Testing CSS media queries in JS

gistfile1.js

if (window.matchMedia('only screen and (max-device-width: 480px)').matches) {
  // Asynchronously load iphone.js
} else if (window.matchMedia('only screen and (min-device-width: 481px) and ' +
                              '(max-device-width: 1024px) and ' +
                              '(orientation: portrait)').matches) {
  // Asynchronously load ipad-portrait.js
}