How to alert the user when closing the window

Hi,

I want to alert the user to save the form, if he has changed any value
in the form and not saved that, in this case when the user leave the
page with out saving i want to show him an alert message asking him to
save the data.

it can be done with the help of onUnload method, but how we can
identify if the user has changed any value in the screen or not,
please is any one have found a solution for this or got any idea it
will be of great help

thanks in advance
Rajin

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