JavaScript alert before page refresh 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: alert without page refresh

// note that its the return false; that prevents the page refresh.
<button onclick="alert('Hello'); return false;">Click</button>