semester-2/php-develop/w6-01-sum-calculator/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>

    <style>
        .container {
            display: flex;
            flex-direction: column;
            text-align: center;
            gap: 1.25em;

            justify-content: center;
            align-items: center;
            height: 100vh;

            margin: 2em;
        }

        #calculate-button {
            width: 40%;
        }
    </style>
    <script>
        /**
         * Get the sum of two numbers and show the answer in the answer input.
         *
         * @param {HTMLInputElement} left
         * @param {HTMLInputElement} right
         * @param {HTMLInputElement} answer
         */
        function sum(left, right, answer) {
            // `+(string) -> number`
            const leftNum = +left.value;
            const rightNum = +right.value;

            // `number + '' -> string`
            answer.value = leftNum + rightNum + "";
        }

        window.onload = function () {
            const leftNumber = document.getElementById("left-number");
            const rightNumber = document.getElementById("right-number");
            const answer = document.getElementById("answer");

            document.getElementById("calculate-button").onclick =
                () => sum(leftNumber, rightNumber, answer);
        }
    </script>
</head>
<body>
<section class="container">
        <div>
            <input type="number" id="left-number" value="6">
            +
            <input type="number" id="right-number" value="3">
            =
            <input type="number" id="answer" value="9">
        </div>

        <button id="calculate-button">計算 Calculate</button>
    </section>
</body>
</html>