15. Check Given number is prime or not.

    <html>
    <head>
        <script>
            function prime() {
                var n = parseInt(document.getElementById("num").value);
                var flag = 0;

                // Numbers less than 2 are not prime
                if (n <= 1) {
                    document.getElementById("out").innerHTML = n + " is not a prime number.";
                    return;
                }

                // Check divisibility from 2 to n / 2
                for (var i = 2; i <= n / 2; i++) {
                    if (n % i === 0) {
                        flag = 1;
                        break;
                    }
                }

                // Display the result
                if (flag === 0) {
                    document.getElementById("out").innerHTML = n + " is a prime number.";
                } else {
                    document.getElementById("out").innerHTML = n + " is not a prime number.";
                }
            }
        </script>
    </head>

    <body>
        <h2>Prime Number Checker</h2>
        Enter the number: <input type="number" id="num">
        <button type="button" onclick="prime()">Check</button>
        <hr>
        <p id="out"></p>
    </body>
    </html>

Output

        No Output Available