[Javascript] Checkbox 전체선택 / 해제

<script language="javascript">
<!--
var checkflag = "false";

function check(field) {
if (checkflag == "false") {
  for (i = 0; i < field.length; i++) {
   field[i].checked = true;
  }
  checkflag = "true";
} else {
  for (i = 0; i < field.length; i++) {
   field[i].checked = false;
  }
  checkflag = "false";
}
}
-->
</script>


<form name="chkform" method="post" action="">
전체선택:<input type=checkbox value="" onClick="this.value=check(this.form.list)">
<hr>
<input type=checkbox name=list value="1">
<input type=checkbox name=list value="2">
<input type=checkbox name=list value="3">
<input type=checkbox name=list value="4">
<input type=checkbox name=list value="5">
</form>