34

I have the following code:

public obs$: Observable<boolean>

<div *ngIf="(obs$ | async) === true || (obs$ | async) === false">
  {{ (obs$ | async) ? 'yes' : 'no' }}
</div>

It works as intended, but the if looks a little verbose.

The problem is that I cannot simply do <div *ngIf="(obs$ | async)">. If I try that, it will work in the case when the observable did not emit a value yet or if the value is true, but it will not work if the value is false, because the if will evaluate to false and the div is not displayed.

I assume the same issue applies if a falsy value is returned, such as an empty string or 0.

Is there a better, easier way of doing that?

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5
  • 3
    You can do (obs$ | async) !== null Commented Mar 22, 2019 at 9:50
  • 2
    Can you please shed some light on when the Observable will not emit a value? Commented Mar 22, 2019 at 9:51
  • 1
    @SachinGupta Oh wow, it is actually that simple. I tried to compare it to undefined which didn't work and I just gave up. Thanks a lot! If you post an answer I will accept it. Commented Mar 22, 2019 at 9:54
  • @SiddAjmera I'm making an API call to get the value, so it sometimes takes a few seconds to emit a value. Commented Mar 22, 2019 at 9:55
  • 1
    You can also pipe the response and return a false in the catchError operator. That way you'd always be certain that a boolean value is emitted. And you won't have to use an *ngIf on the div Commented Mar 22, 2019 at 9:56

4 Answers 4

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60

Here's a way where you can share the subscription and not have to make any custom pipe:

<ng-container *ngIf="{value: obs$ | async} as context">
    {{context.value}}
</ng-container>

The *ngIf will evaluate to true no matter what (I believe) and then you can do your own tests on the value in the context object.

...would be better if they would just implement *ngLet I think!

The thing I like about this solution is that the resulting value is reusable with only the single use of an async pipe and its still pretty concise (even if it isn't perfect).

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8 Comments

To add some clarity the context object is always defined so the ng-container will always show. To check if the Observable has completed use context.value !== null ... thanks, good answer
Is this still the way to go? Its working well, but feels hacky...
@enno.void you could try this: npmjs.com/package/ng-let
@WillyC thanks, i know the ng-let package, but im confused why something like this isnt backed into angular itself.
@enno.void you’re not kidding. Seems like a trivial thing to add and clearly in need.
|
18

You can do (obs$ | async) !== null

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2 Comments

You can't get value of boolean content in Observable with this solution, right?
Its a conditional check to be put in HTML, you wont get the value.
1

You can use "!!" to evalulate to boolean. for example *ngIf = "!!(item$ | async)"

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1 Comment

simple and it works
0

I think the most elegant way should be creating a custom BoolPipe ， and chain it after the async pipe。

@Pipe({name: 'myBool'})
export class MyBoolPipe implements PipeTransform {
  transform(value: boolean, exponent: string): string {
    return !value ? 'yes' : 'no';
  }
}

and init the obj$ in the constructor of your component or service (don't need null check in your HTML template), then chain it with async pipe like this:

<div>{{ obs$ | async | myBool }}</div>
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1 Comment

nothing elegant about that, you are just bloating up the code

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