add event listener and event handler code example

Example 1: javascript onclick event listener

document.getElementById("myBtn").addEventListener("click", function() {
  alert("Hello World!");
});

Example 2: js add event listener

function eventHandler(event) {
  if (event.type == 'fullscreenchange') {
    /* gestire un interruttore a schermo intero */
  } else /* fullscreenerror */ {
    /* gestire un errore di commutazione a schermo intero */
  }
}