从另一个模块开玩笑 mocking function：结果值未定义

Question

I'm practicing testing with Jest, in particular mocking functions.

I am testing a function that returns the doubled parameter. I am testing this function by mocking it and mapping several parameters.

here is the function that i am testing, it is located in the file ("./mock-func.js"):

function doubling(n) {
  return n * 2;
}

exports.module = doubling;

here is the test (in another file):

const mockFunc = require("./mock-func.js");

mockFunc.doubling = jest.fn();

test("map calls functions correctly", () => {
  const a = [10, 20, 30];

  a.map(mockFunc.doubling);

  expect(mockFunc.doubling.mock.calls.length).toBe(3);
  expect(mockFunc.doubling.mock.calls[0]).toEqual([10, 0, a]);
  expect(mockFunc.doubling.mock.calls[1]).toEqual([20, 1, a]);
  expect(mockFunc.doubling.mock.calls[2]).toEqual([30, 2, a]);
  expect(mockFunc.doubling.mock.results[0].value).toBe(20);
  expect(mockFunc.doubling.mock.results[1].value).toBe(40);
  expect(mockFunc.doubling.mock.results[2].value).toBe(60);
});

the problem that i have is that the test passes for the mock calls but not for the results values. It indicates that it returns undefined when the expected values are 20, 40, 60. I can't figure out why the mock calls pass but not the results values.

I am suspecting that it is because the function is located in another module because the test passes if i define the function in the test like so:

const myMockFn = jest.fn(n => n * 2);

test("map calls functions correctly", () => {
  const a = [10, 20, 30];

  a.map(myMockFn);

  expect(myMockFn.mock.calls.length).toBe(3);
  expect(myMockFn.mock.calls[0]).toEqual([10, 0, a]);
  expect(myMockFn.mock.calls[1]).toEqual([20, 1, a]);
  expect(myMockFn.mock.calls[2]).toEqual([30, 2, a]);
  expect(myMockFn.mock.results[0].value).toBe(20);
  expect(myMockFn.mock.results[1].value).toBe(40);
  expect(myMockFn.mock.results[2].value).toBe(60);
});

any help/feedback would be appreciated! thanks!

Answer 1

jest.fn() will create a mock function but it won't call the actual function. You can provide a mock Implementation like you have done in the scenario where the tests are passing.

However, you should not be mocking the module that you are trying to test.

If you need to check the behavior of your function, it would be best to call the function itself and assert for the return value.

const res = doubling(20)
expect(res).toEqual(40)

Something like the above is a clean and crisp test for your module. I'm not sure if there is a reason you are trying to check the arguments to the function but you need to call the actual function or provide a dummy implementation to get some values in the results.