window.innerWidth

Width of the browser window viewport including, if rendered, the vertical scrollbar.

Syntax

var intViewportWidth = window.innerWidth;

Values

The window.innerWidth property is read-only; it has no default value.
window.innerWidth property stores an integer representing a number of pixels.

Notes

The innerWidth property will be supported in any window object like a window, a frame, a frameset or a secondary window.

Assuming a frameset
-------------------

var intFrameWidth = window.innerWidth;// or
var intFrameWidth = self.innerWidth;// will return the 
width of the frame viewport within the frameset
var intFramesetWidth = parent.innerWidth;// will return the
width of the viewport of the closest frameset
var intOuterFramesetWidth = top.innerWidth;// will return the
width of the viewport of the outermost frameset

Standards information

DOM Level 0. Not part of any W3C technical specification or recommendation.