The callback route

server.js

const CLIENT_ID = process.env.CLIENT_ID;
const CLIENT_SECRET = process.env.CLIENT_SECRET;
const REDIRECT = process.env.DISCORD_CALLBACK_URI;

app.get('/discord/callback', async function(req, res) {
  // We must check to ensure that a code was provided for us to use
  if (!req.query.code) res.send('No code provided!');
                                
  const code = req.query.code;
  // We will now create the credentials we will use to get our access and refresh tokens
  const creds = btoa(`${CLIENT_ID}:${CLIENT_SECRET}`);

  try {
    const resp = await fetch(`https://discordapp.com/api/oauth2/token?grant_type=authorization_code&code=${code}&redirect_uri=${REDIRECT}`, {
      method: 'POST',
      headers: {
        Authorization: `Basic ${creds}`
      }
    });
    const token = await resp.json();
    
    res.json(token);
  } catch (err) {
    console.error(err);
    res.send('Some error occured getting your tokens!');
  }
});