使用js写一个简单的计算器

以下是一个简单的计算器示例，使用JavaScript编写：

HTML代码：

<div id="calculator">
  <input type="text" id="screen" readonly>
  <table>
    <tr>
      <td><button onclick="clearScreen()">C</button></td>
      <td><button onclick="toggleSign()">+/-</button></td>
      <td><button onclick="backspace()">←</button></td>
      <td><button onclick="operate('/')">÷</button></td>
    </tr>
    <tr>
      <td><button onclick="appendNumber(7)">7</button></td>
      <td><button onclick="appendNumber(8)">8</button></td>
      <td><button onclick="appendNumber(9)">9</button></td>
      <td><button onclick="operate('*')">x</button></td>
    </tr>
    <tr>
      <td><button onclick="appendNumber(4)">4</button></td>
      <td><button onclick="appendNumber(5)">5</button></td>
      <td><button onclick="appendNumber(6)">6</button></td>
      <td><button onclick="operate('-')">-</button></td>
    </tr>
    <tr>
      <td><button onclick="appendNumber(1)">1</button></td>
      <td><button onclick="appendNumber(2)">2</button></td>
      <td><button onclick="appendNumber(3)">3</button></td>
      <td><button onclick="operate('+')">+</button></td>
    </tr>
    <tr>
      <td><button onclick="appendNumber(0)">0</button></td>
      <td><button onclick="appendDecimal()">.</button></td>
      <td><button onclick="calculate()">=</button></td>
    </tr>
  </table>
</div>

JavaScript代码：

let currentNumber = 0;
let operator = null;
let waitingForSecondNumber = false;

function appendNumber(number) {
  const screen = document.getElementById('screen');
  
  if (waitingForSecondNumber) {
    screen.value = number;
    waitingForSecondNumber = false;
  } else {
    if (screen.value === '0') {
      screen.value = number;
    } else {
      screen.value += number;
    }
  }
  
  currentNumber = parseFloat(screen.value);
}

function appendDecimal() {
  const screen = document.getElementById('screen');
  
  if (waitingForSecondNumber) {
    screen.value = '0.';
    waitingForSecondNumber = false;
  } else {
    if (screen.value.indexOf('.') === -1) {
      screen.value += '.';
    }
  }
  
  currentNumber = parseFloat(screen.value);
}

function clearScreen() {
  const screen = document.getElementById('screen');
  
  screen.value = '0';
  currentNumber = 0;
  operator = null;
  waitingForSecondNumber = false;
}

function toggleSign() {
  const screen = document.getElementById('screen');
  
  screen.value = -parseFloat(screen.value);
  currentNumber = parseFloat(screen.value);
}

function backspace() {
  const screen = document.getElementById('screen');
  
  screen.value = screen.value.slice(0, -1);
  
  if (screen.value === '') {
    screen.value = '0';
  }
  
  currentNumber = parseFloat(screen.value);
}

function operate(newOperator) {
  const screen = document.getElementById('screen');
  
  if (operator !== null && !waitingForSecondNumber) {
    calculate();
  }
  
  waitingForSecondNumber = true;
  operator = newOperator;
}

function calculate() {
  const screen = document.getElementById('screen');
  let result = 0;
  
  switch (operator) {
    case '+':
      result = currentNumber + parseFloat(screen.value);
      break;
    case '-':
      result = currentNumber - parseFloat(screen.value);
      break;
    case '*':
      result = currentNumber * parseFloat(screen.value);
      break;
    case '/':
      result = currentNumber / parseFloat(screen.value);
      break;
  }
  
  screen.value = result.toString();
  currentNumber = result;
  waitingForSecondNumber = true;
}

该计算器包括数字、小数点、正负号、加、减、乘、除、清除、删除和等于按钮。用户可以通过单击数字和运算符按钮来输入表达式，然后单击等于按钮来计算结果。