count

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

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

Parameters

predicate

Optional. Default is undefined. A function that is used to analyze the value and the index and determine whether or not to increment the count. Return true to increment the count, and return false to keep the count the same. If the predicate is not provided, every value will be counted.

Returns

OperatorFunction<T, number>: A function that returns an Observable that emits one number that represents the count of emissions.

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

• max
• min
• reduce