A Cloudflare worker to exchange an OAuth Web Flow code for an access token. Configure your CLIENT_ID and CLIENT_SECRET and you are all set.

worker.js

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request))
})

const CLIENT_ID = '<your client ID>'
const CLIENT_SECRET = '<your client Secret>'

async function handleRequest(request) {
  if (request.method === 'GET') {
    return new Response(`$ curl -XPOST -H'Content-Type: application/json' -d'{"code": "<your oauth code>"}' ${request.url}`)
  }

  const { code } = await request.json()
  return await fetch('https://github.com/login/oauth/access_token', {
    method: "POST",
    headers: {
      'content-type': "application/json",
      accept: "application/json"
    },
    body: JSON.stringify({
      client_id: CLIENT_ID,
      client_secret: CLIENT_SECRET,
      code
    })
  })
}