Hide Fields When Checkbox Clicked

I have put the following code into a sharepoint aspx page. So that when I change the value in a drop down box it runs a function (in this case it displays "Works").

<script language="javascript" type="text/javascript">
function getField(fieldType,fieldTitle) {
var docTags = document.getElementsByTagName(fieldType);
for (var i=0; i < docTags.length; i++) {
if (docTags[i].title == fieldTitle) {
return docTags[i]
}}}

function TestFunctionName() {
alert("Works");
}
getField('Select','DropDownBoxName').onchange = function() {TestFunctionName()};
</script>

What I'd like to be able to do as well is when a check box is changed or clicked on the same thing happens. I dont seem to be able to achieve it though. I've change the line...
getField('Select','DropDownBoxName').onchange = function() {TestFunctionName()};
to...
getField('Input','CheckBoxName').onchange = function() {TestFunctionName()};
and
getField('Input','CheckBoxName').onclick = function() {TestFunctionName()};