| import { Observable } from '../Observable'; |
| import { scan } from './scan'; |
| import { takeLast } from './takeLast'; |
| import { defaultIfEmpty } from './defaultIfEmpty'; |
| import { OperatorFunction, MonoTypeOperatorFunction } from '../types'; |
| import { pipe } from '../util/pipe'; |
| |
| /* tslint:disable:max-line-length */ |
| export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed: R): OperatorFunction<T, R>; |
| export function reduce<T>(accumulator: (acc: T, value: T, index: number) => T, seed?: T): MonoTypeOperatorFunction<T>; |
| export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R): OperatorFunction<T, R>; |
| /* 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: R, value: T, index: number): R} accumulator The accumulator function |
| * called on each source value. |
| * @param {R} [seed] The initial accumulation value. |
| * @return {Observable<R>} 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<T, R>(accumulator: (acc: T | R, value: T, index?: number) => T | R, seed?: T | R): OperatorFunction<T, T | R> { |
| // 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<T>): Observable<T | R> { |
| return pipe(scan(accumulator, seed), takeLast(1), defaultIfEmpty(seed))(source); |
| }; |
| } |
| return function reduceOperatorFunction(source: Observable<T>): Observable<T | R> { |
| return pipe( |
| scan<T, T | R>((acc, value, index) => accumulator(acc, value, index + 1)), |
| takeLast(1), |
| )(source); |
| }; |
| } |