unique username constraint on firestore using nested promises

Question

I am trying to achieve unique username constraint on firestore. I have been experimenting and I couldn't handle it properly. I want to create firebase auth record after making sure username isn't taken, and create firestore record. I first tried to nest then()(for creating auth record) inside another then()(username check inside db) but it got very messy. As you can see I am even experimenting with if() and else's. I would appreciate if someone gave insights.

export const signUp = (newUser) => {
    return (dispatch, getState, {getFirebase, getFirestore}) => {
        const firebase = getFirebase();
        const firestore = getFirestore();
        const user = firestore.collection("users").where("username", "==", newUser.username).get();
        if (!user) {
            firebase.auth().createUserWithEmailAndPassword(
                newUser.email,
                newUser.password
            ).then((response) => {
                return firestore.collection("users").doc(response.user.uid).set({
                    firstName: newUser.firstName,
                    lastName: newUser.lastName,
                    username: newUser.username,
                    createdAt: new Date()
                })
            }).then(() => {
                dispatch({type: "SIGNUP_SUCCESS"});
            }).catch((err) => {
                dispatch({type: "SIGNUP_ERROR", err});
            });
        } else {
            dispatch({type: "SIGNUP_ERROR", err: {message: "username taken"}});
        }
    }
};

Answer 1

Found a way to handle unique usernames by checking "users" collection in firestore.

export const signUp = (newUser) => {
    return (dispatch, getState, {getFirebase, getFirestore}) => {
        const firebase = getFirebase();
        const firestore = getFirestore();
        let exists = null;
        let message = null;
        firestore.collection("users")
            .where("username", "==", newUser.username)
            .get()
            .then((snapshot) => {
                if (!snapshot.empty) {
                    exists = true;
                    message = "Username you chose is unavailable";
                }
            })
            .then(() => {
                if (!exists) {
                    return firebase.auth().createUserWithEmailAndPassword(
                        newUser.email,
                        newUser.password
                    );
                }
            }).then((response) => {
            return firestore.collection("users").doc(response.user.uid).set({
                firstName: newUser.firstName,
                lastName: newUser.lastName,
                username: newUser.username,
                createdAt: new Date()
            })
        }).then(() => {
            dispatch({type: "SIGNUP_SUCCESS"});
        }).catch((err) => {
            if (message) {
                dispatch({type: "SIGNUP_ERROR", err: {message}});
                message = null;
            } else {
                dispatch({type: "SIGNUP_ERROR", err});
            }
        });
    }
};