@reactivex/rxjs/src/operator/reduce.ts

Reactive Extensions for modern JavaScript

import {Observable} from '../Observable';
import {Operator} from '../Operator';
import {Subscriber} from '../Subscriber';

/**
 * 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): R} accumulator The accumulator function
 * called on each source value.
 * @param {R} [seed] The initial accumulation value.
 * @return {Observable<R>} An observable of the accumulated values.
 * @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: R, value: T) => R, seed?: R): Observable<R> {
  return this.lift(new ReduceOperator(accumulator, seed));
}

export interface ReduceSignature<T> {
  <R>(accumulator: (acc: R, value: T) => R, seed?: R): Observable<R>;
}

export class ReduceOperator<T, R> implements Operator<T, R> {

  constructor(private accumulator: (acc: R, value: T) => R, private seed?: R) {
  }

  call(subscriber: Subscriber<R>, source: any): any {
    return source._subscribe(new ReduceSubscriber(subscriber, this.accumulator, this.seed));
  }
}

/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
export class ReduceSubscriber<T, R> extends Subscriber<T> {

  acc: T | R;
  hasSeed: boolean;
  hasValue: boolean = false;

  constructor(destination: Subscriber<R>,
              private accumulator: (acc: R, value: T) => R,
              seed?: R) {
    super(destination);
    this.acc = seed;
    this.accumulator = accumulator;
    this.hasSeed = typeof seed !== 'undefined';
  }

  protected _next(value: T) {
    if (this.hasValue || (this.hasValue = this.hasSeed)) {
      this._tryReduce(value);
    } else {
      this.acc = value;
      this.hasValue = true;
    }
  }

  private _tryReduce(value: T) {
    let result: any;
    try {
      result = this.accumulator(<R>this.acc, value);
    } catch (err) {
      this.destination.error(err);
      return;
    }
    this.acc = result;
  }

  protected _complete() {
    if (this.hasValue || this.hasSeed) {
      this.destination.next(this.acc);
    }
    this.destination.complete();
  }
}