index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Calculator</title>
    <link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" rel="stylesheet">
    <style>
        body {
            background-color: #f0f0f0;
            font-family: Arial, sans-serif;
        }
        
        .calculator {
            max-width: 400px;
            margin: 0 auto;
            background-color: #fff;
            box-shadow: 0px 0px 10px rgba(0, 0, 0, 0.2);
            padding: 20px;
            border-radius: 10px;
        }
        
        .calculator h1 {
            font-size: 24px;
            text-align: center;
        }
        
        .calculator input[type="text"] {
            width: 100%;
            padding: 10px;
            font-size: 20px;
            text-align: right;
            border: none;
            outline: none;
            border-bottom: 1px solid #ccc;
        }
        
        .calculator .buttons {
            display: grid;
            grid-template-columns: repeat(4, 1fr);
            grid-gap: 10px;
            margin-top: 20px;
        }
        
        .calculator button {
            padding: 15px;
            font-size: 18px;
            text-align: center;
            color: #fff;
            border: none;
            border-radius: 5px;
            transition: background-color 0.3s ease;
        }
    </style>
</head>
<body>
    <div class="container mt-5">
        <div class="calculator">
            <h1>Calculator</h1>
            <input type="text" id="result" disabled>
            <div class="buttons">
                <button class="btn btn-warning" onclick="appendToResult('7')">7</button>
                <button class="btn btn-warning" onclick="appendToResult('8')">8</button>
                <button class="btn btn-warning" onclick="appendToResult('9')">9</button>
                <button class="btn btn-success" onclick="appendToResult('/')">/</button>
                <button class="btn btn-warning" onclick="appendToResult('4')">4</button>
                <button class="btn btn-warning" onclick="appendToResult('5')">5</button>
                <button class="btn btn-warning" onclick="appendToResult('6')">6</button>
                <button class="btn btn-success" onclick="appendToResult('*')">*</button>
                <button class="btn btn-warning" onclick="appendToResult('1')">1</button>
                <button class="btn btn-warning" onclick="appendToResult('2')">2</button>
                <button class="btn btn-warning" onclick="appendToResult('3')">3</button>
                <button class="btn btn-success" onclick="appendToResult('-')">-</button>
                <button class="btn btn-danger" onclick="clearResult()">C</button>
                <button class="btn btn-warning" onclick="appendToResult('0')">0</button>
                <button class="btn btn-info" onclick="appendToResult('.')">.</button>
                <button class="btn btn-success" onclick="appendToResult('+')">+</button>
                <button class="btn btn-light" disabled></button>
                <button class="btn btn-light" disabled></button>
                <button class="btn btn-light" disabled></button>
                <button class="btn btn-dark" onclick="calculateResult()">=</button>
            </div>
        </div>
    </div>

    <script>
        let result = document.getElementById('result');

        function appendToResult(value) {
            result.value += value;
        }

        function clearResult() {
            result.value = '';
        }

        function calculateResult() {
            try {
                result.value = eval(result.value);
            } catch (error) {
                result.value = 'Error';
            }
        }
    </script>
</body>
</html>