Property ‘x’ does not exist on type … – Promise syntax – TS2339

I met a problem I didn’t understand with Typescript. I found another topics with that kind of problem but even that i dont know how to resolve my problem.
I can console.log userCredential.user.metadata.b easily, but even that I got a error: Property ‘b’ does not exist on type ‘UserMetadata’. Can u explain me what here is wrong?

Here the part of code

<pre>
 login(email: string, password: string): Promise<any> {
       return this.afAuth.signInWithEmailAndPassword(email, password)
           .then(userCredential => {
               const creationTime: number = userCredential.user.metadata.b; // <-here is a problem
               console.log('creationTinme', creationTime);
               this.handleAuthentication(
                   userCredential.user.email,
                   userCredential.user.uid,
                   'xxx',
                   creationTime
                   // metadata.creationTime expiresInuser.metadata
               );
           })
           .catch(error => {
                console.log(error);
           });
</pre>

Thank you for help in advance

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