playing with my own observable

observable.js

class Observable {
  
  constructor({ initFn, element, eventName }) {
    if (initFn) {
      this._initFn = initFn
    }
    else if (element && eventName) {
      this._event = {
        listener: event => {
          this.next(event)
        },
        element,
        eventName
      }
      this._event.element.addEventListener(eventName, this._event.listener)
    }
    this.listeners = []
  }
  
  static create(initFn) {
    return new Observable({initFn})
  }
  
  static from(input) {
    let initFn
    if (input.constructor === Array) {
      initFn = observer => {
        input.forEach(i => observer.next(i))
      }
    }
    else if (typeof input.then === 'function') {
      initFn = observer => {
        input.then(res => {
          observer.next(res)
        })
      }
    }
    else if (typeof input === 'string' || typeof input[Symbol.iterator] === 'function') {
      initFn = observer => {
        for (let char of input) {
          observer.next(char)
        }
      }
    }
    return new Observable({initFn})
  }
  
  static fromEvent(element, eventName) {
    return new Observable({element, eventName})
  }
  
  subscribe(cb) {
    this.listeners.push(cb)
    
    if (this._initFn)
      this._initFn(this)
      
    return {
      unsubscribe: () => {
        this.listeners = this.listeners.filter(l => l !== cb)
        if (this.listeners.length === 0 && this._event) {
          this._event.element.removeEventListener(this._event.eventName, this._event.listener)
        }
      }
    }
  }
  
  next(value) {
    this.listeners.forEach(cb => cb(value))
  }
  
}

const hello = Observable.create(function(observer) {
  observer.next('Janne');
});

const helloSubscribe = hello.subscribe(val => console.log(val));
hello.next('JANNE')
helloSubscribe.unsubscribe()
hello.next('JANNE')

const array = Observable.from([1,2,3])
array.subscribe(val => console.log(val))

const string = Observable.from('string')
string.subscribe(val => console.log(val))

const promise = Observable.from(new Promise(resolve => resolve('PROMISE')))
promise.subscribe(val => console.log(val))

const map = new Map();
map.set(1, 'Jan');
map.set(2, 'Josef');
const fromMap = Observable.from(map)
fromMap.subscribe(val => console.log(val))

const div = document.createElement('div')
const fromEvent = Observable.fromEvent(div, 'click')
const subscription = fromEvent.subscribe(event => console.log(event))
div.click()
subscription.unsubscribe()