How do I warn users when exiting a browser?

[swindland]

I have a web application that has many forms. If the user clicks on the X at the top right of their browser to close out the browser, it then closes out my forms and the user loses everything. Is there a way to warn the user when they click the X to close the browser that they will lose everything and ask if they want to save their work first before exiting the application?

[curtiss]

You may find some help in this article