mocking.md

skills/engineering/tdd/mocking.md · 1.4 KB · View on GitHub at 4ae2317

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:

// 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:

// 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

Original Markdown source

mocking.md

# 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