HTML קוד:
<script type="text/javascript">
function InputName(){
if (inputname.value == '')
{
alert('Error!');
return false;
}
else
return true;
}
</script>
<form action="bla.php" method="post" onsubmit="InputName()">
<input type="text" name="inputname" />
<br />
<input type="submit" name="submit" value="שלח" />
</form>