Authenticate with Firebase Anonymously Using JavaScript

You can use Firebase Authentication to create and use temporary anonymous accounts to authenticate with Firebase. These temporary anonymous accounts can be used to allow users who haven't yet signed up to your app to work with data protected by security rules. If an anonymous user decides to sign up to your app, you can link their sign-in credentials to the anonymous account so that they can continue to work with their protected data in future sessions.

Before you begin

1. Add Firebase to your JavaScript project.
2. If you haven't yet connected your app to your Firebase project, do so from the Firebase console.
3. Enable anonymous auth:
   1. In the Firebase console, open the Auth section.
   2. On the Sign-in Methods page, enable the Anonymous sign-in method.

Authenticate with Firebase anonymously

When a signed-out user uses an app feature that requires authentication with Firebase, sign in the user anonymously by completing the following steps:

1. Call the signInAnonymously method:

   firebase.auth().signInAnonymously().catch(function(error) {
     // Handle Errors here.
     var errorCode = error.code;
     var errorMessage = error.message;
     // ...
   });
   

   This is also where you can catch and handle errors. For a list of error codes have a look at the Auth Reference Docs.
2. If the signInAnonymously method completes without error, the observer registered in the onAuthStateChanged will trigger and you can get the anonymous user's account data from the User object:

   firebase.auth().onAuthStateChanged(function(user) {
     if (user) {
       // User is signed in.
       var isAnonymous = user.isAnonymous;
       var uid = user.uid;
       // ...
     } else {
       // User is signed out.
       // ...
     }
     // ...
   });

Convert an anonymous account to a permanent account

When an anonymous user signs up to your app, you might want to allow them to continue their work with their new account—for example, you might want to make the items the user added to their shopping cart before they signed up available in their new account's shopping cart. To do so, complete the following steps:

1. When the user signs up, complete the sign-in flow for the user's authentication provider up to, but not including, calling one of the Auth.signInWith methods. For example, get the user's Google ID token, Facebook access token, or email address and password.

2. Get an AuthCredential for the new authentication provider:

   Google Sign-In

   var credential = firebase.auth.GoogleAuthProvider.credential(
       googleUser.getAuthResponse().id_token);
   

   Facebook Login

   var credential = firebase.auth.FacebookAuthProvider.credential(
       response.authResponse.accessToken);
   

   Email-password sign-in

   var credential = firebase.auth.EmailAuthProvider.credential(email, password);
   

3. Pass the AuthCredential object to the sign-in user's link method:

   auth.currentUser.linkWithCredential(credential)
     .then(function(usercred) {
       var user = usercred.user;
       console.log("Anonymous account successfully upgraded", user);
     }).catch(function(error) {
       console.log("Error upgrading anonymous account", error);
     });
   link-multiple-accounts.js

If the call to link succeeds, the user's new account can access the anonymous account's Firebase data.

Next steps

Now that users can authenticate with Firebase, you can control their access to data in your Firebase database using Firebase rules.