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.
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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: 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: T[], value: T, index: number) => T[], seed?: T[]): OperatorFunction<T, T[]>
Parameters
| accumulator |
Type: (acc: T[], value: T, index: number) => T[].
|
| seed |
Optional. Default is undefined.
Type: T[].
|
Returns
OperatorFunction<T, T[]>
|
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 |
Optional. Default is undefined.
Type: R.
|
Returns
OperatorFunction<T, R>
|