Testing Middleware in Express.js with Jest: Handling Next Function Properly
After trying multiple solutions online, I still can't figure this out. I'm stuck on something that should probably be simple... I'm currently trying to write unit tests for a middleware function in my Express.js application using Jest, but I'm running into issues with how to properly handle the `next` function. My middleware is supposed to verify that the user is authenticated before proceeding to the next handler. However, when I run my tests, I get the behavior: `TypeError: next is not a function`. Here's the middleware code: ```javascript const authenticate = (req, res, next) => { if (req.isAuthenticated()) { return next(); } return res.status(401).json({ message: 'Unauthorized' }); }; ``` In my test file, I'm trying to simulate an authenticated request, but it seems like I'm not stubbing the `next` function correctly. Here's what I have tried so far in my test: ```javascript const request = require('supertest'); const express = require('express'); const app = express(); app.use(authenticate); describe('Authentication Middleware', () => { it('should call next() if user is authenticated', async () => { const mockRequest = { isAuthenticated: jest.fn().mockReturnValue(true) }; const mockResponse = {}; const mockNext = jest.fn(); authenticate(mockRequest, mockResponse, mockNext); expect(mockNext).toHaveBeenCalled(); }); it('should return 401 if user is not authenticated', async () => { const mockRequest = { isAuthenticated: jest.fn().mockReturnValue(false) }; const mockResponse = { status: jest.fn().mockReturnThis(), json: jest.fn() }; const mockNext = jest.fn(); authenticate(mockRequest, mockResponse, mockNext); expect(mockResponse.status).toHaveBeenCalledWith(401); expect(mockResponse.json).toHaveBeenCalledWith({ message: 'Unauthorized' }); expect(mockNext).not.toHaveBeenCalled(); }); }); ``` The first test passes, but the second test fails with the `TypeError: next is not a function`. I've verified that both tests are calling the `authenticate` function in the same way. What am I missing here? Is it related to how Iām mocking the `next` function, or something else in my Express setup? Any help would be appreciated! Is there a better approach? I'm using Javascript latest in this project. Could this be a known issue? For context: I'm using Javascript on Windows 10. I'm open to any suggestions.