简单计算器

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" name="num1" id="num1" value="">
<select name="choice" id="choice">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
    <option value="%">%</option>
</select>
<input type="text" name="num2" id="num2">
<input type="button" id="calc" value="计算">
<p id="res"></p>
<script>
    document.getElementById('calc').onclick = function () {
        var num1 = document.getElementById('num1').value
        var num2 = document.getElementById('num2').value
        var choice = document.getElementById('choice').value
        var res = ''
        if (num1 == '' || isNaN(num1 * 1)) res += '第一个输入框必须为数字'
        if (num2 == '' || isNaN(num2 * 1)) res += '第二个输入框必须为数字'
        if ((choice == '/' || choice == '%') && num2 * 1 == 0) res += '第二个输入框不能为零'
        num1 = num1 * 1;
        num2 = num2 * 1;
        if (!res) {
            switch (choice) {
                case '+':
                    res = num1 + num2
                    break
                case '-':
                    res = num1 - num2
                    break
                case '*':
                    res = num1 * num2
                    break
                case '/':
                    res = num1 / num2
                    break
                default:
                    res = num1 % num2
            }
        }
        document.getElementById('res').innerText = res
    }
</script>
</body>
</html>
简单计算器
简单计算器

简单计算器

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