.index()

.index() accepts a DOM element, and returns its index value.

.index() is zero-based.

 
<!DOCTYPE html>
<html>
    <head>
        <script src="http://java2s.com/Book/JavaScriptDemo/jQuery/jquery-1.8.0.min.js">
        </script>
        <script>
            $(function(){
                var item2 = document.getElementById("item2");
                document.writeln($("li").index(item2));
            });
        </script>
    </head>
    <body>
        <ul>
            <li id="item0">A</li>
            <li id="item1">B</li>
            <li id="item2">C</li>
            <li id="item3">D</li>
            <li id="item4">E</li>
            <li id="item5">F</li>
        </ul>
    </body>
</html>

Click to view the demo