areaCircle.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <div>
        반지름: <input id='radius'> <br>
        <button onclick="circle()"> 원의 넓이 구하기 </button>
        <p></p>
        <p id="result"> </p>

    </div>
    <script>
        function circle() {
            let p = Math.PI;
            let r = Number(document.getElementById('radius').value);
            if (isNaN(r) || !r) {
                document.getElementById('result').innerHTML = '빈칸이 있거나 숫자가 아닌 값이 들어있습니다.';
            } else {
                let circleArea = p * Math.pow(r, 2);
                document.getElementById('result').innerHTML = '원의 넓이는 = ' + circleArea;
            }
        }
    </script>
</body>

</html>