On submit form, return false not working

On submit form, return false not working

When I submit the form I got an alert message. When I accept the alert it
will submit the form anyway. Returning false is ignored. Onclick can not
be used. I try with var x = document.forms["form"]["fname"].value; and
still same.
<form id="f" method="post" name="form" onsubmit="return validateForm();"
action="#">
<input type="text" name="fname" id="test" />
<input type="submit" value="submit"/>
</form>
<script type="text/javascript">
function validateForm() {
var x = document.getElementById('test').value;
if (x == null || x == 0 || x == "0") {
alert("Stop");
return false;
}
}
</script>