Responsive JavaScript

checkWidth.js

function checkWidth() {

  var viewportWidth = window.innerWidth;

  // If the viewport width is greater than 640px
  if (viewportWidth > 640) {
    // Execute your code
  } else {
    // Do something else
  }
}

(function () {

  // Calls checkWidth function when page has loaded
  checkWidth();

  // Calls checkWidth function when window has been resized
  window.onresize = checkWidth;

})();