为什么jest.fnmockImplementation在beforeEach范围中声明时看不到reset变量？

问题描述

给出被测模块sut.js

const { dependencyFunc } = require('./dep')

module.exports = () => {
  return dependencyFunc()
}

具有依赖性dep.js

module.exports = {
  dependencyFunc: () => 'we have hit the dependency'
}

和一些测试：

describe('mocking in beforeEach',() => {
  let sut
  let describeScope

  beforeEach(() => {
    let beforeEachScope = false
    describeScope = false

    console.log('running before each',{ beforeEachScope,describeScope })
    jest.setMock('./dep',{
      dependencyFunc: jest.fn().mockImplementation(() => {
        const returnable = { beforeEachScope,describeScope }
        beforeEachScope = true
        describeScope = true
        return returnable
      })
    })
    sut = require('./sut')
  })

  it('first test',() => {
    console.log(sut())
  })

  it('second test',() => {
    console.log(sut())
  })
})

我得到以下输出：

me$ yarn test test.js
yarn run v1.22.5
$ jest test.js
 PASS  ./test.js
  mocking in beforeEach
    ✓ first test (17 ms)
    ✓ second test (2 ms)

  console.log
    running before each { beforeEachScope: false,describeScope: false }

      at Object.<anonymous> (test.js:9:13)

  console.log
    { beforeEachScope: false,describeScope: false }

      at Object.<anonymous> (test.js:22:13)

  console.log
    running before each { beforeEachScope: false,describeScope: false }

      at Object.<anonymous> (test.js:9:13)

  console.log
    { beforeEachScope: true,describeScope: false }

      at Object.<anonymous> (test.js:26:13)

Test Suites: 1 passed,1 total
Tests:       2 passed,2 total
Snapshots:   0 total
Time:        1.248 s,estimated 2 s
Ran all test suites matching /test.js/i.
✨  Done in 3.56s.

我希望两个测试的输出均为{ beforeEachScope: false,describeScope: false }。即，我希望beforeEachScope和describeScope变量都将重置为false，而不管它们是在beforeEach范围还是describe范围中声明的。在我的真实测试中，我认为将它放在beforeEach范围内比较干净，因为在其他地方不需要它。这是怎么回事？开玩笑的东西使用什么范围？

解决方法

我看起来setMock只考虑了给定模块的第一个模拟，而没有在第二个调用中覆盖它。或更确切地说，我相信是require进行了缓存-笑话在运行整个测试套件之前仅清空模块缓存一次（预计the imports will be at the top of the suite在声明要模拟的模块之后）。

您的模拟实现随后从第一个beforeEachScope调用开始对beforeEach进行了关闭。

为什么您可能会想，为什么describeScope上也没有关闭？实际上，确实如此，可能会使您的代码感到困惑的是，beforeEach确实运行了describeScope = false，这总是将其重置为false，然后再记录到任何地方。如果删除该语句，而仅在let describeScope = false范围内初始化describe，您将看到它在第一次调用true之后也将更改为sut()。

如果我们手动解析范围并从执行中删除所有垃圾包装，这就是发生的情况：

let sut
let describeScope

// first test,beforeEach:
let beforeEachScope1 = false
describeScope = false

console.log('running before each 1',{ beforeEachScope1,describeScope }) // false,false as expected
jest.setMock('./dep',{
  dependencyFunc(n) {
    console.log('sut call '+n,describeScope });
    beforeEachScope1 = true
    describeScope = true
  })
})
sut = require('./sut') // will call the function we just created

// first test
sut(1) // still logs false,false

// second test,beforeEach:
let beforeEachScope2 = false // a new variable
describeScope = false // reset from true to false,you shouldn't do this

console.log('running before each 2',{ beforeEachScope2,describeScope }) // logs false,false
jest.setMock('./dep',{
  dependencyFunc(n) {
    // this function is never called
  })
})
sut = require('./sut') // a no-op,sut doesn't change (still calling the first mock)

// second test:
sut(2) // logs true (beforeEachScope1) and false

使用以下内容：

const dependencyFunc = jest.fn();
jest.setMock('./dep',{
  dependencyFunc,})
const sut = require('./sut')

describe('mocking in beforeEach',() => {
  let describeScope = false

  beforeEach(() => {
    let beforeEachScope = false

    console.log('running before each',{ beforeEachScope,describeScope })
    dependencyFunc.mockImplementation(() => {
      const returnable = { beforeEachScope,describeScope }
      beforeEachScope = true
      describeScope = true
      return returnable
    })
  })

  it('first test',() => {
    console.log(sut())
  })

  it('second test',() => {
    console.log(sut())
  })
})

下面的示例演示了缓存和作用域/关闭行为的组合。

let cachedFunction

let varInGlobalClosure

const run = () => {
  varInGlobalClosure = false
  let varInRunClosure = false

  // next line is what jest.mock is doing - caching the function
  cachedFunction = cachedFunction || (() => {
    const returnable = { varInRunClosure,varInGlobalClosure }
    varInRunClosure = true
    varInGlobalClosure = true
    return returnable
  })

  return cachedFunction
}

console.log('first run',run()()) // outputs { varInRunClosure: false,varInGlobalClosure: false }
console.log('second run',run()()) // outputs { varInRunClosure: true,varInGlobalClosure: false }

这是因为我们正在run内创建一个新的闭包，当我们第二次调用varInRunClosure时使用一个新的run，但是缓存的函数仍在使用生成的闭包run第一次运行时，现在在缓存的函数范围之外无法访问。

javascript jestjs scope scope