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Generic OAuth provider.

example
// Using a redirect.
firebase.auth().getRedirectResult().then(function(result) {
  if (result.credential) {
    // This gives you the OAuth Access Token for that provider.
    var token = result.credential.accessToken;
  }
  var user = result.user;
});

// Start a sign in process for an unauthenticated user.
var provider = new firebase.auth.OAuthProvider('google.com');
provider.addScope('profile');
provider.addScope('email');
firebase.auth().signInWithRedirect(provider);
example
// Using a popup.
var provider = new firebase.auth.OAuthProvider('google.com');
provider.addScope('profile');
provider.addScope('email');
firebase.auth().signInWithPopup(provider).then(function(result) {
 // This gives you the OAuth Access Token for that provider.
 var token = result.credential.accessToken;
 // The signed-in user info.
 var user = result.user;
});
see

firebase.auth.Auth.onAuthStateChanged<