HTML 代码：

<input type="text" id="input-box">
<span id="result"></span>

JavaScript 代码：

const inputBox = document.getElementById('input-box');
const result = document.getElementById('result');

// 监听输入框输入事件
inputBox.addEventListener('input', function() {
  const value = inputBox.value.trim();
  if (value === '') {
    result.textContent = '';
  } else {
    try {
      const res = eval(value);
      result.textContent = `= ${res}`;
    } catch (e) {
      result.textContent = '';
    }
  }
});