//error-version

//html
button{$}*4
.output

//js
var buttons = document.querySelectorAll('button')
var output = document.querySelctor('.output')

for(var i=0; i< buttons.length; i++){
	buttons[i].addEventListener('click', function(){
    	ouput.innerText = buttons[i].innerText
    })
}

//console
Uncaught TypeError: Cannot read property 'innerText' of undefined

//es6-version
...
//js
var buttons = document.querySelectorAll('button')
var output = document.querySelctor('.output')

for(let i=0; i< buttons.length; i++){
	buttons[i].addEventListener('click', function(){
    	ouput.innerText = buttons[i].innerText
    })
}

//babel-output

var _loop = function (i) {
	buttons[i].addEventListener('click', function () {
    	output.innerText = buttons[i].innerText
  })
}
for (var i = 0; i < buttons.length; i++) {
	_loop(i)
}