const changeCredential = (() => {
    const credential = firebase.auth.EmailAuthProvider.credential(
        currentUser.email,
        oldPassword
    );
    return currentUser.reauthenticate(credential)
        .then(() => {
            return currentUser.updateEmail(email);
        })
}) ()
.then(() => {
    return (() => {
        const credential = firebase.auth.EmailAuthProvider.credential(
            email,
            oldPassword
        );
        return currentUser.reauthenticate(credential)
            .then(() => {
                return currentUser.updatePassword(newPassword);
            })
    })();
})