<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("button").click(function(){
alert($(".hot").index($("#favorite")));
});
});
</script>
</head>
<body>
<p>Click the button to get the index of the element with id="favorite", relative to the jQuery selector (class="hot"):</p>
<button>Get index</button>
<ul>
<li>Milk</li>
<li class="hot">Tea</li>
<li class="hot" id="favorite">Coffee</li>
</ul>
</body>
</html>