Responsive web design - How to implement JavaScript part?

Responsive web design - How to implement JavaScript part?

I'm preparing to make the web page responsive from a old existing complex
one. https://www.lendingclub.com/browse/browse.action
I use media query for the CSS part, making different CSS work for
different width of the browser. I want to ask that:
How do you deal with the JavaScript in your project, if some functions
only exist in the mobile version (and narrow width browser on PC)?
How to implement that? Do you also get the current window width, if it's
less than 720px then execute it, or something else?
We use YUI in old PC pages, do not include any mobile framework.