Agfdhyk

My purpose is simply to test one function. I cannot figure out how to mock firebase properly. I try to keep the example with axios mocking from Jest docs. I have the following code:

MusicService.js

import { initializeApp } from "firebase/app";

import "firebase/database";



const firebase = initializeApp({

  apiKey: "<API_KEY>",

  authDomain: "<PROJECT_ID>.firebaseapp.com",

  databaseURL: "https://<DATABASE_NAME>.firebaseio.com",

  projectId: "<PROJECT_ID>",

  storageBucket: "<BUCKET>.appspot.com",

  messagingSenderId: "<SENDER_ID>",

});



export class MusicService {

  static getAlbums() {

    return firebase.database().ref("albums").once("value")

    .then(snapshot => Object.values(snapshot.val()));

  }

}

MusicService.test.js

import firebase from 'firebase/app';

import 'firebase/database';

import { MusicService } from './MusicService';



jest.mock('firebase/app');

jest.mock('firebase/database');



test("test", () => {

  firebase.initializeApp.mockImplementation(() => {

    database: jest.fn(() => {

      return {

        ref: jest.fn()

      }

    })

  });



  MusicService.getAlbums();

});

The problem is that I get the following error:

I tried to mock firebase.database.

test("test", () => {

  firebase.mockImplementation(() => {

    return {

      database: {



      }

    }

  });

  MusicService.getAlbums();

});

But in this case I get the error that says:

TypeError: _app.default.mockImplementation is not a function.

I don't expect the working example will be given, but rather could you tell please, what exactly should I mock? The whole firebase library or maybe the part where my function starts - return firebase.database().

I have figured out. I should mock only those modules, a function I am going to test, depends on. For example, I want to test getAlbums function. It uses initializeApp function which is imported from firebase/app module in MusicService.js. So when initializeApp function is being called it should return an object containing database function which in turn returns an object with ref and once functions. Code:

MusicService.test.js.

import { MusicService } from "./FirebaseService";



jest.mock("firebase/app", () => {

  const data = { name: "unnamed" };

  const snapshot = { val: () => data };

  return {

    initializeApp: jest.fn().mockReturnValue({

      database: jest.fn().mockReturnValue({

        ref: jest.fn().mockReturnThis(),

        once: jest.fn(() => Promise.resolve(snapshot))

      })

    })

  };

});



test("getAlbums function returns an array", async () => {

  const data = await MusicService.getAlbums();

  expect(data.constructor).toEqual(Array);

});