67 lines
3.1 KiB
JavaScript
67 lines
3.1 KiB
JavaScript
import { scan } from './scan';
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import { takeLast } from './takeLast';
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import { defaultIfEmpty } from './defaultIfEmpty';
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import { pipe } from '../util/pipe';
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/* tslint:enable:max-line-length */
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/**
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* Applies an accumulator function over the source Observable, and returns the
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* accumulated result when the source completes, given an optional seed value.
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*
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* <span class="informal">Combines together all values emitted on the source,
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* using an accumulator function that knows how to join a new source value into
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* the accumulation from the past.</span>
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*
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* <img src="./img/reduce.png" width="100%">
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*
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* Like
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* [Array.prototype.reduce()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce),
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* `reduce` applies an `accumulator` function against an accumulation and each
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* value of the source Observable (from the past) to reduce it to a single
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* value, emitted on the output Observable. Note that `reduce` will only emit
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* one value, only when the source Observable completes. It is equivalent to
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* applying operator {@link scan} followed by operator {@link last}.
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*
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* Returns an Observable that applies a specified `accumulator` function to each
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* item emitted by the source Observable. If a `seed` value is specified, then
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* that value will be used as the initial value for the accumulator. If no seed
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* value is specified, the first item of the source is used as the seed.
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*
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* @example <caption>Count the number of click events that happened in 5 seconds</caption>
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* var clicksInFiveSeconds = Rx.Observable.fromEvent(document, 'click')
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* .takeUntil(Rx.Observable.interval(5000));
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* var ones = clicksInFiveSeconds.mapTo(1);
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* var seed = 0;
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* var count = ones.reduce((acc, one) => acc + one, seed);
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* count.subscribe(x => console.log(x));
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*
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* @see {@link count}
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* @see {@link expand}
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* @see {@link mergeScan}
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* @see {@link scan}
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*
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* @param {function(acc: R, value: T, index: number): R} accumulator The accumulator function
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* called on each source value.
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* @param {R} [seed] The initial accumulation value.
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* @return {Observable<R>} An Observable that emits a single value that is the
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* result of accumulating the values emitted by the source Observable.
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* @method reduce
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* @owner Observable
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*/
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export function reduce(accumulator, seed) {
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// providing a seed of `undefined` *should* be valid and trigger
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// hasSeed! so don't use `seed !== undefined` checks!
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// For this reason, we have to check it here at the original call site
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// otherwise inside Operator/Subscriber we won't know if `undefined`
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// means they didn't provide anything or if they literally provided `undefined`
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if (arguments.length >= 2) {
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return function reduceOperatorFunctionWithSeed(source) {
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return pipe(scan(accumulator, seed), takeLast(1), defaultIfEmpty(seed))(source);
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};
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}
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return function reduceOperatorFunction(source) {
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return pipe(scan((acc, value, index) => {
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return accumulator(acc, value, index + 1);
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}), takeLast(1))(source);
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};
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}
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//# sourceMappingURL=reduce.js.map
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