| import { scan } from './scan'; |
| import { takeLast } from './takeLast'; |
| import { defaultIfEmpty } from './defaultIfEmpty'; |
| import { pipe } from '../util/pipe'; |
| /* 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> |
| * |
| * <img src="./img/reduce.png" width="100%"> |
| * |
| * 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 <caption>Count the number of click events that happened in 5 seconds</caption> |
| * var clicksInFiveSeconds = Rx.Observable.fromEvent(document, 'click') |
| * .takeUntil(Rx.Observable.interval(5000)); |
| * var ones = clicksInFiveSeconds.mapTo(1); |
| * var seed = 0; |
| * var count = ones.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(accumulator, seed) { |
| // 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) { |
| return pipe(scan(accumulator, seed), takeLast(1), defaultIfEmpty(seed))(source); |
| }; |
| } |
| return function reduceOperatorFunction(source) { |
| return pipe(scan((acc, value, index) => { |
| return accumulator(acc, value, index + 1); |
| }), takeLast(1))(source); |
| }; |
| } |
| //# sourceMappingURL=reduce.js.map |