Full featured test framework:

• Test framework
• Assertion
• Mock

https://facebook.github.io/jest/

npm install --save-dev jest babel-jest

• beforeEach - afterEach - beforeAll - afterAll
• describe: group of test
• test: define a single test

• expect

• Create mock function: const mock = jest.fn()
• Return pre-determined values: - mock.mockReturnValue(3) - mock.mockReturnValueOnce(3)
• Set simple implementation: -sum.mockImplementation((a, b) => 3); - sum.mockImplementationOnce((a, b) => 3);
• Verify how many times function has been called; with what arguments: -expect(mock).toBeCalledWith(1, 2) - mock.mock.calls: array of arguments which this method was called with
• Track objects instantiated from a mock with the 'new' keyword: mock.mock.instances

Jest can mock a module with command jest.mock('<module name>'). After that, require('<module name>') (or import) will return the mock module.

This is good for mocking dependent modules of module under test.

With import, a single variable is used for the mock module. Each test will add data into mock module. We may need to use mock.mockReset() in beforeEach for resetting the data.

A mock version for a module could be placed in __mocks__ folder which is placed in the same folder of the real module.
Jest will use that mocked module implementation instead of its default implementation.

spy = jest.spyOn(object, '<method>')

https://facebook.github.io/jest/docs/jest-object.html#jestspyonobject-methodname

• Track calls to object.method().
• Mock the object.method with spy.mockImplementation(() => customImplementation)

jest

Run test and give the code coverage report

jest --coverage

Watch the file changes and run tests jest --watch

There are some options of tests to be run:

• Run all tests.
• Run tests which are affected by the changes.
• Run selected tests (Pattern).

• Description: With webpack, we can import css file and image file. However, jest cannot run with them.
• Solution: Configure jest to use mock files for css files or image files:

"jest": {
    "moduleNameMapper": {
      "\\.(css|less)$": "<rootDir>/__mocks__/styleMock.js"
    }
  }

Add styleMock.js:

export default {};