Exempel:
<script type="text/javascript">
<!--
function checkForm(o){
var
e=o.elements,
i=e.length,
w=e["s"].options[e["s"].selectedIndex].value;
while(i-->0){
if(e[i].type=="text" && e[i].name==w && e[i].value.length < 1){
alert("Du måste fylla i följande: " + w);
return false;
}
}
};
//-->
</script>
...
<form onsubmit="return checkForm(this)">
<select name="s">
<option value="Mobiltelefon">Mobiltelefon</option>
<option value="Epost">E-post</option>
<option value="Brev">Brev</option>
</select>
<br>
Mobiltelefon:<br>
<input type="text" name="Mobiltelefon"><br>
E-post:<br>
<input type="text" name="Epost"><br>
Brev:<br>
<input type="text" name="Brev"><br>
<input type="submit"><br>
</form>
:)