Untitled

GetElementsByTagName(tag).length
var ul = document.getElementById('listing');
function countLi(){
    alert("There are "+ul.getElementsByTagName('li').length+" Li-Elements in this Documnet!");
}

<input type="button" onclick="countLi()" value="Count the Li's!"/>
<ul id="listing">
    <li>one</li>
    <li>two</li>
    <li>three</li>
</ul>

var ul = document.getElementById('listing');

function countLi(){
    var ul = document.getElementById('listing');
    alert("There are "+ul.getElementsByTagName('li').length+" Li-Elements in this Documnet!");
}

function countLi(){
    var ul = document.getElementById('listing');
    alert("There are "+ul.getElementsByTagName('li').length+" Li-Elements in this Documnet!");
    }