💡 If you want corresponding emissions from multiple observables as they occur, try zip!
⚠ If an inner observable does not complete forkJoin will never emit a value!
This operator is best used when you have a group of observables and only care
about the final emitted value of each. One common use case for this is if you
wish to issue multiple requests on page load (or some other event) and only want
to take action when a response has been received for all. In this way it is
similar to how you might use
Promise.all.
Be aware that if any of the inner observables supplied to forkJoin error you
will lose the value of any other observables that would or have already
completed if you do not catch the
error correctly on the inner observable.
If you are only concerned with all inner observables completing successfully you
can catch the error on the outside.
It's also worth noting that if you have an observable that emits more than one
item, and you are concerned with the previous emissions forkJoin is not the
correct choice. In these cases you may be better off with an operator like
combineLatest or zip.
( StackBlitz )
// RxJS v6.5+
import { ajax } from 'rxjs/ajax';
import { forkJoin } from 'rxjs';
/*
when all observables complete, provide the last
emitted value from each as dictionary
*/
forkJoin(
// as of RxJS 6.5+ we can use a dictionary of sources
{
google: ajax.getJSON('https://api.github.com/users/google'),
microsoft: ajax.getJSON('https://api.github.com/users/microsoft'),
users: ajax.getJSON('https://api.github.com/users')
}
)
// { google: object, microsoft: object, users: array }
.subscribe(console.log);( StackBlitz )
// RxJS v6+
import { interval, forkJoin, of } from 'rxjs';
import { delay, take } from 'rxjs/operators';
const myPromise = val =>
new Promise(resolve =>
setTimeout(() => resolve(`Promise Resolved: ${val}`), 5000)
);
/*
when all observables complete, give the last
emitted value from each as an array
*/
const example = forkJoin({
//emit 'Hello' immediately
sourceOne: of('Hello'),
//emit 'World' after 1 second
sourceTwo: of('World').pipe(delay(1000)),
//emit 0 after 1 second
sourceThree: interval(1000).pipe(take(1)),
//emit 0...1 in 1 second interval
sourceFour: interval(1000).pipe(take(2)),
//promise that resolves to 'Promise Resolved' after 5 seconds
sourceFive: myPromise('RESULT')
});
/*
* Output:
* {
* sourceOne: "Hello",
* sourceTwo: "World",
* sourceThree: 0,
* sourceFour: 1,
* sourceFive: "Promise Resolved: RESULT"
* }
*/
const subscribe = example.subscribe(val => console.log(val));( StackBlitz )
// RxJS v6+
import { mergeMap } from 'rxjs/operators';
import { forkJoin, of } from 'rxjs';
const myPromise = val =>
new Promise(resolve =>
setTimeout(() => resolve(`Promise Resolved: ${val}`), 5000)
);
const source = of([1, 2, 3, 4, 5]);
//emit array of all 5 results
const example = source.pipe(mergeMap(q => forkJoin(...q.map(myPromise))));
/*
output:
[
"Promise Resolved: 1",
"Promise Resolved: 2",
"Promise Resolved: 3",
"Promise Resolved: 4",
"Promise Resolved: 5"
]
*/
const subscribe = example.subscribe(val => console.log(val));( StackBlitz )
// RxJS v6+
import { delay, catchError } from 'rxjs/operators';
import { forkJoin, of, throwError } from 'rxjs';
/*
If any inner observables error, the error result
will be emitted by catchError.
*/
const example = forkJoin({
// emit 'Hello' immediately
sourceOne: of('Hello'),
// emit 'World' after 1 second
sourceTwo: of('World').pipe(delay(1000)),
// throw error
sourceThree: throwError('This will error')
}).pipe(catchError(error => of(error)));
// output: 'This will Error'
const subscribe = example.subscribe(val => console.log(val));( StackBlitz )
// RxJS v6+
import { delay, catchError } from 'rxjs/operators';
import { forkJoin, of, throwError } from 'rxjs';
/*
Emit values from successfully completed
inner observables.
*/
const example = forkJoin({
// emit 'Hello' immediately
sourceOne: of('Hello'),
// emit 'World' after 1 second
sourceTwo: of('World').pipe(delay(1000)),
// throw error
sourceThree: throwError('This will error').pipe(catchError(error => of(error)))
});
/*
* Output:
* {
* sourceOne: "Hello",
* sourceTwo: "World",
* sourceThree: "This will error"
* }
*/
const subscribe = example.subscribe(val => console.log(val));( plunker )
@Injectable()
export class MyService {
makeRequest(value: string, delayDuration: number) {
// simulate http request
return of(`Complete: ${value}`).pipe(
delay(delayDuration)
);
}
}
@Component({
selector: 'my-app',
template: `
<div>
<h2>forkJoin Example</h2>
<ul>
<li> {{propOne}} </li>
<li> {{propTwo}} </li>
<li> {{propThree}} </li>
</ul>
</div>
`,
})
export class App {
public propOne: string;
public propTwo: string;
public propThree: string;
constructor(private _myService: MyService) {}
ngOnInit() {
// simulate 3 requests with different delays
forkJoin({
requestOne: this._myService.makeRequest('Request One', 2000),
requestTwo: this._myService.makeRequest('Request Two', 1000),
requestThree: this._myService.makeRequest('Request Three', 3000)
})
.subscribe(({requestOne, requestTwo, requestThree}) => {
this.propOne = requestOne;
this.propTwo = requestTwo;
this.propThree = requestThree;
});
}
}📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/observable/forkJoin.ts