How does JavaScript object `window.screen.width` detect my ScreenResolution in Ubuntu 10.4

How does JavaScript object window.screen.width detect my ScreenResolution in Firefox开发者_如何学Go under Ubuntu 10.4?

I mean what it accesses in OS to retrieve this data - OS variable or OS config files like a) .xprofile, b) kdm/gdm, c) xorg.conf. ?

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It's almost certainly an implementation detail that could vary from one user-agent (browser) to another.