CODEWITHSUNDEEP
rxjsrxjs5ngrx
balteo
balteo

Reputation: 24689

Interrogation about the RxJS 5 filter() operator

I have an angular 2 router guard whose canActivate method is defined as follows:

canActivate() {
    return this.sessionSigninService.isAuthenticated()
      .filter(isAuthenticated => isAuthenticated !== null)
      .map(isAuthenticated => {
        if (isAuthenticated) {
          return true;
        }
        this.router.navigate(['/signin']);
        return false;
      });
  }

from sessionSigninService:

  isAuthenticated(): Observable<boolean> {
    return this.store.select(fromRoot.getAuthenticated);
  }

FYI, fromRoot.getAuthenticated is initialized with a value of null

When the canActivate method is called and a null value for isAuthenticated enters into the filter() operator, it seems the filter() blocks until the value of isAuthenticated changes to false or true.

Note that this is the desired behavior but I don't understand it...

Can someone please explain this behavior?

Upvotes: 0

Views: 43

Answers (1)

Olaf Horstmann
Olaf Horstmann

Reputation: 16892

Like Array.prototype.filter(), it only emits a value from the source if it passes a criterion function.

filter will only propagate data when the returned value of the filter-method is true - when you have isAuthenticated: null and evaluate null !== null -> it will return false, that's why no data is propagated and the map will not be called.

As soon as you have isAuthenticated: true|false, the filter will evaluate to true, because both true and false are !== null, and propagate data.

Upvotes: 1

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