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copied!<p>The right way to use <code>onbeforeunload</code> to confirm navigating away is to just return the string of the message you want to have shown. <a href="https://developer.mozilla.org/en-US/docs/DOM/window.onbeforeunload" rel="nofollow">MDN has this to say</a>.</p> <blockquote> <p>The function should assign a string value to the returnValue property of the Event object and return the same string</p> </blockquote> <p>So your <code>onbeforeunload</code> function for the behavior you ask for would just be (same as binding it with jquery, especially since onbeforeunload does not support multiple event listeners like other events do)</p> <pre><code>window.onbeforeunload = function() { return "Are you sure?"; }; </code></pre> <p>But ironically, Firefox doesn't support this for security purposes.</p> <blockquote> <p>Note that in Firefox 4 and later the returned string is not displayed to the user. See <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=588292" rel="nofollow">bug 588292</a>.</p> </blockquote> <p><strong>Edit:</strong> I read more of the bug report. The main reason they site for this decision is: "1. Remove the site-supplied text, since this is a security issue (untrusted text in browser dialogs)". I wonder how they feel about an <code>alert</code>?</p> <p>Most modern browsers (Chome, IE 9+ or earlier, and Opera, I know for sure) will ignore anything you try to do in an <code>onbeforeunload</code> function other than returning a string for display. This is to prevent hijacking the users browser, opening popups, overriding navigation attempts by pointing the user another, possibly malicious domain, etc.</p> <p><strong>Edit 2:</strong> I was apparently incorrect. Only <em>some</em> things are disallowed in <code>onbeforeunload</code>, they just happen to mainly be the types of things you tried in your sample code, having to do with event propagation and page navigation. From <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=588292#c97" rel="nofollow">deep in the comments</a> of that FF bug report, user oyasumi posts a link to this jsFiddle with a work around for displaying custom text to FF 4+ users: <a href="http://jsfiddle.net/ecmanaut/hQ3AQ/" rel="nofollow">http://jsfiddle.net/ecmanaut/hQ3AQ/</a> So apparently calling <code>alert()</code> is still allowed, even in <code>onbeforeunload</code>, which hilariously is doing exactly what the was original reason for the 'bug' "fix": displaying custom text in a browser/application/OS/official-looking dialog.</p>

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