JavaScript 实现简易网页计算器

<style>
 * {
 margin: 0;
 padding: 0;
 }
 #panel {
 width: 202px;
 height: 252px;
 margin: 30px auto;
 border: 3px solid #ccc;
 }
 #screen {
 width: 134px;
 height: 40px;
 margin: 5px;
 display: inline-block;
 border: 1px solid #ddd;
 line-height: 40px;
 padding-left: 3px;
 padding-right: 3px;
 text-align: right;
 }
 input[type="button"] {
 width: 40px;
 height: 40px;
 border: 1px solid #ddd;
 margin: 5px;
 font-family: "微软雅黑";
 font-size: 18px;
 font-weight: bold;
 }
 #panel div, p, input {
 float: left;
 }
</style>

<script>
 window.onload = function(){
  //获取计算器面板
  var panel = document.getElementById("panel");
  //为计算面板动态添加单击事件
  panel.onclick = function(e) {
   //参数e用来接收event对象
   //获取所有input元素
   var inputs = e.toElement;
   //获取P元素
   var screen = document.getElementById("screen");
   if(inputs.type == "button") {
    //如果获取到的是input元素，则开始执行运算逻辑
    if (inputs.value == "C") {
     screen.innerHTML = "";
    } else if (inputs.value == "=") {
     try {
      screen.innerHTML = eval(screen.innerHTML);
     } catch (ex) {
      screen.innerHTML = "Error";
     }
    } else {
     screen.innerHTML += inputs.value;
    }
   } else {
    //如果单击的地方不是input元素，则程序不回应
    return;
   }
  }
 }
</script>

<body>
 <div id="panel">
  <div>
   <p id="screen"></p>
   <input type="button" value="C" />
  </div>
  <div>
   <input type="button" value="7" />
   <input type="button" value="8" />
   <input type="button" value="9" />
   <input type="button" value="/" />
   <input type="button" value="4" />
   <input type="button" value="5" />
   <input type="button" value="6" />
   <input type="button" value="*" />
   <input type="button" value="1" />
   <input type="button" value="2" />
   <input type="button" value="3" />
   <input type="button" value="+" />
   <input type="button" value="." />
   <input type="button" value="0" />
   <input type="button" value="-" />
   <input type="button" value="=" />
  </div>
 </div>
</body>