client.js

module.exports = class ProductsClient {   async getById(id) {     const url = `http://localhost:3000/api/products/{id}`;     const response = await fetch(url);     return await response.json();   } } 

manager.js

const ProductsClient = require('./client');  module.exports = class ProductManager {   async getProductToManage(id) {     const productsClient = new ProductsClient();     const productToManage = await productsClient.getById(id)       .catch(err => alert(err));     return productToManage;   } } 

test.js

const ProductsClient = require('./client'); const ProductManager = require('./manager');  jest.mock('./client');  describe('test', () => {   test('111', async () => {     const expectedProduct = {       id: 1,       name: 'football',     };     const mockGetById = jest.fn();     mockGetById.mockResolvedValue(expectedProduct);     ProductsClient.prototype.getById = mockGetById;             const productManager = new ProductManager();       const result = await productManager.getProductToManage(1);      expect(result.name).toBe('football');   }) }) 

在 test.js 中 mock client 完全没问题。一旦把const productsClient = new ProductsClient();拿到class ProductManager外，getById就返回 undefined

const ProductsClient = require('./client');  const productsClient = new ProductsClient();  module.exports = class ProductManager {   async getProductToManage(id) {     const productToManage = await productsClient.getById(id) // undefined       .catch(err => alert(err));     return productToManage;   } }  

jest mock module 文档里也没找到类似的例子。