How do you mock Firebase Firestore methods using Jest?

Question

I have a series of functions, each performing various firestore interactions. How do I use Jest to mock these firestore calls? I would like to avoid using a library.

Answer 1

It's been a while since any activity on this question, but still there's not much material online, here's my solution:

export default class FirestoreMock {
  constructor () {
    // mocked methods that return the class
    this.mockCollection = jest.fn(() => this)
    this.mockWhere = jest.fn(() => this)
    this.mockOrderBy = jest.fn(() => this)

    // methods that return promises
    this.mockAdd = jest.fn(() => Promise.resolve(this._mockAddReturn))
    this.mockGet = jest.fn(() => Promise.resolve(this._mockGetReturn))

    // methods that accepts callbacks
    this.mockOnSnaptshot = jest.fn((success, error) => success(this._mockOnSnaptshotSuccess))

    // return values
    this._mockAddReturn = null
    this._mockGetReturn = null
    this._mockOnSnaptshotSuccess = null
  }

  collection (c) {
    return this.mockCollection(c)
  }

  where (...args) {
    return this.mockWhere(...args)
  }

  orderBy (...args) {
    return this.mockOrderBy(...args)
  }

  add (a) {
    return this.mockAdd(a)
  }

  get () {
    return this.mockGet()
  }

  onSnapshot (success, error) {
    return this.mockOnSnaptshot(success, error)
  }

  set mockAddReturn (val) {
    this._mockAddReturn = val
  }

  set mockGetReturn (val) {
    this._mockGetReturn = val
  }

  set mockOnSnaptshotSuccess (val) {
    this._mockOnSnaptshotSuccess = val
  }

  reset () {
    // reset all the mocked returns
    this._mockAddReturn = null
    this._mockGetReturn = null
    this._mockOnSnaptshotSuccess = null

    // reset all the mocked functions
    this.mockCollection.mockClear()
    this.mockWhere.mockClear()
    this.mockOrderBy.mockClear()
    this.mockAdd.mockClear()
    this.mockGet.mockClear()
  }
}

And here's an example usage:

import FirestoreMock from '../test_helpers/firestore.mock'
import firebase from 'firebase/app'
import 'firebase/firestore'

describe('The Agreement model', () => {
    const firestoreMock = new FirestoreMock()
    beforeEach(() => {
        firebase.firestore = firestoreMock
        firestoreMock.reset()
    })

    it('does something', (done) => {
        firestoreMock.mockAddReturn = { id: 'test-id' }
        firebase.firestore.collection('foobar')
          .add({foo: 'bar'})
          .then(res => {
            expect(firestoreMock.mockCollection).toBeCalledWith('foobar')
            expect(firestoreMock.mockAdd).toBeCalledWith({foo: 'bar'})
            expect(res.id).toEqual('test-id')
            done()
          })
          .catch(done)
    })
})

If there is any interest out there I'm fine with packaging the FirestoreMock implementation so that it can be easily shared

Teo