You can allow users to sign in to your app using multiple authenticationproviders by linking auth provider credentials to an existing user account.Users are identifiable by the same Firebase user ID regardless of theauthentication provider they used to sign in. For example, a user who signed inwith a password can link a Google account and sign in with either method in thefuture. Or, an anonymous user can link a Facebook account and then, later, signin with Facebook to continue using your app.

Before you begin

Add support for two or more authentication providers (possibly includinganonymous authentication) to your app.

Link federated auth provider credentials to a user account

To link credentials from an auth provider such as Google or Facebook to anexisting user account:

1. Sign in the user using any authentication provider or method.
2. Get theAuthProvider object that corresponds to the provider you want to link to the user's account. Examples:

   Web

   import{GoogleAuthProvider,FacebookAuthProvider,TwitterAuthProvider,GithubAuthProvider}from"firebase/auth";constgoogleProvider=newGoogleAuthProvider();constfacebookProvider=newFacebookAuthProvider();consttwitterProvider=newTwitterAuthProvider();constgithubProvider=newGithubAuthProvider();
   auth_get_providers.js

   Web

   vargoogleProvider=newfirebase.auth.GoogleAuthProvider();varfacebookProvider=newfirebase.auth.FacebookAuthProvider();vartwitterProvider=newfirebase.auth.TwitterAuthProvider();vargithubProvider=newfirebase.auth.GithubAuthProvider();
   link-multiple-accounts.js

3. Prompt the user to sign in with the provider you want to link. You can prompt your users to sign in either by opening a pop-up window or by redirecting to the provider's sign-in page. The redirect method is preferred on mobile devices.
   • To sign in with a pop-up window, calllinkWithPopup:

     Web

     import{getAuth,linkWithPopup,GoogleAuthProvider}from"firebase/auth";constprovider=newGoogleAuthProvider();constauth=getAuth();linkWithPopup(auth.currentUser,provider).then((result)=>{// Accounts successfully linked.constcredential=GoogleAuthProvider.credentialFromResult(result);constuser=result.user;// ...}).catch((error)=>{// Handle Errors here.// ...});
     auth_link_with_popup.js

     Web

     auth.currentUser.linkWithPopup(provider).then((result)=>{// Accounts successfully linked.varcredential=result.credential;varuser=result.user;// ...}).catch((error)=>{// Handle Errors here.// ...});
     link-multiple-accounts.js

   • To sign in by redirecting to the provider's sign-in page, calllinkWithRedirect: Follow thebest practices when using `linkWithRedirect`.

     Web

     import{getAuth,linkWithRedirect,GoogleAuthProvider}from"firebase/auth";constprovider=newGoogleAuthProvider();constauth=getAuth();linkWithRedirect(auth.currentUser,provider).then(/* ... */).catch(/* ... */);
     auth_link_with_redirect.js

     Web

     auth.currentUser.linkWithRedirect(provider).then(/* ... */).catch(/* ... */);
     link-multiple-accounts.js

     After the user signs in, they are redirected back to your page. Then, you can retrieve the sign-in result by callinggetRedirectResult when your page loads:

     Web

     import{getRedirectResult}from"firebase/auth";getRedirectResult(auth).then((result)=>{constcredential=GoogleAuthProvider.credentialFromResult(result);if(credential){// Accounts successfully linked.constuser=result.user;// ...}}).catch((error)=>{// Handle Errors here.// ...});
     auth_get_redirect_result.js

     Web

     auth.getRedirectResult().then((result)=>{if(result.credential){// Accounts successfully linked.varcredential=result.credential;varuser=result.user;// ...}}).catch((error)=>{// Handle Errors here.// ...});
     link-multiple-accounts.js

   If the user is signed in successfully, the user's account with the provider is linked to the user's account in your Firebase project.

   Account linking will fail if the credentials are already linked to another user account. In this situation, you must handle merging the accounts and associated data as appropriate for your app:

   Web

   import{getAuth,signInWithCredential,linkWithCredential,OAuthProvider}from"firebase/auth";// The implementation of how you store your user data depends on your applicationconstrepo=newMyUserDataRepo();// Get reference to the currently signed-in userconstauth=getAuth();constprevUser=auth.currentUser;// Get the data which you will want to merge. This should be done now// while the app is still signed in as this user.constprevUserData=repo.get(prevUser);// Delete the user's data now, we will restore it if the merge failsrepo.delete(prevUser);// Sign in user with the account you want to link tosignInWithCredential(auth,newCredential).then((result)=>{console.log("Sign In Success",result);constcurrentUser=result.user;constcurrentUserData=repo.get(currentUser);// Merge prevUser and currentUser data stored in Firebase.// Note: How you handle this is specific to your applicationconstmergedData=repo.merge(prevUserData,currentUserData);constcredential=OAuthProvider.credentialFromResult(result);returnlinkWithCredential(prevUser,credential).then((linkResult)=>{// Sign in with the newly linked credentialconstlinkCredential=OAuthProvider.credentialFromResult(linkResult);returnsignInWithCredential(auth,linkCredential);}).then((signInResult)=>{// Save the merged data to the new userrepo.set(signInResult.user,mergedData);});}).catch((error)=>{// If there are errors we want to undo the data merge/deletionconsole.log("Sign In Error",error);repo.set(prevUser,prevUserData);});
   auth_merge_accounts.js

   Web

   // The implementation of how you store your user data depends on your applicationvarrepo=newMyUserDataRepo();// Get reference to the currently signed-in uservarprevUser=auth.currentUser;// Get the data which you will want to merge. This should be done now// while the app is still signed in as this user.varprevUserData=repo.get(prevUser);// Delete the user's data now, we will restore it if the merge failsrepo.delete(prevUser);// Sign in user with the account you want to link toauth.signInWithCredential(newCredential).then((result)=>{console.log("Sign In Success",result);varcurrentUser=result.user;varcurrentUserData=repo.get(currentUser);// Merge prevUser and currentUser data stored in Firebase.// Note: How you handle this is specific to your applicationvarmergedData=repo.merge(prevUserData,currentUserData);returnprevUser.linkWithCredential(result.credential).then((linkResult)=>{// Sign in with the newly linked credentialreturnauth.signInWithCredential(linkResult.credential);}).then((signInResult)=>{// Save the merged data to the new userrepo.set(signInResult.user,mergedData);});}).catch((error)=>{// If there are errors we want to undo the data merge/deletionconsole.log("Sign In Error",error);repo.set(prevUser,prevUserData);});
   link-multiple-accounts.js

Link email address and password credentials to a user account

To add email address and password credentials to an existing useraccount:

1. Sign in the user using any authentication provider or method.
2. Prompt the user for an email address and new password.
3. Create anAuthCredential object with the email address and password:

   Web

   import{EmailAuthProvider}from"firebase/auth";constcredential=EmailAuthProvider.credential(email,password);
   auth_make_email_credential.js

   Web

   varcredential=firebase.auth.EmailAuthProvider.credential(email,password);
   link-multiple-accounts.js

4. Pass theAuthCredential object to the signed-in user'slinkWithCredential method:

   Web

   import{getAuth,linkWithCredential}from"firebase/auth";constauth=getAuth();linkWithCredential(auth.currentUser,credential).then((usercred)=>{constuser=usercred.user;console.log("Account linking success",user);}).catch((error)=>{console.log("Account linking error",error);});
   auth_simple_link.js

   Web

   auth.currentUser.linkWithCredential(credential).then((usercred)=>{varuser=usercred.user;console.log("Account linking success",user);}).catch((error)=>{console.log("Account linking error",error);});
   link-multiple-accounts.js

   The call tolinkWithCredential will fail if the credentials are already linked to another user account. In this situation, you must handle merging the accounts and associated data as appropriate for your app (see example above).

Unlink an auth provider from a user account

A single Firebase user account can have multiple authentication providers linked to it (for example, email/password, Google, Facebook), which lets the user sign in to the same Firebase account through different methods.

If you unlink an authentication provider from a user's account, they can no longer sign in with that provider.

import{getAuth,unlink}from"firebase/auth";constauth=getAuth();unlink(auth.currentUser,providerId).then(()=>{// Auth provider unlinked from account// ...}).catch((error)=>{// An error happened// ...});
auth_unlink_provider.js

user.unlink(providerId).then(()=>{// Auth provider unlinked from account// ...}).catch((error)=>{// An error happened// ...});
link-multiple-accounts.js

Troubleshooting

If you encounter errors when trying to link multiple accounts, see thedocumentation on verified email addresses.