HTML/JS 简单计算器

<p>简单计算器:</p>
<table border="1" style="position:center;">
  <tr>
    <td>第一个数：</td>
    <td><input type="text" id="first"/></td>
  </tr>
  <tr>
    <td>第二个数：</td>
    <td><input type="text" id="twice"/></td>
  </tr>
  <tr>
    <td colspan="2" >
      &nbsp;
      <button style="width:inherit" onclick="add()">+</button>
      &nbsp;
      <button style="width:inherit" onclick="subtract()">-</button>
      &nbsp;
      <button style="width:inherit" onclick="ride()">*</button>
      &nbsp;
      <button style="width:inherit" onclick="devide()">/</button>
    </td>
  </tr>
  <tr>
    <td colspan="2" rowspan="2">
      <p id="result"></p>
    </td>
  </tr>

</table>

var result_1;
//加法
function add() {
  var a = getFirstNumber();
  var b = getTwiceNumber();
  var re =Number( a) +Number( b);
  sendResult(re);
}


//减法
function subtract() {
  var a = getFirstNumber();
  var b = getTwiceNumber();
  var re = a - b;
  sendResult(re);
}


//乘法
function ride() {
  var a = getFirstNumber();
  var b = getTwiceNumber();
  var re = a * b;
  sendResult(re);
}


//除法
function devide() {
  var a = getFirstNumber();
  var b = getTwiceNumber();
  var re = a / b;
  sendResult(re);
}


//给p标签传值
function sendResult(result_1) {
  var num = document.getElementById("result")
  num.innerHTML = result_1;
}


//获取第一位数字
function getFirstNumber() {
  var firstNumber = document.getElementById("first").value;
  return firstNumber;
}


//获取第二位数字
function getTwiceNumber() {
  var twiceNumber = document.getElementById("twice").value;
  return twiceNumber;
}