Jest testing with mock of Cloud Firestore SDK

Notification.test.js

const admin = require('firebase-admin')

class Notification {
  constructor(doc, firestore = admin.firestore()) {
    this.doc = doc
    this.firestore = firestore
  }

  notify(toUser) {
    const {user} = this.doc.data()

    this.firestore.collection('notifications')
        .doc()
        .set({
          created_at: this.firestore.FieldValue.serverTimestamp(),
          user: {
            id: toUser.id
          }
        })
  }
}

it(`notification parameter is valid`, () => {
  const setParams = jest.fn()
  const firestore = {
    FieldValue: {
      serverTimestamp: () => new Date()
    },
    collection: (name) => ({
      doc: () => ({
        set: setParams
      })
    })
  }

  const stub = {
    user: {name: 'Evan'},
    image: {url: 'http://...'},
  }
  const notification = new Notification({data: () => stub}, firestore)
  notification.notify({id: 'FRIEND_USER_ID'})

  const params = setParams.mock.calls[0][0]
  expect(params.user.id).toBe('FRIEND_USER_ID')
})