getElementsByTagName

node2:/var/www/html/js#cat t13.html 
<li>aaaa</li>
<li>bbbb</li>
<script type="text/javascript" src="jquery-2.2.2.min.js"></script>    
<script type="text/javascript" src="t13.js"></script>
node2:/var/www/html/js#


node2:/var/www/html/js#cat t13.js
var aa=document.getElementsByTagName("li")[0].innerHTML;
console.log(aa);
var bb=document.getElementsByTagName("li")[0].innerHTML;
console.log(bb);