This set of functions enables github authentication for frontend-only applications, like those hosted using Github Pages.

How it works:

• frontend opens a new child window to /api/GithubLogin
• /api/GithubLogin redirects to github with the right keys
• user authenticates in the new window, github redirects to an azure function
• azure function posts back to github to get a auth token, renders a small snippet of javascript
• javascript uses postMessage to pass the token back to the original frontend

See architecture decision records in doc/adr

There are a number of one-time setup steps involved.

• register your frontend with github by creating an oauth app
• create an azure functions application, v2 or above
• deploy to azure and get the URL to your OauthCallback function
• set the github "Authorization callback URL" to that URL
• adjust azure environment variable settings to fill in:
  • GithubOauthClientId - your application's client id from github
  • GithubOauthClientSecret - your application's client secret from github
  • targetOrigin - the origin where your frontend is hosted. This is used in the postMessage call to securely get the token back to your frontend
  • GithubOauthScope - what access your app needs. See Understanding scopes for Oauth apps

You need to something like this in your frontend, where-ever you have your users trying to login:

const githubLoginPopupUrl = 'https://YOUR-AZURE-FUNCTION-DOMAIN/api/GithubLogin'
const expectedOrigin = new URL(githubLoginPopupUrl).origin;

window.addEventListener('message', (event) => {
    if (event.origin === expectedOrigin && event.data.type === 'GITHUB_LOGIN_COMPLETE') {
        console.log('Oauth login complete. token:', event.data.payload.token);
        // do whatever you need to do with the token
    }
}, { capture: false, once: true })

window.open(githubLoginPopupUrl, 'Github authentication');