auth.js

/**
     * Function called when clicking the Login/Logout button.
     */
    // [START buttoncallback]
    function toggleSignIn() {
        if (!firebase.auth().currentUser) {
          // [START createprovider]
          var provider = new firebase.auth.GoogleAuthProvider();
          // [END createprovider]
          
          // [START signin]
          firebase.auth().signInWithRedirect(provider);
          // [END signin]
        } else {
          // [START signout]
          firebase.auth().signOut();
          // [END signout]
        }
        // [START_EXCLUDE]
        document.getElementById('quickstart-sign-in').disabled = true;
        // [END_EXCLUDE]
      }
      // [END buttoncallback]
  
  
      /**
       * initApp handles setting up UI event listeners and registering Firebase auth listeners:
       *  - firebase.auth().onAuthStateChanged: This listener is called when the user is signed in or
       *    out, and that is where we update the UI.
       *  - firebase.auth().getRedirectResult(): This promise completes when the user gets back from
       *    the auth redirect flow. It is where you can get the OAuth access token from the IDP.
       */
      function initApp() {
        // Result from Redirect auth flow.
        // [START getidptoken]
        firebase.auth().getRedirectResult().then(function(result) 
        {
          // The signed-in user info.
          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;
          // [START_EXCLUDE]
          if (errorCode === 'auth/account-exists-with-different-credential') {
            alert('You have already signed up with a different auth provider for that email.');
            // If you are using multiple auth providers on your app you should handle linking
            // the user's accounts here.
          } else {
            console.error(error);
          }
          // [END_EXCLUDE]
        });
        // [END getidptoken]
  
        // Listening for auth state changes.
        // [START authstatelistener]
        firebase.auth().onAuthStateChanged(function(user) {
          if (user) {
            // User is signed in.
            var displayName = user.displayName;
            var email = user.email;
            var emailVerified = user.emailVerified;
            var photoURL = user.photoURL;
            var isAnonymous = user.isAnonymous;
            var uid = user.uid;
            var providerData = user.providerData;
            // [START_EXCLUDE]
            document.getElementById('quickstart-sign-in-status').textContent = 'Signed in';
            document.getElementById('quickstart-sign-in').textContent = 'Sign out';
            document.getElementById('quickstart-account-details').textContent = email;
            // [END_EXCLUDE]
          } else {
            // User is signed out.
            // [START_EXCLUDE]
            document.getElementById('quickstart-sign-in-status').textContent = 'Signed out';
            document.getElementById('quickstart-sign-in').textContent = 'Sign in with Google';
            document.getElementById('quickstart-account-details').textContent = 'Not Logged In';
            // [END_EXCLUDE]
          }
          // [START_EXCLUDE]
          document.getElementById('quickstart-sign-in').disabled = false;
          // [END_EXCLUDE]
        });
        // [END authstatelistener]
  
        document.getElementById('quickstart-sign-in').addEventListener('click', toggleSignIn, false);
      }
  
      window.onload = function() {
        initApp();
      };