How to trace whether page data has been modified by the user.
in Gmail compose, we get alert message if we click on any of link [except links those get loaded in another window.]
Saying "your message has not been sent. Discard your message?"
this gets prompts Even if we hits in the Back button,

i believe, they don't maintain a simple javascript and keep calling same function in every link.
how does it actually happens?

expect demo [code]
Thanks