老师，请检查，还有一个疑问，

来源：4-8 编程练习

慕斯6325516

2019-12-12 10:20:31

11.为什么我alert写在下面也显示undefined,写在operate上面也是，但是这个窗口弹出之后结果又显示正确

2.请老师检查代码，是否需要优化的地方

<body>
  <div>
   <input type="text" id="former" />
     <select id="cal">
     <option value="add">+</option>
     <option value="subtract">-</option>
     <option value="multiply">x</option>
     <option value="divide">÷</option>
     </select>
   <input type="text" id="later" />
   <input type="button" id="btnEqual" value="=" />
   <input type="text" id="resultOutput" readonly="true" />
  </div>
  <script type="text/javascript">
   var cal=document.getElementById('cal');
   var former=document.getElementById('former');
   var later=document.getElementById('later');
   var btnEqual=document.getElementById('btnEqual');
   var result=document.getElementById('resultOutput');
   btnEqual.onclick=function(){
    operate[cal[cal.selectedIndex].value](former.value,later.value);
    alert(operate[cal[cal.selectedIndex].value](former.value,later.value))
//    switch(cal[cal.selectedIndex].value){
//    case "add":
//      operate.add(former.value,later.value);break;
//    case "subtract":
//      operate.subtract(former.value,later.value);break;
//    case "multiply":
//      operate.multiply(former.value,later.value);break;
//    case "divide":
//      operate.divide(former.value,later.value);break;
//
//   }
   };

   var operate={
    add: function (num1, num2){
    result.value= +num1 + +num2;
   },
       subtract: function (num1, num2){
    result.value= num1 - num2;
   },
       multiply: function (num1, num2){
    result.value= num1 * num2;
   },
       divide: function (num1, num2){
    result.value= num1 / num2;
   }
   };


  </script>
</body>