Prompting and preventing user from navigating away/closing a page

Question

As per how StackOverflow does it when you are adding a question or replying to a question;

I'd like to prompt the user with a;

"Are you sure you want to navigate away from this page"
"Press OK to continue or Cancel to stay on this page"

I've tried a couple of js snippets i've found on ze internet, but nothing quite cuts the mustard.

Answer 1

You need to implement window.onbeforeunload and return your message from it as a string:

window.onbeforeunload = function() {
    if (theUserHasStartedEditing) {
        return "You have started writing or editing a post."
    }
}

There's an online demo here: http://www.4guysfromrolla.com/demos/OnBeforeUnloadDemo2.htm