Spies and mocking with Jest

August 19, 2021

Unit testing, in addition to output testing, includes the usage of spies and mocking. Spies are functions that let you spy on the behavior of functions that are called indirectly by some other code. Spy can be created by using jest.fn(). Mocking is injecting test values into the code during the tests. Some of the use cases will be presented below.

  • 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));
  • Rejected async function can be mocked using mockRejectedValue and mockImplementation.
jest.spyOn(calculationService, "calculate").mockRejectedValue(value);
jest.spyOn(calculationService, "calculate")
.mockImplementation(async () => Promise.reject(new Error(errorMessage)));
await expect(calculateSomething(calculationService)).rejects.toThrowError(Error);
  • 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 which are called multiple times can be mocked with different values using mockResolvedValueOnce and mockReturnValueOnce respectively and mockImplementationOnce.
jest.spyOn(calculationService, "calculate").mockResolvedValueOnce(value)
jest.spyOn(calculationService, "calculate").mockReturnValueOnce(value)
jest.spyOn(calculationService, "calculate").mockImplementationOnce((a) => a + 3)
.mockImplementationOnce((a) => a + 5);
  • External modules can be mocked similarly as spies. For the following example let's 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';
// within test case
  • 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.
  • To assert called arguments for a mocked function, an assertion can be done using toHaveBeenCalledWith matcher.
expect(calculationService.calculate).toHaveBeenCalledWith(firstArgument, secondArgument);
  • To assert skipped call for a mocked function, an assertion can be done using not.toHaveBeenCalled matcher.
  • To assert called arguments for the exact call when a mocked function is called multiple times, an assertion can be done using toHaveBeenNthCalledWith matcher.
argumentsList.forEach((argumentList, index) => {
index + 1,
  • Mocks should be reset to their initial implementation before each test case.
beforeEach(() => {


© 2021