Promise.resolve() but for Observables (RxJS5)

Question

Ok so just to learn RxJS better, decided to try my hand at creating a custom Rx operator.

So here is a simple one that works fine:

Rx.Observable.prototype.multiply = function (input) {

    const source = this;

    return Rx.Observable.create(function (obs) {

        return source.subscribe(function(val){
            obs.next(input*val);
        });
    });

};

and we can use it like so:

  const obs = Rx.Observable.interval(1000)
    .multiply(4)
    .forEach(function (v) {
        console.log(v);
    });

however, what if we get something a little more complicated, for example if our operator takes a function instead of a static value.

Rx.Observable.prototype.handleFn = function (fn) {

    const source = this;

    return Rx.Observable.create(function (obs) {

        return source.subscribe(function(val){
            obs.next(fn.call(obs,val));
        });
    });

};

the above is all good and well, but what if we need to handle an Rx.Observable that gets returned from the input function, something like this:

const obs = Rx.Observable.interval(1000)
    .handleFn(function(){
        return Rx.Observable.timer(399);
    })
    .forEach(function (v) {
        console.log(v);
    });

is there some sort of Promise.resolve() but for Observables so that I can resolve the result of Rx.Observable.timer()? Will check out the source code for Rx.Observable.prototype.flatMap etc.!

Answer 1

You could use a .mergeAll(), like this:

Rx.Observable.prototype.handleFn = function (fn) {

    const source = this;

    return Rx.Observable.create(function (obs) {

        return source.subscribe(function(val){
            obs.next(fn.call(obs,val));
        });
    });

};

const obs = Rx.Observable.interval(1000)
    .handleFn(function(){
        return Rx.Observable.timer(150).mapTo(Math.random());
    })
    .mergeAll();

obs.subscribe(x => console.log(x));

see live JSBin here.

---

Option 2: Instead of mergeAll, you could also do the following:

Rx.Observable.prototype.handleFn = function (fn) {
    const source = this;

    return Rx.Observable.create(function (obs) {
        return source.subscribe(function(val){
            fn.call(obs,val).subscribe(x => obs.next(x));
        });
    });
};

---

Additional Note: If you want to see how this is properly implemented, take a look(as you already mentioned yourself) at the source of flatMap, switchMap, concatMap.

Answer 2

So looking at the source, it's written in TypeScript (good on them). But I don't know TS so I transpiled it to ES5. Turns out that flatMap and mergeMap should be equivalent operators, and mergeMap is the only one in the source, and here it is transpiled:

"use strict";
var __extends = (this && this.__extends) || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
};
var subscribeToResult_1 = require("../util/subscribeToResult");
var OuterSubscriber_1 = require("../OuterSubscriber");
/* tslint:disable:max-line-length */
/**
 * Projects each source value to an Observable which is merged in the output
 * Observable.
 *
 * <span class="informal">Maps each value to an Observable, then flattens all of
 * these inner Observables using {@link mergeAll}.</span>
 *
 * <img src="./img/mergeMap.png" width="100%">
 *
 * Returns an Observable that emits items based on applying a function that you
 * supply to each item emitted by the source Observable, where that function
 * returns an Observable, and then merging those resulting Observables and
 * emitting the results of this merger.
 *
 * @example <caption>Map and flatten each letter to an Observable ticking every 1 second</caption>
 * var letters = Rx.Observable.of('a', 'b', 'c');
 * var result = letters.mergeMap(x =>
 *   Rx.Observable.interval(1000).map(i => x+i)
 * );
 * result.subscribe(x => console.log(x));
 *
 * // Results in the following:
 * // a0
 * // b0
 * // c0
 * // a1
 * // b1
 * // c1
 * // continues to list a,b,c with respective ascending integers
 *
 * @see {@link concatMap}
 * @see {@link exhaustMap}
 * @see {@link merge}
 * @see {@link mergeAll}
 * @see {@link mergeMapTo}
 * @see {@link mergeScan}
 * @see {@link switchMap}
 *
 * @param {function(value: T, ?index: number): Observable} project A function
 * that, when applied to an item emitted by the source Observable, returns an
 * Observable.
 * @param {function(outerValue: T, innerValue: I, outerIndex: number, innerIndex: number): any} [resultSelector]
 * A function to produce the value on the output Observable based on the values
 * and the indices of the source (outer) emission and the inner Observable
 * emission. The arguments passed to this function are:
 * - `outerValue`: the value that came from the source
 * - `innerValue`: the value that came from the projected Observable
 * - `outerIndex`: the "index" of the value that came from the source
 * - `innerIndex`: the "index" of the value from the projected Observable
 * @param {number} [concurrent=Number.POSITIVE_INFINITY] Maximum number of input
 * Observables being subscribed to concurrently.
 * @return {Observable} An Observable that emits the result of applying the
 * projection function (and the optional `resultSelector`) to each item emitted
 * by the source Observable and merging the results of the Observables obtained
 * from this transformation.
 * @method mergeMap
 * @owner Observable
 */


function mergeMap(project, resultSelector, concurrent) {
    if (concurrent === void 0) { concurrent = Number.POSITIVE_INFINITY; }
    if (typeof resultSelector === 'number') {
        concurrent = resultSelector;
        resultSelector = null;
    }
    return this.lift(new MergeMapOperator(project, resultSelector, concurrent));
}


exports.mergeMap = mergeMap;


var MergeMapOperator = (function () {
    function MergeMapOperator(project, resultSelector, concurrent) {
        if (concurrent === void 0) { concurrent = Number.POSITIVE_INFINITY; }
        this.project = project;
        this.resultSelector = resultSelector;
        this.concurrent = concurrent;
    }
    MergeMapOperator.prototype.call = function (observer, source) {
        return source.subscribe(new MergeMapSubscriber(observer, this.project, this.resultSelector, this.concurrent));
    };
    return MergeMapOperator;
}());

exports.MergeMapOperator = MergeMapOperator;
/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
var MergeMapSubscriber = (function (_super) {

    __extends(MergeMapSubscriber, _super);

    function MergeMapSubscriber(destination, project, resultSelector, concurrent) {
        if (concurrent === void 0) { concurrent = Number.POSITIVE_INFINITY; }
        var _this = _super.call(this, destination) || this;
        _this.project = project;
        _this.resultSelector = resultSelector;
        _this.concurrent = concurrent;
        _this.hasCompleted = false;
        _this.buffer = [];
        _this.active = 0;
        _this.index = 0;
        return _this;
    }
    MergeMapSubscriber.prototype._next = function (value) {
        if (this.active < this.concurrent) {
            this._tryNext(value);
        }
        else {
            this.buffer.push(value);
        }
    };
    MergeMapSubscriber.prototype._tryNext = function (value) {
        var result;
        var index = this.index++;
        try {
            result = this.project(value, index);
        }
        catch (err) {
            this.destination.error(err);
            return;
        }
        this.active++;
        this._innerSub(result, value, index);
    };


    MergeMapSubscriber.prototype._innerSub = function (ish, value, index) {
        this.add(subscribeToResult_1.subscribeToResult(this, ish, value, index));
    };


    MergeMapSubscriber.prototype._complete = function () {
        this.hasCompleted = true;
        if (this.active === 0 && this.buffer.length === 0) {
            this.destination.complete();
        }
    };


    MergeMapSubscriber.prototype.notifyNext = function (outerValue, innerValue, outerIndex, innerIndex, innerSub) {
        if (this.resultSelector) {
            this._notifyResultSelector(outerValue, innerValue, outerIndex, innerIndex);
        }
        else {
            this.destination.next(innerValue);
        }
    };


    MergeMapSubscriber.prototype._notifyResultSelector = function (outerValue, innerValue, outerIndex, innerIndex) {
        var result;
        try {
            result = this.resultSelector(outerValue, innerValue, outerIndex, innerIndex);
        }
        catch (err) {
            this.destination.error(err);
            return;
        }
        this.destination.next(result);
    };


    MergeMapSubscriber.prototype.notifyComplete = function (innerSub) {
        var buffer = this.buffer;
        this.remove(innerSub);
        this.active--;
        if (buffer.length > 0) {
            this._next(buffer.shift());
        }
        else if (this.active === 0 && this.hasCompleted) {
            this.destination.complete();
        }
    };


    return MergeMapSubscriber;


}(OuterSubscriber_1.OuterSubscriber));
exports.MergeMapSubscriber = MergeMapSubscriber;
//# sourceMappingURL=mergeMap.js.map