$(".test")选取所有class为test的元素

node2:/var/www/html/js#cat a22.html 
<ul class="test">
<div id=test>aaaa</div>
<div id=test>bbbb</div>
<div id=test>cccc</div>
</ul >
<ul clas="tlcb">
<div id=scan>eeee</div>
<div id=scan>ffff</div>
<div id=scan>gggg</div>
</ul>
<script type="text/javascript" src="jquery-2.2.2.min.js"></script>    
<script type="text/javascript" src="a22.js"></script>

node2:/var/www/html/js#cat a22.js
$(".test").css("background","#bbffaa");