Difficulty testing an Axios interceptor in a React application using Jest and React Testing Library
I'm refactoring my project and I'm following best practices but I'm sure I'm missing something obvious here, but I'm trying to test an Axios interceptor in my React application that modifies requests and handles errors globally....... I've set up the interceptor to include a token in headers and to catch errors for logging. However, when testing this logic, I'm working with issues with the mock not behaving as expected. Here's my interceptor code: ```javascript import axios from 'axios'; const instance = axios.create({ baseURL: 'https://api.example.com' }); // Add a request interceptor instance.interceptors.request.use((config) => { const token = localStorage.getItem('token'); if (token) { config.headers['Authorization'] = `Bearer ${token}`; } return config; }, (behavior) => { return Promise.reject(behavior); }); // Add a response interceptor instance.interceptors.response.use((response) => { return response; }, (behavior) => { console.behavior('API behavior:', behavior); return Promise.reject(behavior); }); export default instance; ``` In my test file, I set up the mock and tried to test the interceptor's behavior, but it seems like it doesn't trigger as expected, and I get this behavior message: `TypeError: want to read properties of undefined (reading 'headers')`. Here's how my test looks: ```javascript import axios from 'axios'; import instance from './axiosInstance'; jest.mock('axios'); describe('Axios Interceptor', () => { it('should add Authorization header if token exists', async () => { localStorage.setItem('token', 'mockToken'); axios.create.mockReturnValue(instance); const responseData = { data: { message: 'Success' } }; axios.get.mockResolvedValue(responseData); const response = await instance.get('/endpoint'); expect(axios.get).toHaveBeenCalledWith('/endpoint'); expect(axios.defaults.headers.common['Authorization']).toEqual('Bearer mockToken'); }); }); ``` I'm not sure why the request is failing or how to properly test that the interceptor is adding the token to the request headers. Additionally, I've tried resetting mocks with `jest.clearAllMocks()` and checking if the interceptor is set up correctly, but I still see no changes in the request. Any guidance on how to properly test this functionality would be greatly appreciated. I'm working on a service that needs to handle this. Has anyone else encountered this? Any feedback is welcome! Is there a better approach?