Defining observable from scratch

observable_from_scratch.js

/**************************
Observable from scratch
***************************/

function Observable(subscribe) {
  this._subscribe = subscribe;
}

Observable.prototype.subscribe = function subscribe() {
  if (typeof arguments[0] === "function") {
    return this._subscribe({
      next: arguments[0],
      error: arguments[1],
      complete: arguments[2]
    });
  } else {
    this._subscribe(arguments[0]);
  } 
};

Observable.prototype.map = function map(projectionFunction) {
  var self = this;
  return new Observable(function(observer) {
    return self._subscribe({
      next: (x) => observer.next(projectionFunction(x)),
      error: (err) => observer.error(err),
      complete: () => observer.complete()
    });
  });
};

Observable.prototype.filter = function filter(predictableFunction) {
  var self = this;
  return new Observable(function(observer) {
    self._subscribe({
      next: (x) => {
        if (predictableFunction(x)) {
          observer.next(x);
        }
      },
      error: (err) => observer.error(err),
      complete: () => observer.complete()
    })
  });
};

// fromEvent -> Observable.fromEvent(dom, eventType)
Observable.fromEvent = function(dom, eventName) {
  return new Observable(function(observer) {
    var handler = (e) => observer.next(e);
    dom.addEventListener(eventName, handler);
    
    // subscription object     
    return {
      unsubscribe: () => {
        dom.removeEventListener(eventName, handler);
      }
    };
  });
};

var button = document.querySelector('#btn');
var o = Observable.fromEvent(button, 'click')
         .map((e) => e.type);

o.subscribe({
  next: (e) => console.log(e),
  error: () => {},
  complete: () => console.log('done')
});