haletothewood/behavioural-tdd
Execute a strict Red-Green-Refactor TDD cycle — one requirement at a time — in any language or framework.
97
1.11x
Quality
100%
Does it follow best practices?
Impact
94%
1.11xAverage score across 5 eval scenarios
mocking.mdreferences/
Mocking Guidelines
Mock at system boundaries only
| Mock these | Never mock these |
|---|---|
| External APIs (payment, email, SMS) | Your own classes or modules |
| Databases (or use a test DB instead) | Internal collaborators |
| Time and randomness | Anything you control and can test directly |
| File system (when impractical to use real files) | Private methods |
Use dependency injection
Pass external dependencies in rather than creating them internally. This makes the seam explicit and mockable without touching internals.
// Testable — dependency is injected
function processPayment(order: Order, paymentClient: PaymentClient) {
return paymentClient.charge(order.total);
}
// Hard to mock — dependency created internally
function processPayment(order: Order) {
const client = new StripeClient(process.env.STRIPE_KEY);
return client.charge(order.total);
}Prefer SDK-style interfaces over generic fetchers
Design boundary interfaces as named operations rather than a single generic call. Each operation is independently mockable with no conditional logic.
// GOOD: each operation is independently mockable
const api = {
getUser: (id: string) => fetch(`/users/${id}`),
getOrders: (userId: string) => fetch(`/users/${userId}/orders`),
createOrder: (data: OrderData) =>
fetch('/orders', { method: 'POST', body: JSON.stringify(data) }),
};
// BAD: mocking requires conditional logic inside the mock
const api = {
fetch: (endpoint: string, options?: RequestInit) => fetch(endpoint, options),
};Design interfaces that return results, not side effects
Functions that return values are easier to test than functions that mutate state as a side effect.
// Testable — assert on the return value
function calculateDiscount(cart: Cart): Discount {
return { amount: cart.total * 0.1 };
}
// Harder to test — must inspect mutated state
function applyDiscount(cart: Cart): void {
cart.total -= cart.total * 0.1;
}