firebase_test_helper.ts

const test = require("firebase-functions-test")(
  {
    databaseURL: "https://my-project.firebaseio.com",
    storageBucket: "my-project.appspot.com",
    projectId: "my-project",
  },
  "./testServiceAccountKey.json"
)
const functions = require("../index") // NOTE: This has to be called after `require("firebase-functions-test")(...)`
export { functions }

export const wrap = test.wrap
export const makeDocumentSnapshot = test.firestore.makeDocumentSnapshot

import * as admin from "firebase-admin"
export const db = admin.firestore()

const deleteDoc = async (doc: admin.firestore.DocumentReference | admin.firestore.Firestore) => {
  if (doc instanceof admin.firestore.DocumentReference) {
    console.debug("Deleting doc: " + doc.path)
    await doc.delete()
  }

  const collections = await doc.getCollections()
  await Promise.all(
    collections.map(async collection => {
      const collectionSnapshot = await collection.get()
      const docRefs = []
      collectionSnapshot.docs.forEach(documentSnapshot => docRefs.push(documentSnapshot.ref))
      await Promise.all(docRefs.map(async docRef => await deleteDoc(docRef)))
    })
  )
}

const cleanupFirestore = async () => {
  await deleteDoc(db)
}
export { cleanupFirestore }

after(() => {
  test.cleanup()
})

Example:

import { assert } from "chai"
import "mocha"
import { wrap, db, functions, cleanupFirestore } from "./firebase-test-helpers"

describe("onUserCreate", () => {
  afterEach( async () => {
    await cleanupFirestore()
  })

  it("add data into /users", async () => {
    const uid = "123"
    const email = "user1@example.com"
    await wrap(functions.onUserCreate)({ email, uid })

    const user = (await db.doc(`_users/${uid}`).get()).data()
    assert.equal(user["email"], email)
  })
})