Suppose a user enters some information on the page and tries to close or navigate from the window?without saving the data. You may want to warn the user about possible data loss.

To do this, use the window DOM object’s onbeforeunload event, which is fired just before the unload event. Its sole purpose is to allow users to change their minds about exiting the page. That is to say, it cancels the upcoming unload event. The code is below:

window.onbeforeunload = Func1function Func1(){     //statements to execute     return 'some message';		} 

If you have set the onbeforeunload handler, and later want to cancel the event handling, simply assign it a null value:

window.onbeforeunload = null;

This event handler works in IE4+ for Windows only. All other JavaScript-enabled browsers will not choke on it, because they consider onbeforeunload to be a user-defined custom method of the window object. Perfectly acceptable.