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                        /前端开发
                        
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                        /JavaScript进阶篇

求大神，为什么运行不了

!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
   function count(){
       var m = parseInt(document.getElementById("txt1").value);
       var n = parseInt(document.getElementById("txt2").value);
       var z=document.getElementById("select").value;

       var num;
       switch(z){
           case'+';
           num=m+n;
           break;
           case'-';
           num=m-n;
           break;
           case'*';
           num=m*n;
           break;
           case'/';
           num=m/n;
           break;
       }
       alert(num);
       document.getElementById("fruit").value=num;
   }
</script>
</head>
<body>
   <input type='text' id='txt1' />
   <select id='select'>
  <option value='+'>+</option>
  <option value="-">-</option>
  <option value="*">*</option>
  <option value="/">/</option>
   </select>
   <input type='text' id='txt2' />
   <input type='button' value=' = ' onclick="count()"> 
   <input type='text' id='fruit' />
</body>
</html>