JavaScript实现简易计算器：动态事件注册与基础DOM操作

<!DOCTYPE html>
<html>
 <head>
 <meta charset="utf-8">
 <title>简易计算器</title>
 <script type="text/javascript">
 window.onload = function(){
 document.onclick = function(){
  var ipts = document.getElementsByTagName("input");
  for (var i = 0; i < ipts.length; i++) {//得到值和符号
  if (ipts[i].type == "text") {
  switch (i){
  case 0:
   num1 = ipts[i].value;
   num1 = Number(num1);
   break;
  case 1:
   opt = ipts[i].value;
   break;
  case 2:
   num2 = ipts[i].value;
   num2 = Number(num2);
   break;
  }
  }
  }
  var res;//结果
  switch (opt){//通过得到的值和符号进行计算
  case "+":
  res = num1 + num2;
  break;
  case "-":
  res = num1 - num2;
  break;
  case "*":
  res = num1 * num2;
  break;
  case "/":
  res = num1 / num2;
  break;
  }
  
  var r = document.getElementById("result");
  r.value = res;
 } 
 }
 </script>
 </head>
 <body>
 <input type="text">
 <input type="text">
 <input type="text">=
 <input type="text" id="result">
 <input type="button" value="计算" id="btn">
 </body>
</html>