semester-2/php-develop/w6-03-get-avg/index.html

<!doctype html>
<html lang="zh-hant-tw">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <form id="score-table">
        <div>
            <label>國文</label>
            <input type="number" name="chinese" value="60" required>
        </div>

        <div>
            <label>英文</label>
            <input type="number" name="english" value="60" required>
        </div>

        <div>
            <label>數學</label>
            <input type="number" name="math" value="60" required>
        </div>

        <div>
            <label>平均</label>
            <input type="number" name="avg" id="average-field" readonly>
        </div>

        <button type="submit">計算</button>
    </form>

    <script>
        const $average = document.getElementById("average-field");

        document.getElementById("score-table").onsubmit = function (event) {
            event.preventDefault(); // 不要真的 submit

            const formData = new FormData(this);

            let total = 0;
            let entries = 0;

            for (const [key, value] of formData.entries()) {
                // 不要把平均也算進去。
                if (key === "avg") continue;

                total += Number(value);  // 總分
                entries += 1;   // 科目數
            }

            const average = total / entries;
            $average.value = average;

            if (average >= 60) {
                $average.style.backgroundColor = "#93dded";
            } else {
                $average.style.backgroundColor = "#f7a8a8";
            }
        }
    </script>
</body>
</html>