I found this mocking example floating around on a tutorial page, but I find it confusing that this is used as an example so often.

test("mock return value", () => {
  const mock = jest.fn();
  mock.mockReturnValue("bar");

  expect(mock("foo")).toBe("bar");
  expect(mock).toHaveBeenCalledWith("foo");
});

Does const mock = jest.fn(); ever connect to a real function that needs to be mocked? If so, how does it know which function to mock? What is the use case for a new and random mock like this?

CodePudding user response：

The example in this article only demonstrates the functionality of jest.fn(). The mock object is not used by the code to be tested.

As it says, the mock function provides the below features:

• Capture calls
• Set return values
• Change the implementation

There is a scenario in real code where a function takes an object and calls a method on that object. In this scenario, you can create a mock object and pass it to that function.

E.g.

// A real function need to be tested, it can be imported from a module
function getUserById(dbc, id) {
  return dbc.query('select * from users where id = $1;', [id]);
}

describe('70538368', () => {
  test('should pass', () => {
    const mDbc = {
      query: jest.fn().mockReturnValue({ name: 'teresa teng' }),
    };
    const actual = getUserById(mDbc, '1');
    expect(actual).toEqual({ name: 'teresa teng' });
    expect(mDbc.query).toBeCalledWith('select * from users where id = $1;', ['1']);
  });
});