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mocking

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When to Mock

何時模擬

Mock at system boundaries only:

只在系統邊界模擬：

  • External APIs (payment, email, etc.)
  • Databases (sometimes - prefer test DB)
  • Time/randomness
  • File system (sometimes)
  • 外部 API（付款、電子郵件等）
  • 資料庫（有時——偏好測試資料庫）
  • 時間 / 隨機性
  • 檔案系統（有時）

Don't mock:

不要模擬：

  • Your own classes/modules
  • Internal collaborators
  • Anything you control
  • 你自己的類別 / 模組
  • 內部協作者
  • 任何你能控制的東西

Designing for Mockability

為可模擬性而設計

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

在系統邊界，設計容易模擬的介面：

1. Use dependency injection

1. 使用依賴注入

Pass external dependencies in rather than creating them internally:

把外部相依傳入，而不是在內部建立：

// 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);
}

// 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

2. 偏好 SDK 風格的介面，而不是通用 fetcher

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

為每個外部操作建立特定函式，而不是一個帶條件邏輯的通用函式：

// 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),
};

// 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

SDK 方式的好處： - 每個 mock 回傳一個特定形狀 - 測試設定中沒有條件邏輯 - 更容易看出一個測試觸及哪些端點 - 每個端點都有型別安全