# When to Mock

Mock at **system boundaries** only:

- External APIs (payment, email, etc.)
- Databases (sometimes - prefer test DB)
- Time/randomness
- File system (sometimes)

Don't mock:

- Your own classes/modules
- Internal collaborators
- Anything you control

## Designing for Mockability

At system boundaries, design interfaces that are easy to mock:

**1. Use dependency injection**

Pass external dependencies in rather than creating them internally:

```typescript
// Easy to mock
function processPayment(order, paymentClient) {
  return paymentClient.charge(order.total);
}

// Hard to mock
function processPayment(order) {
  const client = new StripeClient(process.env.STRIPE_KEY);
  return client.charge(order.total);
}
```

**2. Prefer SDK-style interfaces over generic fetchers**

Create specific functions for each external operation instead of one generic function with conditional logic:

```typescript
// GOOD: Each function is independently mockable
const api = {
  getUser: (id) => fetch(`/users/${id}`),
  getOrders: (userId) => fetch(`/users/${userId}/orders`),
  createOrder: (data) => fetch('/orders', { method: 'POST', body: data }),
};

// BAD: Mocking requires conditional logic inside the mock
const api = {
  fetch: (endpoint, options) => fetch(endpoint, options),
};
```

The SDK approach means:
- Each mock returns one specific shape
- No conditional logic in test setup
- Easier to see which endpoints a test exercises
- Type safety per endpoint

## Mocks Must Not Prove Themselves

A mock stands in for a boundary; it must not implement the behavior the test asserts. If the mock decides the outcome, the test proves the mock. Likewise, do not reuse one identical mock for different APIs, and do not let a fixture supply the ordering, callback, or acknowledgement the code under test is supposed to produce. See the [junk patterns](test-audit.md#junk-patterns).