confirmation before closing of tab/browser using javascript code example

Example 1: js ask before close

window.onbeforeunload = function() {
        return "";
  }

Example 2: javascript confirm tab close

<script>
window.onbeforeunload = function (e) {
    e = e || window.event;

    // For IE and Firefox prior to version 4
    if (e) {
        e.returnValue = 'Sure?';
    }

    // For Safari
    return 'Sure?';
};
</script>