From http://blog.nitishkumarsingh.com/create-a-guid-uuid-in-javascript/

guid.js

function getGUID() {
     return 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx'.replace(/[x]/g, function(c) {
         var num = Math.random() *16 | 0, v = c === 'x' ? num : (num&0x3|0x8);
     
         return v.toString(16);
      });
  }
  
function getGUID() {
    function fd() {
        return Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1);
    }
    
    return fd() + fd() + '-' + fd() + '-' + fd() + '-' + fd() + '-' + fd() + fd() + fd();
 }