What are mocks and spies in testing?

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🔹 Mocks in Testing

A mock is a fake implementation of a function, object, or module that you use in place of the real one during tests.

• Purpose: Isolate the unit of code being tested.

• Example: Instead of calling a real database or API, you mock it so the test runs fast and predictably.

Example (Jest mock):

const getUser = jest.fn(() => ({ id: 1, name: "Roja" }));

test("returns mocked user", () => {
  const user = getUser();
  expect(user.name).toBe("Roja");
});

Here, jest.fn() creates a mock function, so you don’t hit a real database.

🔹 Spies in Testing

A spy is a type of mock that records information about how a function was called—such as the arguments, return value, and how many times it was invoked.

• Purpose: Check interactions between functions.

Example (Jest spy):

const math = {
  add: (a, b) => a + b,
};

test("spy on add function", () => {
  const spy = jest.spyOn(math, "add");
  math.add(2, 3);
  expect(spy).toHaveBeenCalledWith(2, 3);
});

Here, jest.spyOn doesn’t replace the function—it just monitors it.

🔹 Difference

• Mock → replaces the real implementation with a fake one.

• Spy → watches the real implementation (or mock) and tracks how it’s used.

✅ In summary:

• Use mocks when you want to fake dependencies and isolate code.

• Use spies when you want to check if certain functions were called correctly.

Read More :

How do you test Express.js APIs?

What is Jest and how is it used?

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