如何用 jest 测试回调函数

Question

我正在尝试创建一个异步函数来创建一个文件并用玩笑对其进行测试。在我的代码中的某个时刻，我从 CreateWriteStream 调用 write，它需要一个回调并且开玩笑地抱怨这个函数没有被覆盖。

我的功能

import { createWriteStream } from 'fs'
import { CreateFileStorage, CreateFileStorageModel } from '../../../../data/protocols/create-file-storage'

export class CreateFileLocalStorage implements CreateFileStorage {
  async create (fileData: CreateFileStorageModel): Promise<boolean> {
    return await new Promise((resolve) => {
      const ws = createWriteStream(fileData.path)

      ws.write(fileData.buffer, (err) => {
        if (err) resolve(false) // jest's saying 'if path not taken | statement not covered'

        resolve(true)
      })
    })
  }
}

这是我的测试：

import { existsSync, stat } from 'fs'
import path from 'path'
import { CreateFileLocalStorage } from './file'

describe('Create File LocalStorage', () => {
  test('Should return false on failure', async () => {
    const createFileLocalStorage = new CreateFileLocalStorage()
    const buffer = Buffer.from('text to test')
    const file = {
      filename: 'test_file_creation.txt',
      path: path.resolve(__dirname, 'test_file_creation.txt'),
      size: buffer.byteLength,
      buffer
    }

    // I don't know what to do here to get that 100% coverage
  })

  // I have one more test here that is working fine, it tests the happy path
})

我的问题是，我如何模拟/测试这段代码以获得 100% 的覆盖率？

因为它是一个回调函数，所以我不知道如何测试它，并且很难在测试中故意制造失败

编辑：我快到了，并设法使测试与 jest.mock 一起工作，但是现在我的第二个测试失败了

jest.mock('fs', () => {
  return {
    createWriteStream: () => {
      return {
        write: (chunk: any, callback?: (error: Error) => void): boolean => {
          callback(new Error())
          return false
        }
      }
    }
  }
})

describe('Create File LocalStorage', () => {
  test('Should return false on failure', async () => {
    const createFileLocalStorage = new CreateFileLocalStorage()
    const buffer = Buffer.from('text to test')
    const file = {
      filename: 'test_file_creation.txt',
      path: path.resolve(__dirname, 'test_file_creation.txt'),
      size: buffer.byteLength,
      buffer
    }

    const promise = createFileLocalStorage.create(file)
    await expect(promise).resolves.toBeFalsy()
  })

  // this one now is failing, using `jest.clearMocks`
  // (actually any clear|reset jest function didn't work
  // the problem is that the module is still mocked at this point
  test('Should save file to disk on success', async () => {
    const createFileLocalStorage = new CreateFileLocalStorage()
    const buffer = Buffer.from('text to test')
    const file = {
      filename: 'test_file_creation.txt',
      path: path.resolve(__dirname, 'test_file_creation.txt'),
      size: buffer.byteLength,
      buffer
    }

    const created = await createFileLocalStorage.create(file)
    expect(created).toBe(true)
    expect(existsSync(file.path)).toBe(true)

    stat(file.path, (err, status) => {
      expect(err).toBeNull()

      const isFile = status.isFile()
      const size = status.size

      expect(isFile).toBe(true)
      expect(size).toBe(file.size)
    })
  })
})

Answer 1

createWriteStream 返回类WriteStream 的实例，因此您可以通过窥探原型来模拟write 实例方法。

jest.spyOn(WriteStream.prototype, 'write').mockImplementation((buffer, cb) => {
  cb('this is an error')
})

expect(createFileLocalStorage.create('...')).resolves.toBeFalsy()