Node.js Backend Development Bootcamp · 课时

Node.js 中的模拟与测试替身

学习如何用模拟对象、桩和间谍替换真实依赖项，从而隔离被测试代码，让单元测试保持快速且结果确定。

Node.js 中的模拟与测试替身 是 CoddyKit 上的免费 Node.js Backend Development Bootcamp 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Node.js Backend Development Bootcamp 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Node.js Backend Development Bootcamp 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

The Problem with Real Dependencies

A unit test should check one piece of logic in isolation. But real code calls databases, APIs, the clock, and the file system — all slow, flaky, and unpredictable.

Test doubles replace those dependencies with controllable fakes.

Types of Test Doubles

Common terms you will hear:

Stub: returns a canned value
Spy: records how it was called
Mock: a stub + spy with built-in expectations
Fake: a lightweight working implementation

Creating a Mock Function

In Jest, jest.fn() creates a mock function. It records every call and lets you assert on them later.

const callback = jest.fn();
callback('hello');
expect(callback).toHaveBeenCalledWith('hello');

Defining Return Values

Tell a mock what to return with mockReturnValue, or for async code use mockResolvedValue to resolve a promise.

const getUser = jest.fn().mockResolvedValue({ id: 1, name: 'Ada' });
const user = await getUser();
expect(user.name).toBe('Ada');

Asserting on Calls

Spies let you verify interactions:

toHaveBeenCalled()
toHaveBeenCalledTimes(n)
toHaveBeenCalledWith(args)

const save = jest.fn();
save({ id: 1 });
expect(save).toHaveBeenCalledTimes(1);

Mocking a Module

To replace an entire imported module, use jest.mock(). Every export becomes an auto-mock you can configure per test.

jest.mock('./mailer');
const mailer = require('./mailer');
mailer.send.mockResolvedValue(true);

Spying on Existing Methods

Sometimes you want to watch a real method without replacing it. jest.spyOn() wraps an existing method so you can assert and optionally override it.

const spy = jest.spyOn(console, 'log');
console.log('hi');
expect(spy).toHaveBeenCalledWith('hi');

Resetting Between Tests

Mocks accumulate state. Reset them between tests so calls from one test do not leak into another.

mockClear() clears call data
mockReset() also clears implementations

afterEach(() => {
  jest.clearAllMocks();
});

Mocking Time

Tests that depend on the current time are flaky. Jest fake timers let you control the clock so timeouts and dates are deterministic.

jest.useFakeTimers();
const fn = jest.fn();
setTimeout(fn, 1000);
jest.advanceTimersByTime(1000);
expect(fn).toHaveBeenCalled();

Dependency Injection Makes Mocking Easy

Code that receives its dependencies as arguments is far easier to test than code that imports them directly. Design for testability.

function createService(db) {
  return { getUser: id => db.find(id) };
}
const fakeDb = { find: jest.fn().mockReturnValue({ id: 1 }) };
const svc = createService(fakeDb);

Don't Over-Mock

Mocking too much produces tests that only confirm your mocks were called — not that the code works. Mock external boundaries (DB, network), but test real logic directly when you can.

Quick Check

Test your understanding of test doubles.

Recap

You learned to isolate code with test doubles:

Create mocks with jest.fn() and configure return values
Replace modules with jest.mock()
Watch real methods with jest.spyOn()
Reset mocks between tests and control time with fake timers
Inject dependencies for testability and avoid over-mocking

Good mocking keeps unit tests fast, reliable, and focused.

常见问题解答

「Node.js 中的模拟与测试替身」课时是免费的吗？

是的 — 「Node.js 中的模拟与测试替身」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Node.js Backend Development Bootcamp 课程的其余内容，请升级到 CoddyKit PRO。 Node.js Backend Development Bootcamp 课程共包含 4 节课。

「Node.js 中的模拟与测试替身」这节课中我会学到什么？

学习如何用模拟对象、桩和间谍替换真实依赖项，从而隔离被测试代码，让单元测试保持快速且结果确定。你通过在浏览器中直接运行的动手代码来练习 Node.js Backend Development Bootcamp，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Node.js Backend Development Bootcamp 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Node.js Backend Development Bootcamp 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「Node.js 中的模拟与测试替身」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Node.js Backend Development Bootcamp 课中编写并运行代码吗？

能。每节 Node.js Backend Development Bootcamp 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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