Property 'accessToken' does not exist on type 'AuthCredential'

You need to cast the AuthCredential to OAuthCredential which has the accessToken property: https://github.com/firebase/firebase-js-sdk/blob/master/packages/auth-types/index.d.ts#L244

As suggested by @bojeil casting worked for me

firebase
      .auth()
      .signInWithPopup(this.provider)
      .then(result => {
        // This gives you a Google Access Token. You can use it to access the Google API.
        const credential = result.credential as firebase.auth.OAuthCredential;
        const token = credential.accessToken;
        // The signed-in user info.
        const user = result.user;

You need to look at the definition of AuthCredential. If it's this one, it does not have an accessToken property.

The error occurs due to typescript definitions

Replace

let token = result.credential.accessToken;

with

let token = (<any>result).credential.accessToken;