takeUntil

signature: takeUntil(notifier: Observable): Observable

Emit values until provided observable emits.

If you only need a specific number of values, try take!

Examples

Example 1: Take values until timer emits

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { interval, timer } from 'rxjs';
import { takeUntil } from 'rxjs/operators';

//emit value every 1s
const source = interval(1000);
//after 5 seconds, emit value
const timer$ = timer(5000);
//when timer emits after 5s, complete source
const example = source.pipe(takeUntil(timer$));
//output: 0,1,2,3
const subscribe = example.subscribe(val => console.log(val));

Example 2: Take the first 5 even numbers

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { interval } from 'rxjs/observable/interval';
import { takeUntil, filter, scan, map, withLatestFrom } from 'rxjs/operators';

//emit value every 1s
const source = interval(1000);
//is number even?
const isEven = val => val % 2 === 0;
//only allow values that are even
const evenSource = source.pipe(filter(isEven));
//keep a running total of the number of even numbers out
const evenNumberCount = evenSource.pipe(scan((acc, _) => acc + 1, 0));
//do not emit until 5 even numbers have been emitted
const fiveEvenNumbers = evenNumberCount.pipe(filter(val => val > 5));

const example = evenSource.pipe(
  //also give me the current even number count for display
  withLatestFrom(evenNumberCount),
  map(([val, count]) => `Even number (${count}) : ${val}`),
  //when five even numbers have been emitted, complete source observable
  takeUntil(fiveEvenNumbers)
);
/*
    Even number (1) : 0,
  Even number (2) : 2
    Even number (3) : 4
    Even number (4) : 6
    Even number (5) : 8
*/
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

• takeUntil - Official docs

• Avoiding takeUntil leaks - Angular in Depth

• Stopping a stream with takeUntil - John Linquist

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