Besides asserting the output of the function call, unit testing includes the usage of spies and mocking. Spies are functions that let you spy on the behavior of functions called indirectly by some other code. Spy can be created by using jest.fn(). Mocking injects test values into the code during the tests. Some of the use cases will be presented below.

• The async function and its resolved value can be mocked using mockResolvedValue. Another way to mock it is by using mockImplementation and providing a function as an argument.

const calculationService = {
  calculate: jest.fn()
};

jest.spyOn(calculationService, 'calculate').mockResolvedValue(value);

jest
  .spyOn(calculationService, 'calculate')
  .mockImplementation(async (a) => Promise.resolve(a));

• A rejected async function can be mocked using mockRejectedValue and mockImplementation.

jest
  .spyOn(calculationService, 'calculate')
  .mockRejectedValue(new Error(errorMessage));

jest
  .spyOn(calculationService, 'calculate')
  .mockImplementation(async () => Promise.reject(new Error(errorMessage)));

await expect(calculateSomething(calculationService)).rejects.toThrowError(
  Error
);

• The sync function and its return value can be mocked using mockReturnValue and mockImplementation.

jest.spyOn(calculationService, 'calculate').mockReturnValue(value);

jest.spyOn(calculationService, 'calculate').mockImplementation((a) => a);

• Chained methods can be mocked using mockReturnThis.

// calculationService.get().calculate();
jest.spyOn(calculationService, 'get').mockReturnThis();

• Async and sync functions called multiple times can be mocked with different values using mockResolvedValueOnce and mockReturnValueOnce, respectively, and mockImplementationOnce.

jest
  .spyOn(calculationService, 'calculate')
  .mockResolvedValueOnce(value)
  .mockResolvedValueOnce(otherValue);

jest
  .spyOn(calculationService, 'calculate')
  .mockReturnValueOnce(value)
  .mockReturnValueOnce(otherValue);

jest
  .spyOn(calculationService, 'calculate')
  .mockImplementationOnce((a) => a + 3)
  .mockImplementationOnce((a) => a + 5);

• External modules can be mocked similarly to spies. For the following example, suppose axios package is already used in one function. The following example represents a test file where axios is mocked using jest.mock().

import axios from 'axios';
jest.mock('axios');

// within test case
axios.get.mockResolvedValue(data);

• Manual mocks are resolved by writing corresponding modules in __mocks__ directory, e.g., fs/promises mock will be stored in __mocks__/fs/promises.js file. fs/promises mock will be resolved using jest.mock() in the test file.

jest.mock('fs/promises');

• To assert called arguments for a mocked function, an assertion can be done using toHaveBeenCalledWith matcher.

const spy = jest.spyOn(calculationService, 'calculate');

expect(spy).toHaveBeenCalledWith(firstArgument, secondArgument);

• To assert skipped call for a mocked function, an assertion can be done using not.toHaveBeenCalled matcher.

const spy = jest.spyOn(calculationService, 'calculate');

expect(spy).not.toHaveBeenCalled();

• To assert called arguments for the exact call when a mocked function is called multiple times, an assertion can be done using toHaveBeenNthCalledWith matcher.

const argumentsList = [0, 1];

argumentsList.forEach((argumentList, index) => {
  expect(calculationService.calculate).toHaveBeenNthCalledWith(
    index + 1,
    argumentList
  );
});

• Methods should be restored to their initial implementation before each test case.

// package.json
"jest": {
  // ...
  "restoreMocks": true
}
// ...

Demo

The demo with the mentioned examples is available here.

Boilerplate

Here is the link to the boilerplate I use for the development.