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The Viewport

Viewport Width

There are two methods to get the viewport width: window.innerWidth and document.documentElement.clientWidth. The former is more accurate. The latter has better browser support.

To get the best of both worlds, try innerWidth first, and fallback to clientWidth if not supported.

var viewportWidth = window.innerWidth || document.documentElement.clientWidth;

Works in all modern browsers, and IE9+. You can extend support back further with polyfills.