For desktop;
Viewport (Firefox and other) = window.innerWidth/Height; (Includes scrollbar)
Viewport (IE) = document.documentElement.clientWidth/Height; (Does not include scrollbar)

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For mobile devices;
Visual viewport (CSS pixels) = window.innerWidth/Height;
Layout viewport (CSS pixels) = document.documentElement.clientWidth/Height;
Device Width/Height (Device pixels) = screen.width/height;