takeWhile

Emits values from the source observable while the predicate returns true. Completes immediately when the predicate returns false.

Marble Diagram

takeWhile(x => x < 5)

Example

//  Timeline:
//
//  num$      1     2     3     4     5     6     1     2 (ignored)
//  taken$    1     2     3     4     | (completes)
//
//  Explanation:
//  - takeWhile emits values while the predicate returns true
//  - Completes immediately when the predicate returns false
//  - No further values are emitted after completion

const num$ = source<number>();

const taken$ = num$.pipe(takeWhile((x) => x < 5));

const valueHistory: number[] = [];

taken$.subscribe((x) => {
    valueHistory.push(x);
});

num$.next(1); // logs: 1

assert.deepStrictEqual(valueHistory, [1]);

num$.next(2); // logs: 2

num$.next(3); // logs: 3

num$.next(4); // logs: 4

assert.deepStrictEqual(valueHistory, [1, 2, 3, 4]);

num$.next(5); // nothing logged (completes)

assert.deepStrictEqual(valueHistory, [1, 2, 3, 4]);

num$.next(6); // nothing logged (already completed)

assert.deepStrictEqual(valueHistory, [1, 2, 3, 4]);

num$.next(1); // logs: 1

assert.deepStrictEqual(valueHistory, [1, 2, 3, 4]);

View source on GitHub