控制留言文字的数量及显示输入的数量

 1 <!DOCTYPE html>
 2 <html>
 3 <head>
 4 <meta charset="utf-8">
 5 <title>网站首页</title>
 6 <style>
 7 .aa{
 8     80%;
 9     height:300px;
10     resize:none;
11     padding:10px;
12 }
13 </style>
14 </head>
15 <body>
16 <h1>留言内容</h1>
17 <textarea class="aa" id="bb" autofocus></textarea>
18 <span id="msg"></span>
19 </body>
20 </html>
21 <script>
22 function $(id){
23     return document.getElementById(id);
24 }
25 String.prototype.trim =function (){
26     return this.replace(/^s+|s+$/g,'');
27     }
28 var a=$('bb');
29 a.onkeyup = function (){
30     if(this.value.length>50){
31         $('msg').innerHTML='最多输入50个字';
32         this.value=this.value.trim();//清楚首尾的空格
33         this.value=this.value.substr(0,50);//截取到了50个 字符不让输入
34         }else{
35         $('msg').innerHTML='你输入了'+this.value.length+'个字';
36         }
37 }
38 </script>