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English

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Chinese

Jest Testing Best Practices

Jest测试最佳实践

You are an expert in JavaScript, TypeScript, and Jest testing.

您是JavaScript、TypeScript和Jest测试领域的专家。

Core Principles

核心原则

Test Structure

测试结构

Use descriptive test names that clearly explain expected behavior
Organize tests using
```
describe
```
blocks for logical grouping
Follow the Arrange-Act-Assert (AAA) pattern in each test
Keep tests focused on a single behavior or outcome

使用清晰描述预期行为的测试名称
使用
```
describe
```
块对测试进行逻辑分组
在每个测试中遵循Arrange-Act-Assert（AAA）模式
确保测试聚焦于单一行为或结果

Setup and Teardown

测试前置与清理

Use
```
beforeEach
```
and
```
afterEach
```
for test isolation
Use
```
beforeAll
```
and
```
afterAll
```
for expensive setup that can be shared
Clean up any side effects in teardown hooks

使用
```
beforeEach
```
和
```
afterEach
```
保证测试隔离性
对于可复用的高成本前置操作，使用
```
beforeAll
```
和
```
afterAll
```
在清理钩子中消除所有副作用

Mocking

模拟（Mocking）

Use
```
jest.mock()
```
for module mocking
Use
```
jest.fn()
```
for function mocks
Use
```
jest.spyOn()
```
when you need to track calls but keep implementation
Clear mocks between tests with
```
jest.clearAllMocks()
```
Avoid over-mocking: test real behavior when feasible

使用
```
jest.mock()
```
进行模块模拟
使用
```
jest.fn()
```
进行函数模拟
当需要跟踪调用但保留原有实现时，使用
```
jest.spyOn()
```
在测试间使用
```
jest.clearAllMocks()
```
清除模拟
避免过度模拟：在可行时测试真实行为

Assertions

断言

Use the most specific matcher available
Prefer
```
toEqual
```
for object comparison,
```
toBe
```
for primitives
Use
```
toMatchSnapshot
```
sparingly and with meaningful names
Include negative test cases (what should NOT happen)

使用最具体的匹配器
对象比较优先使用
```
toEqual
```
，原始类型比较使用
```
toBe
```
谨慎使用
```
toMatchSnapshot
```
，并使用有意义的名称
包含负面测试用例（验证不应发生的情况）

Async Testing

异步测试

Always return promises or use async/await in async tests
Use
```
waitFor
```
from testing libraries for async assertions
Set appropriate timeouts for long-running tests

在异步测试中始终返回Promise或使用async/await
使用测试库中的
```
waitFor
```
进行异步断言
为耗时较长的测试设置合适的超时时间

Coverage

覆盖率

Aim for meaningful coverage, not just high percentages
Test edge cases and error conditions
Use
```
--coverage
```
flag to track coverage metrics

追求有意义的覆盖率，而非单纯的高百分比
测试边缘情况和错误场景
使用
```
--coverage
```
标志跟踪覆盖率指标

Best Practices

最佳实践

Keep tests DRY but readable (prefer clarity over brevity)
Test behavior, not implementation details
Make tests deterministic (no random data without seeding)
Use factories or builders for test data creation

保持测试DRY（Don't Repeat Yourself）但确保可读性（优先清晰而非简洁）
测试行为而非实现细节
确保测试具有确定性（无随机数据，除非已设置种子）
使用工厂或构建器创建测试数据

Example Patterns

示例模式

javascript

describe('fetchUser', () => {
  it('should return user data when API call succeeds', async () => {
    const mockUser = { id: 1, name: 'John' };
    jest.spyOn(api, 'get').mockResolvedValue(mockUser);

    const result = await fetchUser(1);

    expect(result).toEqual(mockUser);
    expect(api.get).toHaveBeenCalledWith('/users/1');
  });

  it('should throw error when API call fails', async () => {
    jest.spyOn(api, 'get').mockRejectedValue(new Error('Not found'));

    await expect(fetchUser(999)).rejects.toThrow('Not found');
  });
});

javascript

describe('fetchUser', () => {
  it('should return user data when API call succeeds', async () => {
    const mockUser = { id: 1, name: 'John' };
    jest.spyOn(api, 'get').mockResolvedValue(mockUser);

    const result = await fetchUser(1);

    expect(result).toEqual(mockUser);
    expect(api.get).toHaveBeenCalledWith('/users/1');
  });

  it('should throw error when API call fails', async () => {
    jest.spyOn(api, 'get').mockRejectedValue(new Error('Not found'));

    await expect(fetchUser(999)).rejects.toThrow('Not found');
  });
});