EventListener_callback.js

const btn = document.getElementById('btn')
const values = [1, 2, 3, 4, 5]

function logger(event) {
  console.log(`Logger : ${event}`)
  btn.removeEventListener('click', logger)
}

// btn.addEventListener('click', logger)
// const btn$ = Rx.Observable.fromEvent(btn, 'click')
// values.forEach(logger)
// btn$.subscribe(logger)