如何在 JEST 中测试外部 function 时模拟内部 function

Question

I am writing an express JS app with TypeScript using functional programming so I don't have any classes and methods in my service.

I have a method called "getData" in one file:

export const getData = async () => {
    const filePath = path.join(__dirname, '../../data/movies.json');
    return MOVIE_METADATA === undefined ? readJsonDataFromFile(filePath) : MOVIE_METADATA;
}

I am using this method in my service in another file like this:

const findMovieById = async (movieId: string) => {
    const movies: any[] = await getData();
    return movies.find((movie) => movie.imdbId === movieId || movie.id.toString() === movieId);
}

I want to test my service method "findMovieById" and I want to mock the inner method "getData" to return a mocked response however I am unable to do that.

I see a lot of people using SpyOn but that works with a class and then a method but I only have a method.

I also tried jest-mock-extended npm package but the problem remains the same.

I would appreciate it if you could help me here or point be to some resources where I can learn how to mock inner functions while testing outer level functions.

Answer 1

The test with mocks of getData can be implemented like this:

import * as dataService from "../getDataService";
import { findMovieById } from "../findMovieService";

it("should work with original implementation", async () => {
  const movie = await findMovieById(1);
  expect(movie.title).toEqual("Movie with id 1 from ORIGINAL function");
});

it("should work with mocked implementation", async () => {
  jest
    .spyOn(dataService, "getData")
    .mockResolvedValue([
      { imdbId: 1, title: "Movie with id 1 from MOCKED function" },
    ]);
  const movie = await findMovieById(1);
  expect(movie.title).not.toEqual("Movie with id 1 from ORIGINAL function");
  expect(movie.title).toEqual("Movie with id 1 from MOCKED function");
});

You just need to spyOn the getData function and mock the implementation with mockResolvedValue .