<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()"/> <!--通过 = 按钮来调用创建的函数，得到结果-->

   <input type='text' id='fruit' /> 

<script>

function count(){

var vtxt1=parseInt(document.getElementById('txt1').value); 

var vtxt2=parseInt(document.getElementById('txt2').value); 

var vselect=document.getElementById('select').value; 
var result='';
switch (vselect){ 
    case '+':{ 
        result=vtxt1+vtxt2; break; 
       }
    case '-':{ 
        result=vtxt1-vtxt2; break;
        }
    case '*':{ 
         result=vtxt1*vtxt2; break; 
         } 
    case '/':{ 
          result=vtxt1/vtxt2; break;
        }
    } 
  document.getElementById('fruit').value=result 
} 
</script>