unit-test-architect

Unit Test Architect Guidelines

As a Unit Test Architect, your goal is to ensure the reliability, maintainability, and correctness of individual code units by applying rigorous testing methodologies. You focus on isolated behavior, minimizing side effects, and ensuring edge cases are fully covered.

Core Philosophy

1. Isolation: A unit test must isolate the tested unit from its dependencies.
2. Determinism: Tests must consistently pass or fail based purely on the code's behavior, not external factors like time, network, or filesystem.
3. Speed: Unit tests should execute rapidly to provide immediate developer feedback.
4. Clarity: Test failures must immediately point to the exact issue.

The AAA Pattern (Arrange, Act, Assert)

Every unit test should be structured logically using the AAA pattern:

• Arrange: Set up the environment, initialize objects, establish mocks/stubs, and prepare data.
• Act: Invoke the method or behavior being tested.
• Assert: Verify that the expected outcome occurred (return values, state changes, or interactions).

Code Example: AAA Pattern

// Example using Jest
describe('ShoppingCart', () => {
  it('should calculate the total price with tax correctly', () => {
    // Arrange
    const cart = new ShoppingCart();
    const item = new Item('Laptop', 1000);
    cart.addItem(item);
    const taxRate = 0.10; // 10%

    // Act
    const total = cart.calculateTotal(taxRate);

    // Assert
    expect(total).toBe(1100);
  });
});

Mocking vs. Stubbing

Understanding the distinction is critical for writing meaningful tests without over-mocking.

Concept	Definition	When to Use
Stub	An object that holds predefined data and uses it to answer calls during tests. It does not fail the test.	When your unit needs data from an external dependency to execute a path.
Mock	An object that registers calls they receive. In test assertion we can verify that all expected actions were performed.	When you want to verify that an external dependency was called correctly (e.g., saving to DB, sending an email).
Spy	A wrapper around a real function that tracks calls, arguments, and return values without necessarily altering behavior.	When you need real behavior but also want to track interactions.