js_numOperate.html

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8"/>
  <title>javascript制作一个简单的计算器</title>  
  <script type="text/javascript">
  	function resultFun(num1,operation,num2){
  		var ops={
        "+":function(x,y){return x+y},
        "-":function(x,y){return x-y},
        "*":function(x,y){return parseFloat(x*y)},
        "/":function(x,y){return y==0?null:x/y}
    	}
  		return (ops[operation]||function(){return null;})(num1,num2);
  	}
    function count(){
     //获取第一个输入框的值
     var txt1=document.getElementById("txt1").value;
     //获取第二个输入框的值
     var txt2=document.getElementById("txt2").value;
     //获取选择框的值
     var select=document.getElementById("select").value;
     //获取通过下拉框来选择的值来改变加减乘除的运算法则
     var result="";
     switch(select){
         case "+": result=parseInt(txt1)+parseInt(txt2); break;
         case "-": result=parseInt(txt1)-parseInt(txt2); break;
         case "*": result=parseInt(txt1)*parseInt(txt2); break;
         case "/": result=(txt2==0?null:parseInt(txt1)/parseInt(txt2));break;
         default : result=null;break;
     }
    	//var result=resultFun(txt1,select,txt2);
     //设置结果输入框的值 
     document.getElementById("res").value=result;
    }
    
   </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()" /> <br />
    <input type='text' id='res' placeholder="计算结果是:" />   
  </body>
 </html> 