如何判断form中某个元素是否存在？存在几个？

Question

try:
<script>
function c(){
if(f.uploadfilename){
alert(document.getElementsByName("uploadfilename").length)
}
else alert(0)
}
</script>
<form name=f>
<input name=s>
<input type=file name=uploadfilename>
<input type=file name=uploadfilename>
<input type=submit onclick="c()">
</form>

Answer 1

function c(){alert(document.getElementsByName("uploadfilename").length)}

Answer 2

<script>
function c(){
if(f.uploadfilename){
alert(f.uploadfilename.length)
}
else alert(0)
}
</script>
<form name=f>
<input name=s>
<input type=file name=uploadfilename>
<input type=file name=uploadfilename>
<input type=submit onclick="c()">
</form>
<input type=file name=uploadfilename>