    function guid(len) {  
            var buf = [],  
                chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789',  
                charlen = chars.length,  
                length = len || 32;  
                  
            for (var i = 0; i < length; i++) {  
                buf[i] = chars.charAt(Math.floor(Math.random() * charlen));  
            }  
              
            return buf.join('');  
        }  
      
    console.log(guid());    
    console.log(guid(15));  