takeWhile

signature: takeWhile(predicate: function(value, index): boolean, inclusive?: boolean): Observable

Emit values until provided expression is false.

💡 When the optional inclusive parameter is set to true it will also emit the first item that didn't pass the predicate.

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Examples

Example 1: Take values under limit

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { of } from 'rxjs';
import { takeWhile } from 'rxjs/operators';
//emit 1,2,3,4,5
const source$ = of(1, 2, 3, 4, 5);
//allow values until value from source is greater than 4, then complete
source$
.pipe(takeWhile(val => val <= 4))
// log: 1,2,3,4
.subscribe(val => console.log(val));

Example 2: (v6.4+) takeWhile with inclusive flag

( StackBlitz )

// RxJS v6.4+
import { of } from 'rxjs';
import { takeWhile, filter } from 'rxjs/operators';
const source$ = of(1, 2, 3, 9);
source$
// with inclusive flag, the value causing the predicate to return false will also be emitted
.pipe(takeWhile(val => val <= 3, true))
// log: 1, 2, 3, 9
.subscribe(console.log);

Example 3: Difference between takeWhile and filter

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { of } from 'rxjs';
import { takeWhile, filter } from 'rxjs/operators';
// emit 3, 3, 3, 9, 1, 4, 5, 8, 96, 3, 66, 3, 3, 3
const source$ = of(3, 3, 3, 9, 1, 4, 5, 8, 96, 3, 66, 3, 3, 3);
// allow values until value from source equals 3, then complete
source$
.pipe(takeWhile(it => it === 3))
// log: 3, 3, 3
.subscribe(val => console.log('takeWhile', val));
source$
.pipe(filter(it => it === 3))
// log: 3, 3, 3, 3, 3, 3, 3
.subscribe(val => console.log('filter', val));

Additional Resources

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