生成计算器，获取无法设置null的属性“value”

<div id="calculator">
  <input type="text" id="display" disabled>
  <div class="buttons">
    <button onclick="input('7')">7</button>
    <button onclick="input('8')">8</button>
    <button onclick="input('9')">9</button>
    <button onclick="input('/')">/</button>
    <button onclick="input('4')">4</button>
    <button onclick="input('5')">5</button>
    <button onclick="input('6')">6</button>
    <button onclick="input('*')">*</button>
    <button onclick="input('1')">1</button>
    <button onclick="input('2')">2</button>
    <button onclick="input('3')">3</button>
    <button onclick="input('-')">-</button>
    <button onclick="input('0')">0</button>
    <button onclick="input('.')">.</button>
    <button onclick="calculate()">=</button>
    <button onclick="input('+')">+</button>
    <button onclick="clearInput()">Clear</button>
  </div>
</div>

function input(value) {
  var display = document.getElementById("display");
  display.value += value;
}

function calculate() {
  var display = document.getElementById("display");
  var result = eval(display.value);
  display.value = result;
}

function clearInput() {
  var display = document.getElementById("display");
  display.value = "";
}