/
reduce.ts
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/
reduce.ts
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import { Observable } from '../Observable';
import { scan } from './scan';
import { takeLast } from './takeLast';
import { defaultIfEmpty } from './defaultIfEmpty';
import { OperatorFunction } from '../types';
import { pipe } from '../util/pipe';
/* tslint:disable:max-line-length */
export function reduce<V, A = V>(accumulator: (acc: A|V, value: V, index: number) => A): OperatorFunction<V, V|A>;
export function reduce<V, A>(accumulator: (acc: A, value: V, index: number) => A, seed: A): OperatorFunction<V, A>;
export function reduce<V, A, S = A>(accumulator: (acc: A|S, value: V, index: number) => A, seed: S): OperatorFunction<V, A>;
/* tslint:enable:max-line-length */
/**
* Applies an accumulator function over the source Observable, and returns the
* accumulated result when the source completes, given an optional seed value.
*
* <span class="informal">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.</span>
*
* ![](reduce.png)
*
* Like
* [Array.prototype.reduce()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/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 {@link scan} followed by operator {@link 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
* ```ts
* 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));
* ```
*
* @see {@link count}
* @see {@link expand}
* @see {@link mergeScan}
* @see {@link scan}
*
* @param {function(acc: A, value: V, index: number): A} accumulator The accumulator function
* called on each source value.
* @param {A} [seed] The initial accumulation value.
* @return {Observable<A>} An Observable that emits a single value that is the
* result of accumulating the values emitted by the source Observable.
* @method reduce
* @owner Observable
*/
export function reduce<V, A>(accumulator: (acc: V | A, value: V, index?: number) => A, seed?: any): OperatorFunction<V, V | A> {
// providing a seed of `undefined` *should* be valid and trigger
// hasSeed! so don't use `seed !== undefined` checks!
// For this reason, we have to check it here at the original call site
// otherwise inside Operator/Subscriber we won't know if `undefined`
// means they didn't provide anything or if they literally provided `undefined`
if (arguments.length >= 2) {
return function reduceOperatorFunctionWithSeed(source: Observable<V>): Observable<V | A> {
return pipe(
scan(accumulator, seed),
takeLast(1),
defaultIfEmpty(seed),
)(source);
};
}
return function reduceOperatorFunction(source: Observable<V>): Observable<V | A> {
return pipe(
scan<V, V | A>((acc, value, index) => accumulator(acc, value, index + 1)),
takeLast(1),
)(source);
};
}