forkJoin

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

When all observables complete emit the last value from each.


:bulb: If you want corresponding emissions from multiple observables as they occur, try zip!


Examples

Example 1: Making variable number of requests

( jsBin | jsFiddle )

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 = Rx.Observable.forkJoin(
  //emit 'Hello' immediately
  Rx.Observable.of('Hello'),
  //emit 'World' after 1 second
  Rx.Observable.of('World').delay(1000),
  //emit 0 after 1 second
  Rx.Observable.interval(1000).take(1),
  //emit 0...1 in 1 second interval
  Rx.Observable.interval(1000).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));

//make 5 requests
const queue = Rx.Observable.of([1,2,3,4,5]);
//emit array of all 5 results
const exampleTwo = queue
  .mergeMap(q => Rx.Observable.forkJoin(...q.map(myPromise)));
/*
  output:
  [
   "Promise Resolved: 1", 
   "Promise Resolved: 2", 
   "Promise Resolved: 3", 
   "Promise Resolved: 4",    
   "Promise Resolved: 5"
  ]
*/
const subscribeTwo = exampleTwo.subscribe(val => console.log(val));

Additional Resources


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

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