简易计算器

作业如下
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>简易计算器</title>
  <style>
    input {height: 30px; line-height: 30px; padding: 0 10px; width: 60px; border: solid 1px #ddd}
    select {height: 30px; line-height: 30px; padding: 0 10px;border: solid 1px #ddd}
    button {background-color: dodgerblue; color: #fff; border: none; height: 30px; line-height: 30px; padding: 0 10px; cursor: pointer}
    #jieguo {margin-top: 10px; color: #f00}
  </style>
</head>
<body>
<input type="text" name="a" id="a" value="">
<select name="y" id="y">
  <option value="+">+</option>
  <option value="-">-</option>
  <option value="*">*</option>
  <option value="/">/</option>
</select>
<input type="text" name="b" id="b" value="2">
<button id="btn">计算</button>
<div id="jieguo">?</div>
<script>
  document.getElementById('btn').onclick=function () {
    var a = document.getElementById('a').value;
    var b = document.getElementById('b').value;
    var y = document.getElementById('y').value;
    a = parseInt(a);
    b = parseInt(b);
    var tiaojian = true; // 输入条件符合
    var n;
      // 输入情况判断
    if (!a){
      n = '第一个值不能为空或者不是数字';
      tiaojian = false
    }
    if (!b){
      var n = 1111;
      alert('第二个值不能为空或者不是数字')
      tiaojian = false
    }
  // if判断法
  if (tiaojian==true){
    if (y=='+'){
      n = a+"+"+b+"="+(a+b);
    }else if (y=='-'){
      n = a+"-"+b+"="+(a-b);
    }else if (y=='*'){
      n = a+"*"+b+"="+(a*b);
    }else if (y=='/'){
      n = a+"/"+b+"="+(a/b);
    }
  }
    document.getElementById('jieguo').innerHTML=n;
  }
</script>
</body>
</html>
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