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Angular: Easily extract a falsy value from an observable using the async pipe ?

maxime1992 profile image Maxime ・3 min read

Whether you've been using Angular for years or if you just got started with it a day ago, chances are you probably know the ngIf directive. It lets you display (create or remove) a DOM element based on a condition.

As simple as it sounds, there's one (native) functionality of ngIf that not many people seems to be aware of.

Table of content

Unwrap observables from the template

As Angular plays really nicely with RxJs, you can take advantage of the ngIf directive when you have an observable by using the async pipe and the as syntax:

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

But what happens when you want to extract a value from an observable using the async pipe if that value is falsy?

For example a boolean that's false, 0 (the number), an empty string, a null or undefined value, etc.

The value wouldn't be displayed as the ngIf condition will be evaluated as falsy.

There's a long standing issue here: https://github.com/angular/angular/issues/15280

How to unwrap falsy values from an observable?

The main proposal in the issue above is to have an ngLet directive that'd have the same capabilities as ngIf but would always display the template even if the value is falsy.

Would that ngLet directive be the silver bullet for that situation and if so, how can we deal without it in the meantime?

Wrap the value into an object

Wrap the value within the observable into an object using the map operator.

Example:

const wrappedValue$ = falsyValue$.pipe(map(value => ({ value })));

We could do slightly better by having a proper type and a custom rxjs operator. This would reduce the boilerplate, code duplication and improve the readability. Not bad!

But modifying our data stream because of a display issue, it feels like we can/should do better. And we can by using... only Angular!

Use ngIf itself!

The Angular compiler let you do that wrapping directly from your template:

<div *ngIf="{ falsyValue: falsyValue$ | async } as data">
  {{ data.falsyValue}}
</div>

It's apparently not a well known functionality but it's really useful for multiple reasons:

  • In the above example, the data object is always defined so no matter what's in that object, the ngIf condition will always be evaluated as true. This can replace the ngLet directive

  • We can push things further by subscribing to multiple properties at once, no need for nested div or ng-container!

Before:

<ng-container *ngIf="wrappedValue1$ | async as wrappedValue1">
  <ng-container *ngIf="wrappedValue2$ | async as wrappedValue2">
    <ng-container *ngIf="wrappedValue3$ | async as wrappedValue3">
      {{ wrappedValue1 }} {{ wrappedValue2 }} {{ wrappedValue3 }}
    </ng-container>
  </ng-container>
</ng-container>

After:

<div
  *ngIf="{
    value1: value1$ | async,
    value2: value2$ | async,
    value3: value3$ | async
  }"
>
  {{ value1 }} {{ value2 }} {{ value3 }}
</div>

Much cleaner isn't it?

Conclusion

I think that having an ngLet directive might bring clarity about what we're trying to achieve and more importantly, it might be useful when we have only one observable to manage to be able to do the following instead of wrapping it into an object:

<div *ngLet="value$ | async as value">
  {{ value }}
</div>

But for now, the workaround is trivial and shouldn't stop you to do that transformation template side instead of within your TS code.

Happy coding!

Found a typo?

If you've found a typo, a sentence that could be improved or anything else that should be updated on this blog post, you can access it through a git repository and make a pull request. Instead of posting a comment, please go directly to https://github.com/maxime1992/my-dev.to and open a new pull request with your changes.

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Maxime

@maxime1992

Software engineer. Trying to share more on dev.to about my discoveries and side projects. Favourite stack: Typescript, Angular, NestJs.

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