Mocking Modules and Functions in TypeScript
Use jest.mock with correct type casting strategies.
Mocking Modules and Functions in TypeScript is a free TypeScript Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 4 lessons in total.
Welcome
jest.fn() with Types
const mockFn = jest.fn<number, [string]>();
mockFn('hello'); // typed calljest.mock() and TypeScript
jest.mock('./api');
import { fetchUser } from './api';
const mockFetch = fetchUser as jest.MockedFunction<typeof fetchUser>;jest.mocked() Helper
jest.mock('./api');
import { fetchUser } from './api';
const mockFetch = jest.mocked(fetchUser);
mockFetch.mockResolvedValue({ id: 1, name: 'Alice' });Mocking Return Values
mockFetch.mockResolvedValue({ id: 1, name: 'Alice' }); // must match User typeSpying on Methods
const spy = jest.spyOn(userService, 'findUser');
spy.mockResolvedValue(mockUser);Mocking Entire Modules
jest.mock('./db', () => ({
findUser: jest.fn(),
createUser: jest.fn(),
}));Auto Mocking
const mockDb = jest.createMockFromModule<typeof import('./db')>('./db');Partial Mocks
jest.mock('./email', () => ({ ...jest.requireActual('./email'), sendEmail: jest.fn() }));Clearing Mocks
afterEach(() => { jest.clearAllMocks(); });
// Or globally: jest.config clearMocks: trueType-Safe Mock Objects
const mockRepo: jest.Mocked<UserRepository> = {
findById: jest.fn(),
save: jest.fn(),
delete: jest.fn(),
};Quick Check
Recap
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Frequently Asked Questions
Is the “Mocking Modules and Functions in TypeScript” lesson free?
Yes — the full text of “Mocking Modules and Functions in TypeScript” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 4 lessons in total.
What will I learn in “Mocking Modules and Functions in TypeScript”?
Use jest.mock with correct type casting strategies. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start TypeScript Academy?
No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.
How long does the “Mocking Modules and Functions in TypeScript” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this TypeScript Academy lesson?
Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Jest Setup for TypeScript with ts-jest
- Writing Type-Safe Unit Tests
- Mocking Modules and Functions in TypeScript
- Type Testing with expectTypeOf and tsd