window.matchMedia

gistfile1.js

var MM = window.matchMedia("(max-width: 400px)"),
    docbody = document.body;

function doMediaCheck () {
  if (MM.matches) {
    docbody.innerHTML = 'less than 400';
    docbody.className = 'orange';
  } else {
    docbody.innerHTML = 'more than 400';
    docbody.className = '';
  }
}

window.onresize = function () {
  doMediaCheck();
};

doMediaCheck();