window.beforeunload event code example

Example 1: js before unload

window.addEventListener('beforeunload', (event) => {
  // Cancel the event as stated by the standard.
  event.preventDefault();
  // Chrome requires returnValue to be set.
  event.returnValue = '';
});

Example 2: javascript beforeunload

window.addEventListener("beforeunload", function(event) { ... });
window.onbeforeunload = function(event) { ... };