CODEWITHSUNDEEP
angularrxjsangular9
Miguel Moura
Miguel Moura

Reputation: 39394

Compare values from two Observables and return an Observable

I am checking a condition and getting an Observable<boolean>:

let result = this.service.getCity().pipe(
  map((city: string) => city && city.toLowerCase() === ???) 
);

I need to check if the city equals: 

this.authenticationService.getCity():

This method also returns and Observable<boolean>.

How can I combine the two observables and use both values in a condition?

I need to return an Observable at the end.

Upvotes: 2

Views: 2478

Answers (2)

Jonathan Stellwag
Jonathan Stellwag

Reputation: 4267

Alternative to forkjoin

If observable completion is not the trigger to compare both observables, you can use switchMap

const { from, of } = rxjs;
const { map, switchMap, tap, take } = rxjs.operators;

const serviceCity$ = () => from(['Berlin', 'London', 'Paris']);
const authentificationCity$ = () => of('london');

const result$ = serviceCity$().pipe(
  map(city => city && city.toLowerCase()),
  switchMap(city => authentificationCity$().pipe(
    map(authCity => city === authCity),
    take(1),
  ))
);

result$.subscribe(v => console.log('authentification city equals london: ', v));
<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/6.5.3/rxjs.umd.min.js"></script>

Info

  • serviceCity$ and authentificationCity$ equal your service.getCity() and this.authenticationService.getCity()
  • I assumed both functions return a string and you want to compare both strings. If you want to compare two pre-mapped values then explain me and I will update the code.
  • result$ will fire whenever serviceCity$ emits followed by an emit of authentificationCity$
  • take(1) avoids the result$ being triggered by authentificationCity$ instead of serviceCity$

Upvotes: 2

penleychan
penleychan

Reputation: 5470

Assuming this.service.getCity() and this.authenticationService.getCity() returns singular object/string based on how you named it.

You can use forkJoin operator. Here's an example

const source1 = of("CityName");
const source2 = of("CityName");

forkJoin({
  source1,
  source2
}).pipe(
  map((x: {source1, source2}) => x.source1 === x.source2)
).subscribe(result => console.log(result));

StackBlitz: https://stackblitz.com/edit/rxjs-luncbd

Upvotes: 1

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