forkJoin

signature: forkJoin(...args, selector : function): Observable

When all observables complete, emit the last emitted value from each.

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!

Why use forkJoin?

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 receieved 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 better off with an operator like combineLatest or zip.

Examples

Example 1: Observables completing after different durations

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { delay, take } from 'rxjs/operators';
import { forkJoin, of, interval } from 'rxjs';

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
  of('Hello'),
  //emit 'World' after 1 second
  of('World').pipe(delay(1000)),
  //emit 0 after 1 second
  interval(1000).pipe(take(1)),
  //emit 0...1 in 1 second interval
  interval(1000).pipe(take(2)),
  //promise that resolves to 'Promise Resolved' after 5 seconds
  myPromise('RESULT')
);
//output: ["Hello", "World", 0, 1, "Promise Resolved: RESULT"]
const subscribe = example.subscribe(val => console.log(val));

Example 2: Making a variable number of requests

( StackBlitz | jsBin | jsFiddle )

// 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));

Example 3: Handling errors on outside

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { delay, catchError } from 'rxjs/operators';
import { forkJoin, of, throwError } from 'rxjs';

/*
  when all observables complete, give the last
  emitted value from each as an array
*/
const example = forkJoin(
  //emit 'Hello' immediately
  of('Hello'),
  //emit 'World' after 1 second
  of('World').pipe(delay(1000)),
  // throw error
  _throw('This will error')
).pipe(catchError(error => of(error)));
//output: 'This will Error'
const subscribe = example.subscribe(val => console.log(val));

Example 4: Getting successful results when one inner observable errors

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { delay, catchError } from 'rxjs/operators';
import { forkJoin, of, throwError } from 'rxjs';

/*
  when all observables complete, give the last
  emitted value from each as an array
*/
const example = forkJoin(
  //emit 'Hello' immediately
  of('Hello'),
  //emit 'World' after 1 second
  of('World').pipe(delay(1000)),
  // throw error
  _throw('This will error').pipe(catchError(error => of(error)))
);
//output: ["Hello", "World", "This will error"]
const subscribe = example.subscribe(val => console.log(val));

Example 5: forkJoin in Angular

( 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(
      this._myService.makeRequest('Request One', 2000),
      this._myService.makeRequest('Request Two', 1000),
      this._myService.makeRequest('Request Three', 3000)
    )
    .subscribe(([res1, res2, res3]) => {
      this.propOne = res1;
      this.propTwo = res2;
      this.propThree = res3;
    });
  }
}

Additional Resources

• forkJoin - Official docs

Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/observable/ForkJoinObservable.ts