Form valid

Fx:

<script>
function tjek(f) {
if (f.navn.value == "" || f.ipris.value == "" || f.upris.value == "" || f.vol.value == "") {
alert("Du skal indtaste noget i alle felterne!");
return false;
}
return true;
}
</script>

<form name="data" method="post" action="blablabla.cgi" onSubmit="return tjek(this);">
Navn: <input type="text" name="navn">
<br>Ipris: <input type="text" name="ipris">
<br>Upris: <input type="text" name="upris">
<br>Vol: <input type="text" name="vol">
<br><input type="submit" value="Insend">
</form>