I have a NodeJS application and I'm trying to write some tests. Currently, I'm trying to stub Firebase. Current code:

sinon.stub(firebase, 'initializeApp');
const db = firebase.firestore();

const stub = sinon.stub(db, 'collection').returns({
  doc: sinon.stub().returns({
    set: sinon.stub().returns(null)
  })    
})

I'm trying to stub initializeApp but I'm getting:

FirebaseError: Firebase: No Firebase App '[DEFAULT]' has been created - call Firebase App.initializeApp() (app/no-app).

How do I stub initializeApp?