<script>
function  randomChar(num)  {
  var  x="0123456789qwertyuioplkjhgfdsazxcvbnm";
  var  tmp="";
  for(var  i=0;i<num;i++)  {
  tmp  +=  x.charAt(Math.ceil(Math.random()*100000000)%x.length);
  }
  return  tmp;
}
</script>