Applies an accumulator function over the source Observable, and returns each intermediate result, with an optional seed value.

scan<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: T | 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 of the accumulated values.

Description

It's like reduce, but emits the current accumulation whenever the source emits a value.

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. Is similar to reduce, but emits the intermediate accumulations.

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

import { fromEvent } from 'rxjs';
import { scan, mapTo } from 'rxjs/operators';

const clicks = fromEvent(document, 'click');
const ones = clicks.pipe(mapTo(1));
const seed = 0;
const count = ones.pipe(scan((acc, one) => acc + one, seed));
count.subscribe(x => console.log(x));

Overloads

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

Parameters

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

Returns

OperatorFunction<T, R>

scan(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>

scan(accumulator: (acc: R, value: T, index: number) => R): OperatorFunction<T, R>

Parameters

accumulator

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

Returns

OperatorFunction<T, R>

See Also

• expand
• mergeScan
• reduce