やり方

画面で作ってみました
<!DOCTYPE html>
<html lang="ja">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>数字変換ツール</title>
    <style>
        body {
            font-family: Arial, sans-serif;
            padding: 20px;
        }
        input, button {
            font-size: 16px;
            margin: 10px 0;
        }
        .output {
            font-size: 20px;
            font-weight: bold;
            color: blue;
        }
    </style>
</head>
<body>
    <h1>半角数字を全角数字に変換</h1>
    <label for="input-number">半角数字を入力してください:</label>
    <input type="text" id="input-number">
    <button onclick="convertToFullWidth()">変換</button>
    <p>変換後の数字: <span class="output" id="output-number"></span></p>

    <script>
        function toFullWidthNumber(numStr) {
            const offset = '０'.charCodeAt(0) - '0'.charCodeAt(0);
            return numStr.replace(/[0-9]/g, function(ch) {
                return String.fromCharCode(ch.charCodeAt(0) + offset);
            });
        }

        function convertToFullWidth() {
            const inputNumber = document.getElementById('input-number').value;
            const outputNumber = toFullWidthNumber(inputNumber);
            document.getElementById('output-number').textContent = outputNumber;
        }
    </script>
</body>
</html>