👀 Views: 35 💬 Answers: 1 📅 Created: 2025-06-01

jest unit-testing dependency-injection JavaScript

I need help solving I'm trying to figure out I'm stuck on something that should probably be simple. I've looked through the documentation and I'm still confused about I'm currently working on a Node.js application using Express and I want to ensure my unit tests are properly isolated. I’m utilizing Jest for testing and applying dependency injection to mock services. However, I'm running into issues where my unit tests are still dependent on the actual implementations of these services instead of the mocks. Here's a simplified version of my code: ```javascript // userService.js class UserService { constructor(databaseService) { this.databaseService = databaseService; } async getUser(id) { return await this.databaseService.findUserById(id); } } // databaseService.js class DatabaseService { async findUserById(id) { // Simulate a database call return { id, name: 'John Doe' }; } } // userService.test.js const UserService = require('./userService'); const DatabaseService = require('./databaseService'); const mockDatabaseService = { findUserById: jest.fn() }; describe('UserService', () => { it('should return user data', async () => { mockDatabaseService.findUserById.mockResolvedValue({ id: 1, name: 'Jane Doe' }); const userService = new UserService(mockDatabaseService); const user = await userService.getUser(1); expect(user).toEqual({ id: 1, name: 'Jane Doe' }); expect(mockDatabaseService.findUserById).toHaveBeenCalledWith(1); }); }); ``` Despite my best efforts, the unit tests sometimes unexpected result when I modify the actual `DatabaseService`, leading to unexpected errors. I believe this might be due to how I’m configuring the mocks or possibly how Jest is handling the imports. I am using Jest version 27.0.6. I would really appreciate any insights into how I can ensure my tests are truly isolated from the actual implementations, and if there are any best practices for setting up mocks in Jest to prevent these cross-dependencies. Am I missing something obvious? My team is using Javascript for this service. I'm open to any suggestions.