JS限制并且显示textarea字数

转自：https://www.cnblogs.com/shinepolo/articles/1373113.html
 1 <script type="text/javascript">

     function show(a,b) {
                var maxl = 400
        var s = document.getElementById(a).value.length + 1;
        if (s > maxl)
            document.getElementById(a).value = document.getElementById(a).value
                    .substr(0, maxl - 1)
        else
            document.getElementById(b).innerHTML = "已输入：" + s + "/" + maxl
                    + " 字符"
       }
</script>


<s:textarea onkeydown="show();" id="a"></s:textarea>
<div><span id ="b">已输入字符：</span></div>