Confirmation before closing of tab/browser

Try this:

<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>

Here is a working jsFiddle

Try this:

<script>
    window.onbeforeunload = function(e) {
       return 'Dialog text here.';
    };
</script>

more info here MDN.