIf directive, simply include the *ngIf on the parent element:

    <div *ngIf="items$ | async as items">
      <ul *ngFor="let item of items">
        <li>{{item.name}}</li>
      </ul>
      <div *ngIf="!items.length">No items</div>
    </div>

If the parent element already has a structural directive in place, consider using an intermediate element like <ng-container>.

How to utilize the async pipe on an observable<Object> and connect it to a local variable in the HTML using Angular

Answer №1

Answer №2

Answer №3

Answer №4

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