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60 lines
1.4 KiB
Markdown
60 lines
1.4 KiB
Markdown
# When to Mock
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Mock at **system boundaries** only:
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- External APIs (payment, email, etc.)
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- Databases (sometimes - prefer test DB)
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- Time/randomness
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- File system (sometimes)
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Don't mock:
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- Your own classes/modules
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- Internal collaborators
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- Anything you control
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## Designing for Mockability
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At system boundaries, design interfaces that are easy to mock:
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**1. Use dependency injection**
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Pass external dependencies in rather than creating them internally:
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```typescript
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// Easy to mock
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function processPayment(order, paymentClient) {
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return paymentClient.charge(order.total);
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}
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// Hard to mock
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function processPayment(order) {
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const client = new StripeClient(process.env.STRIPE_KEY);
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return client.charge(order.total);
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}
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```
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**2. Prefer SDK-style interfaces over generic fetchers**
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Create specific functions for each external operation instead of one generic function with conditional logic:
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```typescript
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// GOOD: Each function is independently mockable
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const api = {
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getUser: (id) => fetch(`/users/${id}`),
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getOrders: (userId) => fetch(`/users/${userId}/orders`),
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createOrder: (data) => fetch('/orders', { method: 'POST', body: data }),
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};
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// BAD: Mocking requires conditional logic inside the mock
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const api = {
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fetch: (endpoint, options) => fetch(endpoint, options),
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};
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```
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The SDK approach means:
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- Each mock returns one specific shape
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- No conditional logic in test setup
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- Easier to see which endpoints a test exercises
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- Type safety per endpoint
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