onclick vs addEventListener - https://simonewebdesign.it/onclick-vs-addeventlistener/

onclicktest.html

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

<script>

var btn = document.getElementById('btn');

btn.onclick = function() { 
  // will be overwritten! 
  console.log('[onclick] foo'); 
}

btn.onclick = function() { 
  
  console.log('[onclick] bar');
}

btn.addEventListener('click', function() {
  // won't work on IE < 9  
  console.log('[EventListener] foo');
});

btn.addEventListener('click', function() {
  // won't work on IE < 9
  console.log('[EventListener] bar');
});

</script>

super clear