sample simple

app.js

var find = (a) => document.querySelectorAll(a)
var titleNode = find('.content h1')[0]

const onLoad = function(){
  initButton()
  find('.content p:last-child')[0].classList.add('red')
}

function initButton(e){
  var clicked_times = 0
  var button = find('#clicky')[0]
  button.onclick = (e) => {
    titleNode.textContent =  Math.random() * 9000 
    e.srcElement.value = `Clicked: ${++clicked_times}`
  }
}



document.addEventListener("DOMContentLoaded", onLoad);