Event observable

basic.js

function listen(fn) {
  const gen = fn();

  function listener(ev) {
    let { done } = gen.next(ev);

    if(done) {
      this.removeEventListener(ev.type, listener);
      gen.return();
    }
  }

  return { listener };
}

const subscription = listen(function*() {
  let event;
  while(event = yield) {
    console.log('An event!', event);
  }
});

const el = document.querySelector('#btn');
el.addEventListener('click', subscription.listener);

observable.js

const obs = observable();

obs.subscribe(function*() {
  let event;
  while(event = yield) {
    console.log('An event!', event);
  }
});

const el = document.querySelector('#btn');
el.addEventListener('click', obs.listener);

// Implementation
function observable() {
  let observers = [];
  function subscribe(fn) {
    observers.push(fn());
  }

  function listener(ev) {
    for(let i = 0; i < observers.length; i++) {
      let gen = observers[i];
      let { done } = gen.next(ev);
      if(done) {
        gen.return();
        observers.splice(i, 1);
        i--;
      }
    }

    if(observers.length === 0) {
      this.removeEventListener(ev.type, listener);
    }
  }

  return { listener, subscribe };
}