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>