Writing Type-Safe Unit Tests
Annotate test data and assertions with TypeScript types.
Writing Type-Safe Unit Tests is a free TypeScript Academy lesson on CoddyKit — lesson 2 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
Typing Test Data
import type { User } from '../src/types';
const mockUser: User = { id: 1, name: 'Alice', email: 'a@b.com' };
// TypeScript errors if User interface changesTyped Assertions
const result = add(1, 2);
expect(result).toBe(3);
const str: string = result.toString(); // result is typed as numberTyping Return Values
const user: User | null = await findUser(1);
expect(user).not.toBeNull();
expect(user!.name).toBe('Alice');Typed Test Helpers
function createUser(overrides?: Partial<User>): User {
return { id: 1, name: 'Test User', email: 'test@example.com', ...overrides };
}beforeEach Typing
let user: User;
beforeEach(() => { user = createUser(); });
test('has a name', () => { expect(user.name).toBeDefined(); });Typed Async Tests
test('fetches user', async () => {
const user: User = await getUser(1);
expect(user.id).toBe(1);
});Error Testing with Types
await expect(getUser(-1)).rejects.toThrow(NotFoundError);Generic Test Utilities
function expectType<T>(value: T): T { return value; }
expectType<User>(response.data); // compile error if wrong typeSnapshot Testing with Types
const user: User = { id: 1, name: 'Alice' };
expect(user).toMatchSnapshot();Test Coverage and Types
Quick Check
Recap
Frequently asked questions
Is the “Writing Type-Safe Unit Tests” lesson free?
Yes — the full text of “Writing Type-Safe Unit Tests” 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 “Writing Type-Safe Unit Tests”?
Annotate test data and assertions with TypeScript types. 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 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Writing Type-Safe Unit Tests” 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.