👀 Views: 440 💬 Answers: 1 📅 Created: 2025-06-06

jest typescript unit-testing TypeScript

Does anyone know how to I've searched everywhere and can't find a clear answer. I'm sure I'm missing something obvious here, but I'm working on a TypeScript project using Jest for unit testing, and I'm running into an scenario when trying to mock a method from a class. I have a class called `UserService` that interacts with a `DatabaseService` to fetch users from a database. Here’s a simplified version of my code: ```typescript class DatabaseService { getUser(id: number): User | null { // logic to get user from DB } } class UserService { constructor(private dbService: DatabaseService) {} fetchUser(id: number): User | null { return this.dbService.getUser(id); } } ``` In my test file, I want to mock the `getUser` method of `DatabaseService`. Here’s how I attempted to do this: ```typescript import { UserService } from './UserService'; import { DatabaseService } from './DatabaseService'; jest.mock('./DatabaseService'); test('fetchUser should return a user object', () => { const mockDatabaseService = new DatabaseService() as jest.Mocked<DatabaseService>; mockDatabaseService.getUser.mockReturnValue({ id: 1, name: 'John Doe' }); const userService = new UserService(mockDatabaseService); const user = userService.fetchUser(1); expect(user).toEqual({ id: 1, name: 'John Doe' }); }); ``` When I run my tests, I get the following behavior: ``` TypeError: mockDatabaseService.getUser.mockReturnValue is not a function ``` I've checked the Jest documentation and confirmed I’m using Jest version 27.0.0. It seems like my mock isn't being recognized as a jest mock object. What am I missing in my mocking implementation? Any best practices or design patterns for mocking methods in TypeScript with Jest that I should consider? This is part of a larger web app I'm building. I'm working in a Windows 11 environment. Could someone point me to the right documentation? I'm working on a microservice that needs to handle this.