API / rxjs/index

forkJoin

Joins last values emitted by passed Observables.

forkJoin<T>(...sources: Array<ObservableInput<T> | ObservableInput<T>[] | Function>): Observable<T[]>

Parameters

sources

Any number of Observables provided either as an array or as an arguments passed directly to the operator.

Returns

Observable<T[]>: Observable emitting either an array of last values emitted by passed Observables or value from project function.

Description

Wait for Observables to complete and then combine last values they emitted.

forkJoin is an operator that takes any number of Observables which can be passed either as an array or directly as arguments. If no input Observables are provided, resulting stream will complete immediately.

forkJoin will wait for all passed Observables to complete and then it will emit an array with last values from corresponding Observables. So if you pass n Observables to the operator, resulting array will have n values, where first value is the last thing emitted by the first Observable, second value is the last thing emitted by the second Observable and so on. That means forkJoin will not emit more than once and it will complete after that. If you need to emit combined values not only at the end of lifecycle of passed Observables, but also throughout it, try out combineLatest or zip instead.

In order for resulting array to have the same length as the number of input Observables, whenever any of that Observables completes without emitting any value, forkJoin will complete at that moment as well and it will not emit anything either, even if it already has some last values from other Observables. Conversely, if there is an Observable that never completes, forkJoin will never complete as well, unless at any point some other Observable completes without emitting value, which brings us back to the previous case. Overall, in order for forkJoin to emit a value, all Observables passed as arguments have to emit something at least once and complete.

If any input Observable errors at some point, forkJoin will error as well and all other Observables will be immediately unsubscribed.

Optionally forkJoin accepts project function, that will be called with values which normally would land in emitted array. Whatever is returned by project function, will appear in output Observable instead. This means that default project can be thought of as a function that takes all its arguments and puts them into an array. Note that project function will be called only when output Observable is supposed to emit a result.

Examples

Use forkJoin with operator emitting immediately

import { forkJoin, of } from 'rxjs'; const observable = forkJoin( of(1, 2, 3, 4), of(5, 6, 7, 8), ); observable.subscribe( value => console.log(value), err => {}, () => console.log('This is how it ends!'), ); // Logs: // [4, 8] // "This is how it ends!"

Use forkJoin with operator emitting after some time

import { forkJoin, interval } from 'rxjs'; import { take } from 'rxjs/operators'; const observable = forkJoin( interval(1000).pipe(take(3)), // emit 0, 1, 2 every second and complete interval(500).pipe(take(4)), // emit 0, 1, 2, 3 every half a second and complete ); observable.subscribe( value => console.log(value), err => {}, () => console.log('This is how it ends!'), ); // Logs: // [2, 3] after 3 seconds // "This is how it ends!" immediately after

Use forkJoin with project function

import { forkJoin, interval } from 'rxjs'; import { take } from 'rxjs/operators'; const observable = forkJoin( interval(1000).pipe(take(3)), // emit 0, 1, 2 every second and complete interval(500).pipe(take(4)), // emit 0, 1, 2, 3 every half a second and complete ).pipe( map(([n, m]) => n + m), ); observable.subscribe( value => console.log(value), err => {}, () => console.log('This is how it ends!'), ); // Logs: // 5 after 3 seconds // "This is how it ends!" immediately after

Overloads

forkJoin(sources: [ObservableInput<T>]): Observable<T[]>

Parameters

sources

Type: [ObservableInput].

Returns

Observable<T[]>
forkJoin(sources: [ObservableInput<T>, ObservableInput<T2>]): Observable<[T, T2]>

Parameters

sources

Type: [ObservableInput, ObservableInput].

Returns

Observable<[T, T2]>
forkJoin(sources: [ObservableInput<T>, ObservableInput<T2>, ObservableInput<T3>]): Observable<[T, T2, T3]>

Parameters

sources

Type: [ObservableInput, ObservableInput, ObservableInput].

Returns

Observable<[T, T2, T3]>
forkJoin(sources: [ObservableInput<T>, ObservableInput<T2>, ObservableInput<T3>, ObservableInput<T4>]): Observable<[T, T2, T3, T4]>

Parameters

sources

Type: [ObservableInput, ObservableInput, ObservableInput, ObservableInput].

Returns

Observable<[T, T2, T3, T4]>
forkJoin(sources: [ObservableInput<T>, ObservableInput<T2>, ObservableInput<T3>, ObservableInput<T4>, ObservableInput<T5>]): Observable<[T, T2, T3, T4, T5]>

Parameters

sources

Type: [ObservableInput, ObservableInput, ObservableInput, ObservableInput, ObservableInput].

Returns

Observable<[T, T2, T3, T4, T5]>
forkJoin(sources: [ObservableInput<T>, ObservableInput<T2>, ObservableInput<T3>, ObservableInput<T4>, ObservableInput<T5>, ObservableInput<T6>]): Observable<[T, T2, T3, T4, T5, T6]>

Parameters

sources

Type: [ObservableInput, ObservableInput, ObservableInput, ObservableInput, ObservableInput, ObservableInput].

Returns

Observable<[T, T2, T3, T4, T5, T6]>
forkJoin(sources: Array<ObservableInput<T>>): Observable<T[]>

Parameters

sources

Type: Array>.

Returns

Observable<T[]>
forkJoin(v1: ObservableInput<T>): Observable<T[]>

Parameters

v1

Type: ObservableInput.

Returns

Observable<T[]>
forkJoin(v1: ObservableInput<T>, v2: ObservableInput<T2>): Observable<[T, T2]>

Parameters

v1

Type: ObservableInput.
v2

Type: ObservableInput.

Returns

Observable<[T, T2]>
forkJoin(v1: ObservableInput<T>, v2: ObservableInput<T2>, v3: ObservableInput<T3>): Observable<[T, T2, T3]>

Parameters

v1

Type: ObservableInput.
v2

Type: ObservableInput.
v3

Type: ObservableInput.

Returns

Observable<[T, T2, T3]>
forkJoin(v1: ObservableInput<T>, v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>): Observable<[T, T2, T3, T4]>

Parameters

v1

Type: ObservableInput.
v2

Type: ObservableInput.
v3

Type: ObservableInput.
v4

Type: ObservableInput.

Returns

Observable<[T, T2, T3, T4]>
forkJoin(v1: ObservableInput<T>, v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>, v5: ObservableInput<T5>): Observable<[T, T2, T3, T4, T5]>

Parameters

v1

Type: ObservableInput.
v2

Type: ObservableInput.
v3

Type: ObservableInput.
v4

Type: ObservableInput.
v5

Type: ObservableInput.

Returns

Observable<[T, T2, T3, T4, T5]>
forkJoin(v1: ObservableInput<T>, v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>, v5: ObservableInput<T5>, v6: ObservableInput<T6>): Observable<[T, T2, T3, T4, T5, T6]>

Parameters

v1

Type: ObservableInput.
v2

Type: ObservableInput.
v3

Type: ObservableInput.
v4

Type: ObservableInput.
v5

Type: ObservableInput.
v6

Type: ObservableInput.

Returns

Observable<[T, T2, T3, T4, T5, T6]>
forkJoin(...args: Array<ObservableInput<any> | Function>): Observable<any>

Parameters

args

Type: Array | Function>.

Returns

Observable<any>
forkJoin(...sources: ObservableInput<T>[]): Observable<T[]>

Parameters

sources

Type: ObservableInput[].

Returns

Observable<T[]>

See Also