index.html

<html lang='en'>

<head>
    <meta charset='utf-8' />
    <title>Discord OAUTH</title>

    <style>
        * {
            text-align: center;
            margin: 0px auto;
        }

        #welcome_txt {
            font-size: 24px;
        }

        #login-link {
            background-color: whitesmoke;
            border-radius: 8px;
            color: black;
            padding: 15px 30px;
            text-align: center;
            text-decoration: none;
            display: inline-block;
            font-size: 20px;
        }

        #login-link:hover {
            cursor: pointer;
        }
    </style>
</head>

<body>
    <br>
    <a id='login-link' href='[DISCORD OAUTH LINK HERE]'>Login with Discord</a><br><br>
    <p id='welcome_txt'></p>

    <script>
        window.onload = function () {
            if (location.href.indexOf("code") !== -1) { // Detect if you logged in or not
                const code = location.href.substring(location.href.indexOf("code") + 5, location.href.length); // Get the code OAUTH gives you
                const req = new XMLHttpRequest(); // Create a new XMLHttpRequest
                req.open("POST", "http://localhost:80/user"); // Open the XMLHttpRequest; CHANGE THE PORT TO THE PORT YOU HAVE AS YOUR VARIABLE IN OAUTH.js.
                req.send(code); // Send the code in the request
                req.onload = () => { // Will run when the request is loaded
                    if (req.status === 500) { // Error
                        document.getElementById('welcome_txt').innerText = `There was an error with that. Please try logging in again. Error Code: ${req.status}`;
                    } else if (req.status === 200) { // Successful
                        document.getElementById("welcome_txt").innerText = `Welcome, ${req.responseText}!`
                    } else { // Other
                        document.getElementById('welcome_txt').innerText = `An error occured. Please try logging in again. Error Code: ${req.status}`;
                    }

                }
            }
        }
    </script>
</body>

</html>