API / rxjs/operators

reduce

Applies an accumulator function over the source Observable, and returns the accumulated result when the source completes, given an optional seed value.

reduce<T, R>(accumulator: (acc: R, value: T, index?: number) => R, seed?: R): OperatorFunction<T, R>

Parameters

accumulator

The accumulator function called on each source value.
seed

Optional. Default is undefined.

The initial accumulation value.

Returns

OperatorFunction<T, R>: An Observable that emits a single value that is the result of accumulating the values emitted by the source Observable.

Description

Combines together all values emitted on the source, using an accumulator function that knows how to join a new source value into the accumulation from the past.

Like Array.prototype.reduce(), reduce applies an accumulator function against an accumulation and each value of the source Observable (from the past) to reduce it to a single value, emitted on the output Observable. Note that reduce will only emit one value, only when the source Observable completes. It is equivalent to applying operator scan followed by operator last.

Returns an Observable that applies a specified accumulator function to each item emitted by the source Observable. If a seed value is specified, then that value will be used as the initial value for the accumulator. If no seed value is specified, the first item of the source is used as the seed.

Example

Count the number of click events that happened in 5 seconds

import { fromEvent, interval } from 'rxjs'; import { reduce, takeUntil, mapTo } from 'rxjs/operators'; const clicksInFiveSeconds = fromEvent(document, 'click').pipe( takeUntil(interval(5000)), ); const ones = clicksInFiveSeconds.pipe(mapTo(1)); const seed = 0; const count = ones.pipe(reduce((acc, one) => acc + one, seed)); count.subscribe(x => console.log(x));

Overloads

reduce(accumulator: (acc: T, value: T, index: number) => T, seed?: T): MonoTypeOperatorFunction<T>

Parameters

accumulator

Type: (acc: T, value: T, index: number) => T.
seed

Optional. Default is undefined.

Type: T.

Returns

MonoTypeOperatorFunction<T>
reduce(accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): OperatorFunction<T, T[]>

Parameters

accumulator

Type: (acc: T[], value: T, index: number) => T[].
seed

Type: T[].

Returns

OperatorFunction<T, T[]>
reduce(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction<T, R>

Parameters

accumulator

Type: (acc: R, value: T, index: number) => R.
seed

Optional. Default is undefined.

Type: R.

Returns

OperatorFunction<T, R>

See Also