Mastering Jest Testing with TypeScript and Node.js

Build solid testing skills with Jest, TypeScript, and Node - from setup to mocks, spies, and production error handling

What you'll learn

• Set up and configure Jest for a TypeScript-based Node.js project
• Write unit and integration tests using Jest’s core methods and matchers
• Use test doubles such as mocks, stubs, fakes, and spies effectively
• Implement and test robust error handling for real-world applications

Requirements

• Basic understanding of JavaScript and TypeScript. You should be comfortable with functions, variables, and basic types)
• Familiarity with Node.js and using the terminal/command line
• A code editor like VS Code and Node.js installed on your machine
• No prior testing experience is required — we'll start from the ground up!

Description

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.

Who this course is for:

This course is perfect for JavaScript or TypeScript developers who want to learn how to write high-quality, automated tests using Jest in Node.js projects. Whether you're a beginner to testing or a developer who’s written a few tests but never felt confident with mocking, spying, or handling real-world errors — this course is for you.