TypeScript Academy · Lesson

Writing Type-Safe Unit Tests

Annotate test data and assertions with TypeScript types.

Lesson 2 of 413 steps

Writing Type-Safe Unit Tests is a free TypeScript Academy lesson on CoddyKit. This is 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, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 4 lessons in total.

Welcome

Type-safe tests catch bugs in test code itself and ensure your test data matches your production types.

Typing Test Data

Use your types for test data — TypeScript will catch stale test data when types change.

import type { User } from '../src/types';
const mockUser: User = { id: 1, name: 'Alice', email: 'a@b.com' };
// TypeScript errors if User interface changes

Typed Assertions

expect() works with typed values. TypeScript narrows the type through assertions.

const result = add(1, 2);
expect(result).toBe(3);
const str: string = result.toString(); // result is typed as number

Typing Return Values

Annotate expected return types to catch type changes.

const user: User | null = await findUser(1);
expect(user).not.toBeNull();
expect(user!.name).toBe('Alice');

Typed Test Helpers

Create typed test factory functions for building test data.

function createUser(overrides?: Partial<User>): User {
  return { id: 1, name: 'Test User', email: 'test@example.com', ...overrides };
}

beforeEach Typing

Type variables initialized in beforeEach correctly.

let user: User;
beforeEach(() => { user = createUser(); });
test('has a name', () => { expect(user.name).toBeDefined(); });

Typed Async Tests

Annotate async test functions and their return types.

test('fetches user', async () => {
  const user: User = await getUser(1);
  expect(user.id).toBe(1);
});

Error Testing with Types

Test that functions throw the correct typed error.

await expect(getUser(-1)).rejects.toThrow(NotFoundError);

Generic Test Utilities

Build generic typed test utilities for common assertions.

function expectType<T>(value: T): T { return value; }
expectType<User>(response.data); // compile error if wrong type

Snapshot Testing with Types

TypeScript types help write accurate snapshot tests.

const user: User = { id: 1, name: 'Alice' };
expect(user).toMatchSnapshot();

Test Coverage and Types

TypeScript ensures test utilities maintain the same interface as production code.

Quick Check

What is the benefit of typing test factory functions with `Partial`?

Answer it in the interactive lesson

Recap

Writing Type-Safe Unit Tests: mastered the key concepts and patterns.

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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. 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 “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, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

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.

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

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