concatAll

signature: concatAll(): Observable

Collect observables and subscribe to next when previous completes.

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⚠ Be wary of backpressure when the source emits at a faster pace than inner observables complete!

💡 In many cases you can use concatMap as a single operator instead!

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Examples

( example tests )

Example 1: concatAll with observable

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { map, concatAll } from 'rxjs/operators';
import { of, interval } from 'rxjs';

//emit a value every 2 seconds
const source = interval(2000);
const example = source.pipe(
  //for demonstration, add 10 to and return as observable
  map(val => of(val + 10)),
  //merge values from inner observable
  concatAll()
);
//output: 'Example with Basic Observable 10', 'Example with Basic Observable 11'...
const subscribe = example.subscribe(val =>
  console.log('Example with Basic Observable:', val)
);

Example 2: concatAll with promise

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { map, concatAll } from 'rxjs/operators';
import { interval } from 'rxjs';

//create and resolve basic promise
const samplePromise = val => new Promise(resolve => resolve(val));
//emit a value every 2 seconds
const source = interval(2000);

const example = source.pipe(
  map(val => samplePromise(val)),
  //merge values from resolved promise
  concatAll()
);
//output: 'Example with Promise 0', 'Example with Promise 1'...
const subscribe = example.subscribe(val =>
  console.log('Example with Promise:', val)
);

Example 3: Delay while inner observables complete

( StackBlitz | jsBin | jsFiddle )

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

const obs1 = interval(1000).pipe(take(5));
const obs2 = interval(500).pipe(take(2));
const obs3 = interval(2000).pipe(take(1));
//emit three observables
const source = of(obs1, obs2, obs3);
//subscribe to each inner observable in order when previous completes
const example = source.pipe(concatAll());
/*
  output: 0,1,2,3,4,0,1,0
  How it works...
  Subscribes to each inner observable and emit values, when complete subscribe to next
  obs1: 0,1,2,3,4 (complete)
  obs2: 0,1 (complete)
  obs3: 0 (complete)
*/

const subscribe = example.subscribe(val => console.log(val));

Related Recipes

• Progress Bar

Additional Resources

• concatAll 📰 - Official docs

• concatAll - In Depth Dev Reference

• Flatten a higher order observable with concatAll in RxJS 🎥 💵 - André Staltz

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📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/concatAll.ts