window.innerWidth vs document.documentElement.clientWidth

Solution 1:

According to the W3C specification (17 March 2016):

The innerWidth attribute must return the viewport width including the size of a rendered scroll bar (if any), or zero if there is no viewport.

...

The clientWidth attribute must run these steps:

1. If the element has no associated CSS layout box or if the CSS layout box is inline, return zero.
2. If the element is the root element and the element's document is not in quirks mode, or if the element is the HTML body element and the element's document is in quirks mode, return the viewport width excluding the size of a rendered scroll bar (if any).
3. Return the width of the padding edge excluding the width of any rendered scrollbar between the padding edge and the border edge, ignoring any transforms that apply to the element and its ancestors.

Solution 2:

I am using this:

    window.innerWidth && document.documentElement.clientWidth ? 
Math.min(window.innerWidth, document.documentElement.clientWidth) : 
window.innerWidth || 
document.documentElement.clientWidth || 
document.getElementsByTagName('body')[0].clientWidth;

It covers cases where the scrollbar is not taken into consideration and has mobile support.