Cannot read property 'arrayUnion' of undefined

I am trying to add an element to an existing array in Firebase. I am using react. I have looked at other mentions of this issue on here and the solution is always exactly what I am already doing so help would be much appreciated. This only needs to update if the user selects a team to join and I've checked that teams is truthy

My firebase setup:

import firebase from 'firebase/app'
import 'firebase/functions'
import 'firebase/firestore'
import 'firebase/auth'
import 'firebase/storage'

// For Firebase JS SDK v7.20.0 and later, measurementId is optional
const firebaseConfig = {
  apiKey: process.env.REACT_APP_API_KEY,
  authDomain: process.env.REACT_APP_AUTH_DOMAIN,
  databaseURL: process.env.REACT_APP_DATABASE_URL,
  projectId: process.env.REACT_APP_PROJECT_ID,
  storageBucket: process.env.REACT_APP_STORAGE_BUCKET,
  messagingSenderId: process.env.REACT_APP_MESSAGE_SENDER_ID,
  appId: process.env.REACT_APP_ID,
  measurementId: process.env.REACT_APP_MEASUREMENT_ID
};

const firebaseApp = firebase.initializeApp(firebaseConfig)

const db = firebaseApp.firestore()
const auth = firebaseApp.auth()
const storage = firebaseApp.storage()
// const provider = new firebase.auth.GoogleAuthProvider()

export { db, storage, auth }

my function:

import { db, storage } from '../../firebase'
...
...
...

if(teams) {
      db.collection('teams').doc(team.id).update({
        teamMembers: db.FieldValue.arrayUnion(currentUser.id)
      })
    }

The Error:

Uncaught (in promise) TypeError: Cannot read property 'arrayUnion' of undefined
    at userInfoFirebase (SecondStep.js:179)
    at submitFinalData (SecondStep.js:200)
    at handleSubmit (SecondStep.js:281)
    at HTMLUnknownElement.callCallback (react-dom.development.js:3945)
    at Object.invokeGuardedCallbackDev (react-dom.development.js:3994)
    at invokeGuardedCallback (react-dom.development.js:4056)
    at invokeGuardedCallbackAndCatchFirstError (react-dom.development.js:4070)
    at executeDispatch (react-dom.development.js:8243)
    at processDispatchQueueItemsInOrder (react-dom.development.js:8275)
    at processDispatchQueue (react-dom.development.js:8288)
    at dispatchEventsForPlugins (react-dom.development.js:8299)
    at react-dom.development.js:8508
    at batchedEventUpdates$1 (react-dom.development.js:22396)
    at batchedEventUpdates (react-dom.development.js:3745)
    at dispatchEventForPluginEventSystem (react-dom.development.js:8507)
    at attemptToDispatchEvent (react-dom.development.js:6005)
    at dispatchEvent (react-dom.development.js:5924)
    at unstable_runWithPriority (scheduler.development.js:646)
    at runWithPriority$1 (react-dom.development.js:11276)
    at discreteUpdates$1 (react-dom.development.js:22413)
    at discreteUpdates (react-dom.development.js:3756)
    at dispatchDiscreteEvent (react-dom.development.js:5889)

Your db = firebase.firestore(), but FieldValue is defined on firebase.firestore (without ()).

You'll also need to export the firebase namespace with:

export { firebase, db, storage, auth }

Import it with:

import { firebase, db, storage } from '../../firebase'

And then use it with:

teamMembers: firebase.firestore.FieldValue.arrayUnion(currentUser.id)

Alternatively, you can export arrayUnion or FieldValue itself, and use those.