API / rxjs/operators

count

Counts the number of emissions on the source and emits that number when the source completes.

count<T>(predicate?: (value: T, index: number, source: Observable<T>) => boolean): OperatorFunction<T, number>

Parameters

predicate

Optional. Default is undefined.

A boolean function to select what values are to be counted. It is provided with arguments of:

  • value: the value from the source Observable.
  • index: the (zero-based) "index" of the value from the source Observable.
  • source: the source Observable instance itself.

Returns

OperatorFunction<T, number>: An Observable of one number that represents the count as described above.

Description

Tells how many values were emitted, when the source completes.

count transforms an Observable that emits values into an Observable that emits a single value that represents the number of values emitted by the source Observable. If the source Observable terminates with an error, count will pass this error notification along without emitting a value first. If the source Observable does not terminate at all, count will neither emit a value nor terminate. This operator takes an optional predicate function as argument, in which case the output emission will represent the number of source values that matched true with the predicate.

Examples

Counts how many seconds have passed before the first click happened

import { fromEvent, interval } from 'rxjs'; import { count, takeUntil } from 'rxjs/operators'; const seconds = interval(1000); const clicks = fromEvent(document, 'click'); const secondsBeforeClick = seconds.pipe(takeUntil(clicks)); const result = secondsBeforeClick.pipe(count()); result.subscribe(x => console.log(x));

Counts how many odd numbers are there between 1 and 7

import { range } from 'rxjs'; import { count } from 'rxjs/operators'; const numbers = range(1, 7); const result = numbers.pipe(count(i => i % 2 === 1)); result.subscribe(x => console.log(x)); // Results in: // 4

See Also