TIL - Episode 3. Mocking node modules. | Alvin Crespo - Entrepreneur | Technology Leader

October 31, 2019

TIL - Episode 3. Mocking node modules.

In order to mock the node fs module with Jest, we need to make whats called a Manual Mock.
- https://github.com/echobind/eb-scripts/pull/8/files#diff-3dbad05e428063061d9a3a717674a5f1
  - The important thing to note is that this needs to exist alongside the node_modules directory

If the module you are mocking is a Node module (e.g.: lodash), the mock should be placed in the mocks directory adjacent to node_modules

— Mocking Node Modules, Jest Documentation

Once you add the mock, the template for your test would look something like this

//  src/utils/my.test.ts

afterEach(() => {
  // reset your modules after each test
  jest.resetModules()
})

test("some test that requires the non-mocked node module", () => {
  jest.dontMock("fs")

  // add your test here
})

test("some test that mocks the fs module in order to test the expected outcome", () => {
  jest.mock("fs")

  // add your test here
})

If you want to test out some global node object, such as process - you can do the following:

test("some test that mocks the global process object", () => {
  jest.spyOn(process, "cwd").mockImplementation(() => "well..hello there!")

  // add your test here
})

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