[JavaScript] Button enabled wenn txtfeld nicht leer

<tr>
 <td>Vorname:</td>
 <td><input type="text" size="20" maxlength="30" name="Vorname" ID="deinname"></td>
</tr>
<tr>
 <td></td>
 <td class="center">
 <button ID="send" name="absenden" type="submit">Absenden</button>
 <button name="Zur&uuml;cksetzten" type="reset">Zur&uuml;cksetzen</button>
 </td>
</tr>

<script type="text/javascript">
document.getElementById('Send').disabled = true;
var mega = document.getElementById('deinname')

 if(mega == "")
 {
 document.getElementById('Send').disabled = True;
 }
 else
 {
 document.getElementById('Send').disabled = False;
 }
</script>

<script type="text/javascript">
function validateForm() {
var d=document.feedback;
var error="";
 if(d.vorname.value=="") error+="Vorname ";
 if(d.nachname.value=="") error+="Nachname ";
 if(d.textfeld.value=="") error+="Textfeld ";
 if(d.mail.value.indexOf('@')==-1) error+="eMail ";
 if(error!="") {
alert("Folgende Felder korrigieren: "+error);
return false;
}
else return true;
}
</script>