test-patterns

Test Patterns

You are applying proven testing patterns to write maintainable, reliable tests. These patterns help ensure tests are readable, focused, and trustworthy.

Pattern Selection Guide

Use this to choose the right pattern for your situation:

• Structuring a single test? → Arrange-Act-Assert (AAA) or Given-When-Then
• Writing behavior/feature specs? → Given-When-Then (BDD style)
• Repeating test setup data? → Test Data Builders
• Many variations of complex objects? → Object Mother
• Testing same logic with many inputs? → Parameterized Tests
• Shared setup/teardown across tests? → Test Fixtures
• Verifying a dependency was called? → Spy
• Replacing an external dependency? → Test Doubles (Stub / Mock / Fake)

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Pattern Combination Workflows

Patterns rarely stand alone — here's how to combine them for common scenarios:

Unit tests (isolated logic): Fixtures for setup → AAA structure → Stubs/Mocks for dependencies → Parameterized Tests for multiple input cases

Integration tests (service + external dependencies): Fixtures for setup → AAA structure → Fakes for external services (e.g. in-memory DB) → Spies to verify interaction points

BDD / feature specs: Given-When-Then → Object Mother or Test Data Builders for scenario data → Fakes for infrastructure

High-variation logic (validators, calculators, formatters): Parameterized Tests → Test Data Builders to construct each case → AAA structure within each case

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Core Pattern: Arrange-Act-Assert (AAA)

Structure every test with three distinct phases:

// Arrange - Set up test data and dependencies
const user = createTestUser({ role: 'admin' });
const service = new UserService(mockRepository);

// Act - Execute the code under test
const result = await service.updateRole(user.id, 'member');

// Assert - Verify the expected outcome
expect(result.role).toBe('member');
expect(mockRepository.save).toHaveBeenCalledWith(user);

Guidelines:

• Keep sections visually separated (blank lines or comments)
• Arrange should be minimal - only what's needed for this test
• Act should be a single operation
• Assert should verify one logical concept