catching browser navigation (prior to moving away)

[SpaceMarine]

hello,

in my web apps a user is free to close the browser or click a
hyperlink to go somewhere else, prior to completing data-entry on the
webform.

how can i capture this event & warn them? (on Gmail if you try closing
the browser or clicking away while composing a message in its form,
that it will toss up a javascript alert dialog).


thanks!
sm