takeWhile

takeWhile(predicate) will emit values from source while they satisfy given predicate, and will complete once emitted value dissatisfies the predicate:

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const { rxObserver } = require('api/v0.3');
const { timer } = require('rxjs');
const { takeWhile } = require('rxjs/operators');

const source$ = timer(0, 100);

source$.subscribe(rxObserver('source$'));
source$.pipe(
    takeWhile(n => n < 5)
  )
  .subscribe(rxObserver('takeWhile(n < 5)'));
⚠️ Execution time is limited to 1000ms
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Also see take and takeUntil operators