老师，option这里不需要做事件监听吗？

来源：4-14 项目作业

慕UI4313976

2021-05-03 14:13:28

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <input type="text" id="num1">
    <select name="" id="option">
        <option value="+">加</option>
        <option value="-">减</option>
        <option value="*">乘</option>
        <option value="/">除</option>
    </select>
    <input type="text" id="num2">
    <input type="button" value="等于" id="btn">
    <input type="text" id="result">

    <script>
        var num1 = document.getElementById('num1'),
            option = document.getElementById('option'),
            num2 = document.getElementById('num2'),
            btn = document.getElementById('btn'),
            result = document.getElementById('result');

        btn.addEventListener('click', function () {
            switch (option.value) {
                case '+':
                    result.value = Number(num1.value) + Number(num2.value);
                    break;
                case '-':
                    result.value = Number(num1.value) - Number(num2.value);
                    break;
                case '*':
                    result.value = Number(num1.value) * Number(num2.value);
                    break;
                case '/':
                    result.value = Number(num1.value) / Number(num2.value);
                    break;

                default:
                    alert('出错了')
                    break;
            }
        })

    </script>
</body>

</html>