checkbox_value

<input type="checkbox" value="satu" name="check[]" onclick="addlist(this, 'txt1')>Satu
<input type="checkbox" value="dua" name="check[]" onclick="addlist(this, 'txt1')>Dua

<textarea id="txt1"><textarea>

<script type="text/javascript">

function addlist(checkobj, output) {

   var checkgrup    = checkobj.form[checkobj.name];
   var cheklength   = checkgrup.length;
   var list         = new Array();

   if (checklength > 1) {

      for ( var i = 0; i < checklength; i++) {

         if (checkgrup[i].checked) {

            list[list.length] = checkgrup[i].value;

         }

      }

   } else {

      if (checkgrup.checked) {

         list[list.length] = checkgrup.value;

      }

   }

   document.getElementById(output).value = list.join(', ');

   return

}
</script>