You are applying proven testing patterns to write maintainable, reliable tests. These patterns help ensure tests are readable, focused, and trustworthy.
Use this to choose the right pattern for your situation:
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
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:
For behavior-focused tests:
describe('Shopping Cart', () => {
describe('when adding an item', () => {
it('should increase the item count', () => {
// Given
const cart = new Cart();
// When
cart.add({ id: '1', quantity: 2 });
// Then
expect(cart.itemCount).toBe(2);
});
});
});
Create flexible test data without repetition:
// Builder function
function createTestOrder(overrides = {}) {
return {
id: 'order-123',
status: 'pending',
items: [],
total: 0,
...overrides
};
}
// Usage
const completedOrder = createTestOrder({ status: 'completed', total: 99.99 });
const emptyOrder = createTestOrder({ items: [] });
Factory for complex test objects:
class TestUserFactory {
static admin() {
return new User({ role: 'admin', permissions: ALL_PERMISSIONS });
}
static guest() {
return new User({ role: 'guest', permissions: [] });
}
static withSubscription(tier) {
return new User({ subscription: { tier, active: true } });
}
}
Test multiple cases efficiently:
describe('isValidEmail', () => {
const validCases = [
'[email protected]',
'[email protected]',
'[email protected]'
];
const invalidCases = [
'',
'not-an-email',
'@no-local.com',
'no-domain@'
];
test.each(validCases)('should accept valid email: %s', (email) => {
expect(isValidEmail(email)).toBe(true);
});
test.each(invalidCases)('should reject invalid email: %s', (email) => {
expect(isValidEmail(email)).toBe(false);
});
});
Reusable test setup:
describe('OrderService', () => {
let service;
let mockPaymentGateway;
let mockInventory;
beforeEach(() => {
mockPaymentGateway = createMockPaymentGateway();
mockInventory = createMockInventory();
service = new OrderService(mockPaymentGateway, mockInventory);
});
afterEach(() => {
jest.clearAllMocks();
});
});
Verify interactions without implementation:
it('should send notification on order completion', async () => {
const notifySpy = jest.spyOn(notificationService, 'send');
await orderService.complete(orderId);
expect(notifySpy).toHaveBeenCalledWith({
type: 'order_completed',
orderId: orderId
});
});
Choose the right type:
| Type | When to Use |
|---|---|
| Stub | Need predictable, canned return values |
| Mock | Need to assert a dependency was called correctly |
| Spy | Partial mocking — observe calls on a real object |
| Fake | Need a working lightweight substitute (e.g. in-memory DB) |
Ensure tests don't affect each other:
Test names should describe:
Good examples:
shouldReturnEmptyArrayWhenNoItemsExistthrowsErrorWhenUserNotAuthenticatedcalculatesDiscountForPremiumMemberssrc/
services/
UserService.ts
UserService.test.ts # Co-located tests
tests/
integration/
api.test.ts # Integration tests
e2e/
checkout.spec.ts # End-to-end tests
For each test:
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).