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TypeScript Academy · Lesson

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 — 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, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

Jest module mocking with TypeScript requires correct type casting strategies to avoid any-typed mocks.

jest.fn() with Types

Type a jest mock function with its signature.
const mockFn = jest.fn<number, [string]>();
mockFn('hello'); // typed call

jest.mock() and TypeScript

jest.mock is untyped by default. Cast to jest.MockedFunction for typed access.
jest.mock('./api');
import { fetchUser } from './api';
const mockFetch = fetchUser as jest.MockedFunction<typeof fetchUser>;

jest.mocked() Helper

Jest 27+ provides jest.mocked() to type mocked modules cleanly.
jest.mock('./api');
import { fetchUser } from './api';
const mockFetch = jest.mocked(fetchUser);
mockFetch.mockResolvedValue({ id: 1, name: 'Alice' });

Mocking Return Values

Type mockReturnValue and mockResolvedValue correctly.
mockFetch.mockResolvedValue({ id: 1, name: 'Alice' }); // must match User type

Spying on Methods

Use jest.spyOn for typed method spying.
const spy = jest.spyOn(userService, 'findUser');
spy.mockResolvedValue(mockUser);

Mocking Entire Modules

Mock an entire module with manual factory functions.
jest.mock('./db', () => ({
  findUser: jest.fn(),
  createUser: jest.fn(),
}));

Auto Mocking

jest.createMockFromModule creates a mock from an existing module.
const mockDb = jest.createMockFromModule<typeof import('./db')>('./db');

Partial Mocks

Mock only some methods of a module.
jest.mock('./email', () => ({ ...jest.requireActual('./email'), sendEmail: jest.fn() }));

Clearing Mocks

Reset mocks between tests to prevent state leakage.
afterEach(() => { jest.clearAllMocks(); });
// Or globally: jest.config clearMocks: true

Type-Safe Mock Objects

Create typed mock objects for dependency injection tests.
const mockRepo: jest.Mocked<UserRepository> = {
  findById: jest.fn(),
  save: jest.fn(),
  delete: jest.fn(),
};

Quick Check

What does `jest.mocked(fn)` do in TypeScript?

Recap

Mocking Modules and Functions in TypeScript: mastered the key concepts and patterns.

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, and the TypeScript Academy course includes 4 lessons in total. 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.

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; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

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

  1. Jest Setup for TypeScript with ts-jest
  2. Writing Type-Safe Unit Tests
  3. Mocking Modules and Functions in TypeScript
  4. Type Testing with expectTypeOf and tsd
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