<form name="test-form" action="" method="">
<input type="checkbox" name="kk">
<form>
<script type="text/javascript">
var oForm = document.forms['test-form'],
eles = oForm.elements['kk'];
alert(eles.length); // undefined
alert(eles.nodeType); // 1
</script>

<script type="text/javascript">
var oForm = document.forms['test-form'],
eles = YAHOO.util.Dom.getElementsBy(function(el) {
return el.type === 'checkbox' && el.name === 'kk';
}, 'input', oForm);
alert(eles.length); // 1
</script>

<form name="hehe">
<input type="checkbox" name="haha" />
</form>
<form name="hehe2">
<input type="checkbox" name="haha" />
<input type="checkbox" name="haha" />
</form>
<script type="text/javascript">
document.write(document.hehe.haha.length);
document.write('<br />');
document.write(document.hehe2.haha.length);
</script>