[javascript] Do something if screen width is less than 960 px

I would suggest (jQuery needed) :

/*
 * windowSize
 * call this function to get windowSize any time
 */
function windowSize() {
  windowHeight = window.innerHeight ? window.innerHeight : $(window).height();
  windowWidth = window.innerWidth ? window.innerWidth : $(window).width();

}

//Init Function of init it wherever you like...
windowSize();

// For example, get window size on window resize
$(window).resize(function() {
  windowSize();
  console.log('width is :', windowWidth, 'Height is :', windowHeight);
  if (windowWidth < 768) {
    console.log('width is under 768px !');
  }
});

Added in CodePen : http://codepen.io/moabi/pen/QNRqpY?editors=0011

Then you can get easily window's width with the var : windowWidth and Height with : windowHeight

otherwise, get a js library : http://wicky.nillia.ms/enquire.js/