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GitHub auth provider.

GitHub requires an OAuth 2.0 redirect, so you can either handle the redirect directly, or use the signInWithPopup handler:

example
// Using a redirect.
firebase.auth().getRedirectResult().then(function(result) {
  if (result.credential) {
    // This gives you a GitHub Access Token.
    var token = result.credential.accessToken;
  }
  var user = result.user;
}).catch(function(error) {
  // Handle Errors here.
  var errorCode = error.code;
  var errorMessage = error.message;
  // The email of the user's account used.
  var email = error.email;
  // The firebase.auth.AuthCredential type that was used.
  var credential = error.credential;
  if (errorCode === 'auth/account-exists-with-different-credential') {
    alert('You have signed up with a different provider for that email.');
    // Handle linking here if your app allows it.
  } else {
    console.error(error);
  }
});

// Start a sign in process for an unauthenticated user.
var provider = new firebase.auth.GithubAuthProvider();
provider.addScope('repo');
firebase.auth().signInWithRedirect(provider);
example
// With popup.