JS实现文字倒计数

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>My Practice</title>
    <script src="jquery-3.2.1.js"></script>
</head>
<body>
    <div>
        <textarea onkeyup="javascript:checkWord(this);" onmousedown="" ></textarea>(<span id="ResidueCount">10</span>)
    </div>
    <span>test</span>

    <script type="text/javascript">
        var str = "";
        var maxLength = 10;
        function checkWord(c) {
            str = c.value;
            //console.log(str + "    " + str.length);
            var len = maxLength - str.length;
            if (len >= 0) {
                $("#ResidueCount").text(len);
            }
            else {
                c.value = str.substring(0, maxLength);
            }
        }
    </script>
</body>
</html>
参考：http://www.studyofnet.com/news/529.html