js 操作checkItem。选中与取消

<input type=checkbox name=mm value=a onclick="checkItem('mm')"><br>
<input type=checkbox name=mm value=b onclick="checkItem('mm')"><br>
<input type=checkbox name=mm value=c onclick="checkItem('mm')"><br>
<input type=checkbox name=mm value=d onclick="checkItem('mm')"><br>
<input type=checkbox name=mm value=e onclick="checkItem('mm')"><br>
<input id="Text1" type="text" /><br>

<script language=Javascript>
function checkAll(e, itemName)
{
  var aa = document.getElementsByName(itemName);
  for (var i=0; i<aa.length; i++)
   aa[i].checked = e.checked;
}
function checkItem(itemName)
{
Text1.value="";
  var aa = document.getElementsByName(itemName);
  for (var i=0; i<aa.length; i++)
  {
  if(aa[i].checked) Text1.value=Text1.value+aa[i].value
  }

//if(e.checked) Text1.value=e.toString();

//  var all = document.getElementsByName(allName)[0];
//  if(!e.checked) all.checked = false;
//  else
//  {
//    var aa = document.getElementsByName(e.name);
//    for (var i=0; i<aa.length; i++)
//     if(!aa[i].checked) return;
//    all.checked = true;
//  }
}
</script>