$("#test")选取id为test的元素

node2:/var/www/html/js#cat a21.html 
<div id=test>aaaa</div>
<div id=test>bbbb</div>
<div id=test>cccc</div>
<script type="text/javascript" src="jquery-2.2.2.min.js"></script>    
<script type="text/javascript" src="a21.js"></script>

node2:/var/www/html/js#cat a21.js 
$("#test").css("background","#bbffaa");