How to run a javascript when browser process is killed?

I hav onUnload="cleanup()" defined in my html body tag. The cleanup() method in javascript is

f开发者_如何学Pythonunction cleanup() {--clean up session--}.

But when the browser is terminated by killing the process using task manager, I want to do the same action as cleanup. How can this be done?

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You cannot. If the process is forcefully killed, nobody cares about your scripts.