How to let JS know about your media queries?

Based on https://adactio.com/journal/5429. View source for details.

A bit hacky, but useful if you:

Don't want to duplicate your media queries inside JS (with Window.matchMedia() for example, which doesn't work in IE9, even though media queries do)
Don't want to worry about pixels inside JS, when using em in CSS (document.documentElement.clientWidth returns pixels)
Don't care about how you've called your different breakpoints (in Sass or whatever) and just want to do something different for a certain range of them

Just what MQ you're currently in when you need it, or if you're on a 'small' screen or not.

Current size: (unknown)