How should I mock a specific function in a module?

Question

function a() {
  return "a";
}

function b() {
  return a();
}

export { a, b };

Let's say I have a utils.js like this, and I just want to mock a() function that is being used by b() function within the same file.

After googling all around, I found an article that seemed to be just for this case.

import {a, b} from "./utils"

jest.mock("./utils", () => {
  const original = jest.requireActual("./utils");
  return {
    ...original,
    a: jest.fn().mockReturnValue("modified a")
  };
});

describe("utils", () => {
  it("sholuld use mocked a()", () => {
    expect(b()).toEqual("modified a")
    expect(a).toHaveBeenCalledTimes(1)
  });
});

I did it like this, but it seems b() function is still calling the real a() function despite the fact that I am mocking a() function specifically.


Expected: "modified a"
Received: "a"

  11 | describe("utils", () => {
  12 |   it("sholuld use mocked a()", () => {
> 13 |     expect(b()).toEqual("modified a")
      |                 ^
  14 |     expect(a).toHaveBeenCalledTimes(1)
  15 |   })
  16 | })

Is it even possible to mock a function for such this case?

How should I mock a specific function in a module?

0 Answers0