开玩笑：spyOn 测试失败，即使（异步）function 正在执行

Question

I'm trying to spyOn an async function that's called within a submodule. This is something I've done many times before, so I can't work out why it's failing: Here's the (simplified) code:

routes.js:

const express = require('express');
const router = express.Router();
const { fetchSamples } = require('./controllers.js');

router.get('/fetch-samples', fetchSamples);

controllers.js

const { fetchSamplesFromDb } = require('./services');

exports.fetchSamples = (req, res) => {
  const data = await fetchSamplesFromDb(req.query.params);
  res.status(200).json(data);
};

services.js

exports.fetchSamplesFromDb = async params => {
  console.log('I get called!');
  const x = await xyz; // Other stuff....
};

And the failing test:

const request = require('supertest');
const app = require('../app.js'); // express web server
const services = require('../services.js');

it('responds with 200 when successful', async () => {
  const spy = jest.spyOn(services, 'fetchSample');
  const response = await request(app).get('/fetch-samples');
  expect(response.status).toBe(200); // PASSES
  expect(spy).toHaveBeenCalled(); // FAILS
});

I can't work out why the spy isn't called. I've wondered if it's because it's async, but I haven't been able to get the test to pass. Would really appreciate some pointers!

Answer 1

After you require the module under test, before mocking use the jest.spyOn() method, the services module is also required and deconstructed with the original fetchSamplesFromDB method.

It's late when you use jest.spyOn() method to mock fetchSamplesFromDB method in test case function. You can use the service method like this:

const services = require('./services');

exports.fetchSamples = async (req, res) => {
  const data = await services.fetchSamplesFromDb(req.query.params);
  res.status(200).json(data);
};

In this way, jest.spyOn(services, 'fetchSamplesFromDb') will work.