testing

Testing SOP

Test Pyramid

        /\
       /e2e\       ← Few, slow, test full user journeys
      /──────\
     /integr. \    ← Some, test component interactions
    /──────────\
   /  unit tests \ ← Many, fast, test single units in isolation
  /──────────────\

What to Test — Behavioral Coverage Checklist

For every function/module, think through:

• Happy path: normal inputs → expected output
• Edge cases: empty string, zero, null, undefined, empty array, max integer
• Error cases: invalid input, missing required field, network failure
• Boundary values: min/max allowed values, exact boundary, just over/under
• Side effects: does it correctly modify state, call dependencies?

Test Structure — Arrange, Act, Assert

describe("UserService.createUser", () => {
  it("should hash the password before saving", async () => {
    // ARRANGE
    const mockRepo = { save: jest.fn().mockResolvedValue({ id: "1" }) };
    const service = new UserService(mockRepo);
    const plainPassword = "secret123";

    // ACT
    await service.createUser({ email: "[email protected]", password: plainPassword });

    // ASSERT
    const savedUser = mockRepo.save.mock.calls[0][0];
    expect(savedUser.password).not.toBe(plainPassword);
    expect(savedUser.password).toMatch(/^\$2[aby]\$/); // bcrypt hash
  });

  it("should throw if email already exists", async () => {
    // ARRANGE
    const mockRepo = { save: jest.fn().mockRejectedValue(new DuplicateKeyError()) };
    const service = new UserService(mockRepo);

    // ACT & ASSERT
    await expect(
      service.createUser({ email: "[email protected]", password: "pass" })
    ).rejects.toThrow("Email already in use");
  });
});

Naming Tests

it("should <expected behavior> when <condition>")
it("should throw <error> if <invalid condition>")
it("should return <value> given <input>")

Mocking Strategy

// Mock external dependencies, not internal logic
jest.mock("../services/EmailService");        // External service
jest.mock("../repositories/UserRepository"); // Database layer

// Do NOT mock:
// - The unit under test itself
// - Simple utility functions
// - Pure functions with no side effects