hein: formularfeld (de)aktivieren mit javascript?

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<html>
<head>
<title></title>
<script type="text/javascript">

function SetRead(){

df = document.testform;

if(df.cb_test.checked){
    df.textfeld.readOnly = true;
  }else{
     df.textfeld.readOnly = false;
  }
}
</script>

</head>
<body text="#000000" bgcolor="#FFFFFF" link="#FF0000" alink="#FF0000" vlink="#FF0000">

<form name="testform">
<textarea name="textfeld" cols="" rows=""></textarea><br />
deaktivieren:<input type="Checkbox" name="cb_test" onclick="SetRead()">
</form>
</body>
</html>

gruß hein