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安装和移除

写测试的时候你经常需要在运行测试前做一些准备工作，和在运行测试后进行一些收尾工作。 Jest 提供辅助函数来处理这个问题。

Repeating Setup

如果你有一些要为多次测试重复设置的工作，你可以使用 beforeEach 和 afterEach。

例如，我们考虑一些与城市信息数据库进行交互的测试。你必须在每个测试之前调用方法 initializeCityDatabase() ，同时必须在每个测试后，调用方法 clearCityDatabase()。你可以这样做：

beforeEach(() => {
  initializeCityDatabase();
});

afterEach(() => {
  clearCityDatabase();
});

test('city database has Vienna', () => {
  expect(isCity('Vienna')).toBeTruthy();
});

test('city database has San Juan', () => {
  expect(isCity('San Juan')).toBeTruthy();
});

beforeEach 和 afterEach 能够通过与异步代码测试相同的方式处理异步代码 — — 他们可以采取 done 参数或返回一个 promise。例如，如果 initializeCityDatabase() 返回解决数据库初始化时的 promise ，我们会想返回这一 promise︰

beforeEach(() => {
  return initializeCityDatabase();
});

一次性设置

在某些情况下，你只需要在文件的开头做一次设置。如果这个通用设置是异步的，就比较麻烦，因为没办法每个用例都设置一遍，这样性能还很差。 Jest provides beforeAll and afterAll hooks to handle this situation.

For example, if both initializeCityDatabase() and clearCityDatabase() returned promises, and the city database could be reused between tests, we could change our test code to:

beforeAll(() => {
  return initializeCityDatabase();
});

afterAll(() => {
  return clearCityDatabase();
});

test('city database has Vienna', () => {
  expect(isCity('Vienna')).toBeTruthy();
});

test('city database has San Juan', () => {
  expect(isCity('San Juan')).toBeTruthy();
});

作用域

顶层的 before* 和 after* hook 函数会应用于文件中的每一条测试在 describe 块中声明的 hook 函数，只会作用于 describe 中的测试

比如说，我们不仅有一个城市的数据库，还有一个食品数据库。我们可以为不同的测试做不同的设置︰

// Applies to all tests in this file
beforeEach(() => {
  return initializeCityDatabase();
});

test('city database has Vienna', () => {
  expect(isCity('Vienna')).toBeTruthy();
});

test('city database has San Juan', () => {
  expect(isCity('San Juan')).toBeTruthy();
});

describe('matching cities to foods', () => {
  // Applies only to tests in this describe block
  beforeEach(() => {
    return initializeFoodDatabase();
  });

  test('Vienna <3 veal', () => {
    expect(isValidCityFoodPair('Vienna', 'Wiener Schnitzel')).toBe(true);
  });

  test('San Juan <3 plantains', () => {
    expect(isValidCityFoodPair('San Juan', 'Mofongo')).toBe(true);
  });
});

注意顶级的beforeEach 会比 describe 中的beforeEach 执行的更早。下面的代码也许能帮助你理解所有 hook 的执行顺序。

beforeAll(() => console.log('1 - beforeAll'));
afterAll(() => console.log('1 - afterAll'));
beforeEach(() => console.log('1 - beforeEach'));
afterEach(() => console.log('1 - afterEach'));

test('', () => console.log('1 - test'));

describe('Scoped / Nested block', () => {
  beforeAll(() => console.log('2 - beforeAll'));
  afterAll(() => console.log('2 - afterAll'));
  beforeEach(() => console.log('2 - beforeEach'));
  afterEach(() => console.log('2 - afterEach'));

  test('', () => console.log('2 - test'));
});

// 1 - beforeAll
// 1 - beforeEach
// 1 - test
// 1 - afterEach
// 2 - beforeAll
// 1 - beforeEach
// 2 - beforeEach
// 2 - test
// 2 - afterEach
// 1 - afterEach
// 2 - afterAll
// 1 - afterAll

执行顺序

Jest 会在所有真正的测试开始之前执行测试文件里所有的 describe 处理程序（handlers）。 This is another reason to do setup and teardown inside before* and after* handlers rather than inside the describe blocks. Once the describe blocks are complete, by default Jest runs all the tests serially in the order they were encountered in the collection phase, waiting for each to finish and be tidied up before moving on.

考虑以下示例测试文件和输出：

describe('describe outer', () => {
  console.log('describe outer-a');

  describe('describe inner 1', () => {
    console.log('describe inner 1');

    test('test 1', () => console.log('test 1'));
  });

  console.log('describe outer-b');

  test('test 2', () => console.log('test 2'));

  describe('describe inner 2', () => {
    console.log('describe inner 2');

    test('test 3', () => console.log('test 3'));
  });

  console.log('describe outer-c');
});

// describe outer-a
// describe inner 1
// describe outer-b
// describe inner 2
// describe outer-c
// test 1
// test 2
// test 3

Just like the describe and test blocks Jest calls the before* and after* hooks in the order of declaration. Note that the after* hooks of the enclosing scope are called first. For example, here is how you can set up and tear down resources which depend on each other:

beforeEach(() => console.log('connection setup'));
beforeEach(() => console.log('database setup'));

afterEach(() => console.log('database teardown'));
afterEach(() => console.log('connection teardown'));

test('test 1', () => console.log('test 1'));

describe('extra', () => {
  beforeEach(() => console.log('extra database setup'));
  afterEach(() => console.log('extra database teardown'));

  test('test 2', () => console.log('test 2'));
});

// connection setup
// database setup
// test 1
// database teardown
// connection teardown

// connection setup
// database setup
// extra database setup
// test 2
// extra database teardown
// database teardown
// connection teardown

note

If you are using jasmine2 test runner, take into account that it calls the after* hooks in the reverse order of declaration. To have identical output, the above example should be altered like this:

  beforeEach(() => console.log('connection setup'));
+ afterEach(() => console.log('connection teardown'));

  beforeEach(() => console.log('database setup'));
+ afterEach(() => console.log('database teardown'));

- afterEach(() => console.log('database teardown'));
- afterEach(() => console.log('connection teardown'));

  // ...

通用建议

如果测试失败，第一件要检查的事就是，当仅运行这条测试时，它是否仍然失败。 To run only one test with Jest, temporarily change that test command to a test.only:

test.only('this will be the only test that runs', () => {
  expect(true).toBe(false);
});

test('this test will not run', () => {
  expect('A').toBe('A');
});

如果你有一个测试，当它作为一个更大的用例中的一部分时，经常运行失败，但是当你单独运行它时，并不会失败，所以最好考虑其他测试对这个测试的影响。通常可以通过修改 beforeEach 来清除一些共享的状态来修复这种问题。如果你不确定哪些分享的状态被修改了，你可以尝试在 beforeEach 打印出来看看。

Repeating Setup​

一次性设置​

作用域​

执行顺序​

通用建议​

Repeating Setup

一次性设置

作用域

执行顺序

通用建议