un esempio:
codice:
<html>
<head>
<script language="javascript">
  function check()
  {
    for(i=0;i<document.myform.elements.length;i++)
       {
         document.myform.elements[i].checked=true;
       }
  }
</script>
</head>
<body>
<form name="myform">
 <input type="checkbox" name="uno" />
 <input type="checkbox" name="due" />
 <input type="checkbox" name="uno" />
 <input type="button" value="clicca" onclick="check()" />
 </form>
</body>
</html>