It should be getElementsByClassName not getElementByClassName. i.e. Get Elements not Element. Unlike id of an element classname does not have to be unique for the document. There could be many elements with the same class name and function returns all of them.
It returns an array of all the elements with that class name. If you need to access a certain element you need to use the index.
<script type="text/javascript">
function getInfo() {
var myElements = document.getElementsByClassName("contentMiddle");
if(myElements != null)
{
alert(myElements[0].className);
}
else
{
alert("No elements found !");
}
}
</script>
HTML:
<input onclick="getInfo()" type="button" value="ClickMe" />
