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 mockfunction processPayment(order, paymentClient) {  return paymentClient.charge(order.total);}
// Hard to mockfunction 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 mockableconst 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 mockconst 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