exhaustMap

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exhaustMap
signature: exhaustMap(project: function, resultSelector: function): Observable
Map to inner observable, ignore other values until that observable completes.
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Examples
Example 1: exhaustMap with interval
( Stackblitz | jsBin | jsFiddle )
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// RxJS v6+
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import { interval, merge, of } from 'rxjs';
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import { delay, take, exhaustMap } from 'rxjs/operators';
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​
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const sourceInterval = interval(1000);
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const delayedInterval = sourceInterval.pipe(delay(10), take(4));
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​
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const exhaustSub = merge(
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  // delay 10ms, then start interval emitting 4 values
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  delayedInterval,
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  // emit immediately
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  of(true)
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)
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  .pipe(exhaustMap(_ => sourceInterval.pipe(take(5))))
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  /*
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   *  The first emitted value (of(true)) will be mapped
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   *  to an interval observable emitting 1 value every
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   *  second, completing after 5.
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   *  Because the emissions from the delayed interval
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   *  fall while this observable is still active they will be ignored.
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   *
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   *  Contrast this with concatMap which would queue,
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   *  switchMap which would switch to a new inner observable each emission,
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   *  and mergeMap which would maintain a new subscription for each emitted value.
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   */
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  // output: 0, 1, 2, 3, 4
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  .subscribe(val => console.log(val));
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Example 2: Another exhaustMap with interval
( Stackblitz | jsBin | jsFiddle )
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// RxJS v6+
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import { interval } from 'rxjs';
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import { exhaustMap, tap, take } from 'rxjs/operators';
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​
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const firstInterval = interval(1000).pipe(take(10));
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const secondInterval = interval(1000).pipe(take(2));
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​
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const exhaustSub = firstInterval
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  .pipe(
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    exhaustMap(f => {
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      console.log(`Emission Corrected of first interval: ${f}`);
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      return secondInterval;
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    })
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  )
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  /*
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                When we subscribed to the first interval, it starts to emit a values (starting 0).
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                This value is mapped to the second interval which then begins to emit (starting 0).  
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                While the second interval is active, values from the first interval are ignored.
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                We can see this when firstInterval emits number 3,6, and so on...
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​
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                  Output:
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                  Emission of first interval: 0
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                  0
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                  1
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                  Emission of first interval: 3
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                  0
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                  1
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                  Emission of first interval: 6
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                  0
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                  1
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                  Emission of first interval: 9
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                  0
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                  1
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              */
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  .subscribe(s => console.log(s));
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Related Recipes
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Outside Examples
exhaustMap for login effect in @ngrx example app
( Source )
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@Effect()
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  login$ = this.actions$.pipe(
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    ofType(AuthActionTypes.Login),
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    map((action: Login) => action.payload),
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    exhaustMap((auth: Authenticate) =>
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      this.authService
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        .login(auth)
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        .pipe(
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          map(user => new LoginSuccess({ user })),
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          catchError(error => of(new LoginFailure(error)))
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        )
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    )
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  );
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Additional Resources
​exhaustMap 📰 - Official docs
​exhaustMap - In Depth Dev Reference
📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/exhaustMap.ts​
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Last modified 6mo ago
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Contents
Map to inner observable, ignore other values until that observable completes.