Lazy Load Import Firebase (dynamic import)

firebase.js

Promise.all([
  import('firebase/app'),
  import('firebase/database'),
  import('firebase/auth'),
])
  .then(x => x[0].default)
  .then(firebase => {
    const config = {
      apiKey: '',
      authDomain: '',
      databaseURL: '',
      projectId: '',
      storageBucket: '',
      messagingSenderId: '',
    }

    firebase.initializeApp(config)

    if (global.firebase) {
      return global.firebase
    } else if (!firebase) {
      return Promise.reject(new Error('loading error'))
    } else {
      global.firebase = firebase

      const googleAuthProvider = new firebase.auth.GoogleAuthProvider()
      googleAuthProvider.addScope('https://www.googleapis.com/auth/userinfo.email')

      global.firebase.googleAuthProvider = googleAuthProvider
      
      return global.firebase ? global.firebase : firebase
    }
  })
  .catch(err => {
    throw new Error(err)
  })

@Jaseibert Thanks this works great.

To add on, I also made my own firebase types/interface files (I am using typescript), then I re-wrote firebase code I needed to use which I couldn't wait for (such as Timestamp) in my own files without the bloat. Lastly I made sure I absolutely never imported from firebase/(any of them) outside of the root firestore files were created from your guide.

The second step of the optimization I explained above took and extra 125kB+ off the bundle. After optimizing just firebase my first load JS shared by all went from 320kB to 125kB! (I use functions, storage, firestore, and auth)

You can use this eslint rule to prevent importing firebase directly:
"no-restricted-imports": ["error", "firebase/app", "firebase/firestore", "firebase/storage", "firebase/auth", "firebase/functions"]