Unit Testing vs Integration Testing

Unit vs Integration Testing

Both verify your code, but at different scopes. Unit tests check individual pieces in isolation; integration tests check how pieces work together.

Comparison

Aspect	Unit	Integration
Scope	One function	Multiple parts
Speed	Very fast	Slower
Dependencies	Mocked	Real
Catches	Logic bugs	Interface bugs

Unit Test Example

test("calculates total", () => {
  expect(calculateTotal([10, 20])).toBe(30);
});

Integration Test Example

test("creates user in database", async () => {
  const res = await request(app)
    .post("/users")
    .send({ name: "Sara" });
  expect(res.status).toBe(201);
  // Tests route + controller + database together
});

The Testing Pyramid

      /  E2E (few)
     /    Integration (some)
    /____  Unit (many)

Write lots of fast unit tests, fewer integration tests, and minimal E2E tests.

FAQs

Which should I write first?

Unit tests — fast feedback while developing. More in our Testing section.

What is mocking?

Replacing real dependencies with fakes to isolate the code under test.