javascript alert refresh page code example

Example 1: javascript before reload page alert

<script type="text/javascript">
 window.onbeforeunload = function () {
  return 'Are you really want to perform the action?';
 }
</script>

Example 2: javascript alert on refresh

window.onbeforeunload = function() {
  return "Data will be lost if you leave the page, are you sure?";
};