How can I create a JavaScript warning when a user tries to leave the page?

I have a form that I'd like visitors to fill out. Once the form is started, I want a warning message to show when they try and leave the page and the form was not submitted. Something like "You have not submitted the form, are you sure you want to loose your information? [cancel] [ok]"

I have gone though and done quite a bit of testing and I can get everything to work except, when the user chooses to stay, the page refreshes and all the form data is lost.

Any ideas on how I can show a warning and not loose form data?

Thanks

 

Use an onunload event. Here's a link with an example

 

Unfortunately, the page which you suggest here also doesn't add any value as when you close the page, it gve you "ok" "cancel". Click on cancel and still the page will close.

 

https://developer.mozilla.org/en/DOM/window.onbeforeunload

 

This worked perfectly! I'm not exactly sure how the code works, but it works!!

Thanks a bunch!