onclickvsaddEventListener

HTML

<button id="btn">Click me!</button>

JavaScript

var btn = document.getElementById('btn');
btn.onclick = function() { 
  // will be overwritten! 
  console.log('[onclick] 바'); 
}
btn.onclick = function() { 
  
  console.log('[onclick] 보');
}
btn.addEventListener('click', function() {
  console.log('[EventListener] 바');
});
btn.addEventListener('click', function() {
  console.log('[EventListener] 보');
});