js监听input输入字符变化

　　<p class="text-input">
        <input type="text" id="username" autoComplete=‘off‘ maxlength="30">
        <span class="js_limit">
            <em>0</em>/<span>30</span>
        </span>
    </p>

    <script src="jquery-1.7.2.min.js"></script>
    <script>
        $(function(){
            $(‘#username‘).bind(‘input propertychange‘, function() {
                $(‘.js_limit em‘).html($(this).val().length);
            });
        })
    </script>