How to write tests for javascript with variable responses to different viewport width sizes

I have javascript code that displays a different set of UI for different viewport widths - and I'd like to put in some automated tests for them to help in regression, but my que开发者_如何学Pythonstion is, how do i do this if I'm using qunit or Jasmine? in fact, any other methods for that matter?

more specifically, how do i set the viewport widths (or emulate them) when doing my tests?