takeWhile

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

Emit values until provided expression is false.

Examples

Example 1: Take values under limit

( jsBin | jsFiddle )

import { of } from 'rxjs/observable/of';
import { takeWhile } '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
const example = source.pipe(takeWhile(val => val <= 4));
//output: 1,2,3,4
const subscribe = example.subscribe(val => console.log(val));
Example 2: Difference between takeWhile() and filter()

( jsBin | jsFiddle )

import { of } from 'rxjs/observable/of';
import { takeWhile, filter } '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
// output: [3, 3, 3]
source
  .pipe(takeWhile(it => it === 3))
  .subscribe(val => console.log('takeWhile', val))


// output: [3, 3, 3, 3, 3, 3, 3]
source
  .pipe(filter(it => it === 3))
  .subscribe(val => console.log('filter', val));

Additional Resources


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

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