An ES6 Observable example

Observable.js

class Observable {
  constructor(fn) {
    this.fn = fn
  }

  subscribe(next, error, complete) {
    if (typeof next === "function") {
      return this.fn({
        next,
        error: error || (() => {}),
        complete: complete || (() => {})
      })
    } else {
      return this.fn(next)
    }
  }

  map(fn) {
    return new Observable(observer => {
      return this.subscribe(
        val => observer.next(fn(val)),
        error => observer.error(error),
        () => observer.complete()
      )
    })
  }

  mergeMap(fn) {
    return new Observable(observer => {
      return this.subscribe(
        val => {
          fn(val).subscribe(
            val => observer.next(fn(val)),
            error => observer.error(error),
            () => observer.complete()
          )
        },
        error => observer.error(error),
        () => observer.complete()
      )
    })
  }

  static fromArray(array) {
    return new Observable(observer => {
      array.forEach(val => observer.next(val))
      observer.complete()
    })
  }

  static fromEvent(el, event) {
    return new Observable(observer => {
      const handler = e => observer.next(e)
      el.addEventListener(event, handler)

      return () => { el.removeEventListener(event, handler) }
    })
  }

  static fromPromise(promise) {
    return new Observable(observer => {
      promise
        .then(val => {
          observer.next(val)
          observer.complete()
        })
        .catch(e => {
          observer.error(e)
        })
    })
  }
}