check uncheck all checkboxes javascript

<SCRIPT LANGUAGE="JavaScript">
function checkbox (field) {
for (i = 0; i < field.length; i++) {
if(field[i].checked == true) {
uncheckAll(field);
}
else {
checkAll(field);
}
}
}
function checkAll(field)
{
for (i = 0; i < field.length; i++)
field[i].checked = true ;
}

function uncheckAll(field)
{
for (i = 0; i < field.length; i++)
field[i].checked = false ;
}

</script>

<form name="myform" action="checkboxselect.php" method="post">
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<input type="checkbox" name="CheckAll" value="Check All"

 onClick="checkbox(document.myform.list1)">Select all<br>
<input type="submit" value="submit">
<?php echo $listvalue =$_REQUEST['list'];
while (list ($key,$val) = @each ($listvalue)) {
echo "$val,";
}
?>
<br>
</form>