A few lines of js code to realize self adaptation

Write the following lines in javascript:

(function (doc, win, undefined) {
      var docEl = doc.documentElement,
       resizeEvt = 'orientationchange' in win? 'orientationchange' : 'resize',
       recalc = function () {
        var clientWidth = docEl.clientWidth;
        if (clientWidth === undefined) return;
        docEl.style.fontSize = 20 * (clientWidth / 320) + 'px';
       };
      if (doc.addEventListener === undefined) return;
      win.addEventListener(resizeEvt, recalc, false);
      doc.addEventListener('DOMContentLoaded', recalc, false)
     })(document, window);