Mastering Jest Testing with TypeScript and Node.js

This course is your complete guide to mastering testing in modern Node.js applications using Jest and TypeScript. Whether you're testing utility functions or handling complex service errors, this course walks you through everything with practical examples and real-world patterns.

We begin with setting up a Node.js project from scratch, configuring TypeScript and Jest, and writing your first test. You’ll learn the structure of test files, naming conventions, and how to run and debug specific tests. We'll use the __tests__ folder and also explore organizing tests next to modules.

You’ll work with core Jest methods and patterns:

• Test Structuring: describe(), it(), test()

• Assertions & Matchers: expect(), .toBe(), .toEqual(), .toHaveLength(), .toBeTruthy(), .toBeFalsy()

• Testing Concepts: System Under Test (SUT), Arrange-Act-Assert (AAA)

• Lifecycle Hooks: beforeEach(), afterEach(), beforeAll(), afterAll()

We’ll write tests for:

• Utility functions: add(), subtract(), isEven(), createUser(), createJwtToken()

• Class methods: Calculator.add(), subtract(), isPositive()

Then you'll dive into test doubles:

• Dummy, Stub, Fake, Mock, and Spy

• Use of jest.fn(), jest.mock(), jest.spyOn(), mockReturnValue(), mockResolvedValue(), mockImplementation()

• Mock and spy reset methods: jest.clearAllMocks(), jest.resetAllMocks(), jest.restoreAllMocks()

Advanced error handling includes:

• Declarative style: expect(...).rejects.toThrow()

• Imperative style: try/catch, fail('Should have thrown an error')

• Enterprise-level practices using:

  • CustomError.throwError(), AppCodes, HttpCodes, CustomLogger

You’ll test real-world scenarios like:

• Mocking services (UserRepository, NewsletterService)

• Validating error responses and logging

• Testing invalid inputs and API failure cases

By the end, you’ll confidently write robust, clean, and well-structured tests for any Node.js TypeScript project.