main.html

<!DOCTYPE html>
<head>
    <title>Grade Summary</title>
    <link rel="stylesheet" href="site.css">
</head>
<body>
    <h4>Grade Summary App</h4>
    <h1>Enter Grades</h1>

    <div class="form-layout">
        <div class="form-element">
            <input id="grade" />
        </div>
        <div class="form-element">
            <input type="button" onclick="add()" value="Add" />
        </div>
    </div>
    <div class="form-layout">
        <button id="showSummary" onclick="showSummary()">Show Summary</button>
    </div>
    <div class="form-layout">
        <button id="clear" onclick="clear()">Clear</button>
    </div>

    <script type="application/javascript">
        const grades = [];

        function add() {
            let grade = document.getElementById("grade").value;
            grades.push(grade);
            document.getElementById("grade").value="";
            document.getElementById("grade").focus();
        }

        function clear() {
            grades = [];
        }

        function showSummary() {
            localStorage.setItem("gradeArray", grades);
            window.open("summary.html", "_self");
        }

    </script>
</body>