jest mock 返回 undefined

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。