observable.js

const compose = (f, g) => x => f(g(x));
const identity = x => x;
const noop = () => {};

function Disposable(fn) {
    this.dispose = fn;
}

Disposable.of = fn => new Disposable(fn);

function Observer(onNext = identity, onComplete = noop) {
    this.onNext = onNext;
    this.onComplete = onComplete;
}

Observer.of = (onNext, onComplete) => new Observer(onNext, onComplete);

Observer.prototype.map = function (onNext) {
    return Observer.of(compose(onNext, this.onNext), this.onComplete);
}

function Observable(fn, observer = Observer.of()) {
    this.fn = fn;
    this.observer = observer;
}

Observable.of = (fn, observer) =>  new Observable(fn, observer);

Observable.prototype.subscribe = function (onNext, onComplete) {
    return Disposable.of(this.fn(Observer.of(this.observer.map(onNext).onNext, onComplete)));
}

Observable.prototype.map = function(onNext) {
    return Observable.of(this.fn, this.observer.map(onNext));
}


const fromEvent = (source, event) => {
    return Observable.of(observer => {
       source.addEventListener(event, observer.onNext);

       return () => {
           source.removeEventListener(event, observer.onNext);
       } 
    });
}

const fromArray = arr => {
    return Observable.of(observer => {
       arr.forEach(observer.onNext);
       observer.onComplete(); 
    });
}

// Very basic example:

const basicObservable = Observable.of(observer => {
    observer.onNext(1);
    observer.onNext(2);
    observer.onNext(3);
    observer.onComplete();
    return () => console.log('clean up/dispose code here');
});

basicObservable
.map(x => x * 2)
.subscribe(
    x => console.log('x', x),
    () => console.log('basicObservable complete')
);


// Observable array

fromArray([5, 6, 7, 8])
.map(x => x + 1)
.subscribe(
    x => console.log('Array value ', x),
    () => console.log('Array complete')
)


// Mouse move example:

const observable = fromEvent(document, 'mousemove');

const mouseSubscription = observable
.map(e => e.clientX)
.subscribe(
    x => console.log('mouse ', x)
);

setTimeout(() => {
    console.log('disposing of mouse');
    mouseSubscription.dispose();
}, 5000);