concatMapTo

signature: concatMapTo(observable: Observable, resultSelector: function): Observable

Subscribe to provided observable when previous completes, emit values.

Examples

Example 1: Map to basic observable

( jsBin | jsFiddle )

//emit value every 2 seconds
const interval = Rx.Observable.interval(2000);
const message = Rx.Observable.of('Second(s) elapsed!');
//when interval emits, subscribe to message until complete, merge for result
const example = interval.concatMapTo(message, (time, msg) => `${time} ${msg}`);
//log values
//output: '0 Second(s) elapsed', '1 Second(s) elapsed'
const subscribe = example.subscribe(val => console.log(val));
Example 2: Map to observable that emits at slower pace

( jsBin | jsFiddle )

import { interval } from 'rxjs/observable/interval';
import { concatMapTo, take } from 'rxjs/operators';
//emit value every 2 seconds
const interval = interval(2000);
//emit value every second for 5 seconds
const source = interval(1000).pipe(take(5));
/*
  ***Be Careful***: In situations like this where the source emits at a faster pace
  than the inner observable completes, memory issues can arise.
  (interval emits every 1 second, basicTimer completes every 5)
*/
//basicTimer will complete after 5 seconds, emitting 0,1,2,3,4
const example = interval.pipe(
  concatMapTo(
    source,
    (firstInterval, secondInterval) => `${firstInterval} ${secondInterval}`
  )
);
/*
  output: 0 0
          0 1
          0 2
          0 3
          0 4
          1 0
          1 1
          continued...

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

Additional Resources


:file_folder: Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/patching/operator/concatMapTo.ts

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