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 is Testing

Testing is a crucial part of software development that ensures code behaves as expected, remains reliable, and prevents regressions. By writing tests, developers can catch errors early, improve code maintainability, and confidently make changes without breaking existing functionality. Different types of testing, such as unit tests, integration tests, and end-to-end tests, help validate various aspects of an application, ensuring a smooth and bug-free user experience.

What is Jest

Jest is a powerful, easy-to-use JavaScript testing framework designed for simplicity and efficiency. It provides built-in assertions, test runners, and mocking capabilities, making it an excellent choice for both frontend and backend testing. With TypeScript support through ts-jest, Jest enables developers to write type-safe tests that integrate seamlessly into modern development workflows. Its zero-configuration setup, fast execution, and strong community support make it a go-to solution for testing JavaScript and TypeScript applications.

Course Outline

Module 1: Setting Up Jest for TypeScript Projects

• Create a new project folder and initialize a Node.js project.
• Install necessary dependencies for TypeScript and Jest.
• Configure Jest with a preset for TypeScript and set the appropriate test environment.
• Update TypeScript configuration for compatibility with Jest.
• Write a simple function and create a corresponding test file.
• Structure test cases using standard Jest methods.
• Run tests using Jest and integrate test execution into the project setup.

Module 2: Organizing and Writing Jest Tests

• Overview of the project structure and test file locations.
• Jest automatically detects test files inside the __tests__ directory.
• Naming conventions for test files (.test.js|ts vs .spec.js|ts).
• Different approaches to organizing test files:
  • Placing tests next to the corresponding modules.
  • Keeping tests in a dedicated __tests__ directory.
• Running a single test file for debugging.
• Using focused tests (it.only()) to narrow down issues.
• Logging within test cases to debug test failures.
• Writing Jest tests using:
  • describe() for grouping tests.
  • it() and test() for defining test cases.
  • expect() for assertions.
  • .toBe() for strict equality comparisons.
  • .toEqual() for deep object comparisons.
• Understanding the System Under Test (SUT) concept.
• Using the Arrange-Act-Assert (AAA) pattern in tests.
• Writing test cases for utility functions:
  • Testing arithmetic functions like add() and subtract().
  • Checking boolean functions like isEven().
  • Verifying object creation functions like createUser().
  • Testing asynchronous functions like createJwtToken().
• Understanding and using Jest matchers:
  • .toBe() for primitive comparisons.
  • .toEqual() for deep object comparisons.
  • .toHaveLength() for array length checks.
  • .toBeTruthy() and .toBeFalsy() for boolean assertions.
• Writing tests for class-based modules:
  • Creating test cases for Calculator class.
  • Verifying class methods like add(), subtract(), and isPositive().
• Using Jest lifecycle hooks for efficient test setup:
  • beforeEach() for resetting test conditions before each test.
  • afterEach() for cleaning up after each test.
  • beforeAll() for setup before running test suites.
  • afterAll() for cleanup after all tests run.
• Comparing manual setup vs. using Jest hooks:
  • Benefits of reducing repetition with beforeEach().
  • Maintaining consistent test states across multiple test cases.

Module 3: Using Test Doubles (Mocks, Stubs, Fakes, Spies)

• Understanding Test Doubles and their role in testing.
• Types of test doubles:
  • Dummy - Used as placeholders in tests.
  • Stub - Provides predefined responses without real logic.
  • Fake - A working but simplified implementation of a component.
  • Mock - A complete override of a dependency.
  • Spy - Observes function calls while keeping the original behavior.
• Writing tests using Dummy Test Doubles:
  • Creating a test case with a dummy object.
  • Using dummy data to test parameter dependencies.
• Using Stubs and Fakes:
  • Creating a Stub to control method return values.
  • Implementing a Fake to simulate real behavior in memory.
  • Writing tests for an OrderService using stubs and fakes.
• Implementing Spies in Jest:
  • Using jest.spyOn() to track function calls.
  • Overriding function return values dynamically with mockReturnValue().
  • Mocking asynchronous functions with mockResolvedValue().
  • Throwing errors inside spies with mockImplementation().
  • Resetting and restoring spies using:
    • jest.clearAllMocks()
    • jest.resetAllMocks()
    • jest.restoreAllMocks()
• Working with Mocks in Jest:
  • Creating mock functions using jest.fn().
  • Mocking an entire module using jest.mock().
  • Understanding Jest's auto-hoisting behavior for mocks.
  • Ensuring all required methods are mocked to prevent undefined errors.
• Writing tests for a User Service:
  • Mocking database interactions (UserRepository).
  • Mocking external services (NewsletterService).
  • Handling errors in API calls using mocks and spies.
• Comparing Mocks vs. Spies:
  • Mocks replace the function entirely.
  • Spies track calls but can retain the original behavior.
• Best practices for managing test doubles:
  • When to use stubs, fakes, mocks, and spies.
  • Avoiding unnecessary complexity in tests.
  • Keeping test cases isolated and independent.

Module 4: Error Handling

• Understand the importance of testing error scenarios in production-grade applications.

• Learn how to handle and test both expected and unexpected errors using Jest.

• Error Testing Strategies:

  • Compare two main approaches:
    • Declarative Style with expect().rejects.toThrow() for cleaner syntax.
    • Imperative Style using try-catch blocks for detailed and custom assertions.
  • Use fail('Should have thrown an error') to prevent false positives in async test cases.

• Test Implementation Details:

  • Spy on service methods (createUser, subscribeUser) with jest.spyOn().
  • Mock return values or rejections using mockResolvedValue() and mockImplementation().

• Enterprise Error Management:

  • Integrate custom utility modules for robust error handling:
    • CustomError for structured exception throwing.
    • AppCodes and HttpCodes for standardized error identification.
    • CustomLogger for logging contextual data.
  • Simulate and test enterprise-level error propagation in the UserService.

• Verifying Error Parameters:

  • Spy on CustomError.throwError() and assert its call with:
    • Proper HTTP status code (HttpCodes.INTERNAL_SERVER_ERROR)
    • Application-level code (AppCodes.REGISTER_USER_FAILED)
    • Descriptive message ('failed to register user')

• Test Cleanup and Mock Management:

  • Use jest.restoreAllMocks() in afterEach() to ensure clean test runs.
  • Silence logs during tests by mocking CustomLogger.error and CustomLogger.info with jest.fn().

• Real-world Scenarios Covered:

  • Handle invalid user input (e.g. missing name).
  • Mock failed service interactions (e.g. subscription failure).
  • Validate logger and error service behavior.