                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /JavaScript进阶篇

各位大神请教一下，点=号出不来结果是什么问题？

<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
   function count(){

    //获取第一个输入框的值
    var num1=document.getElementById('txt1').value;
//获取第二个输入框的值
var num2=document.getElementById('txt2').value;
//获取选择框的值
var select=document.getElementById('select').value;

var sum;
//获取通过下拉框来选择的值来改变加减乘除的运算法则
if(select=='+'){
     sum=parseInt(num1)+parseInt(num2);
}else if(select=='-'){
     sum=parseInt(num1)-parseInt(num2);
}else if(select=='*'){
     sum=parseInt(num1)*parseInt(num2);
}else if(select=='/'){
     sum=parseInt(num1)/parseInt(num20;
}else{
     document.write("请选择一个");
}
    //设置结果输入框的值
   document.getElementById('fruit').value=sum;
   }
</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>