node.js - 当模拟 es6 类时，Jest 无法读取未定义的属性 'instances'

Question

尝试模拟 ES6 类，只是为了获得一个我可以监视的虚拟模拟。我的代码似乎非常严格地遵循文档，但是在我的虚拟对象上调用 .mock 时出现此错误:

TypeError: Cannot read property 'instances' of undefined

jest.mock('../../../adapters/Cache')
const Fizz = require('../Fizz')
const Cache = require('../../../adapters/Cache')

const fizz = new Fizz()

describe('CACHE', () => {
  it('should return a mock', () => {
    //This is the line that fails
    const mockCache = Cache.mock.instances[0]

    const mockRetrieveRecords = mockCache.retrieveRecords
    fizz.doStuff()
    expect(mockRetrieveRecords).toHaveBeenCalledTimes(1)
  })
})

Answer 1

jest.mock('../../../adapters/Cache') 将仅使用 undefined 来模拟模块。要模拟返回类的模块，您可以创建一个仅返回一个返回模拟实例的函数的模拟。要设置使 retrieveRecords 成为您可以在测试中访问的 spy ，您必须使用空 spy 模拟该模块，导入它并设置真正的模拟:

jest.mock('../../../adapters/Cache', () => jest.fn())
const Cache = require('../../../adapters/Cache')
const Fizz = require('../Fizz')

const retrieveRecords = jest.fn()
Cache.mockImplementation(() => ({retrieveRecords})



describe('CACHE', () => {
  it('should return a mock', () => {
    const fizz = new Fizz()
    fizz.doStuff()
    expect(retrieveRecords).toHaveBeenCalledTimes(1)
  })
})