Jest for Unit Testing

Jest is a JavaScript testing framework that provides the tools required to write and execute automated tests for JavaScript applications.

Automated tests let developers verify that code behaves as expected by defining inputs, executing the code under controlled conditions, and comparing its actual output or behavior against an expected result.

Unit tests focus on small pieces of application logic in isolation. When the code under test depends on external functions or objects, test doubles such as stubs, spies, and mocks can be used to replace those dependencies with controlled alternatives.

Writing reliable unit tests makes it easier to detect regressions, change existing code confidently, and verify both successful and failure scenarios without repeatedly testing everything by hand.

Learning objectives

In this module, you'll learn how to write automated unit tests for JavaScript applications using Jest.

After completing this module, you will be able to:

• Write and execute unit tests using Jest.
• Verify expected results using assertions and matchers.
• Test primitive values, arrays, and objects.
• Test fulfilled and rejected promises and callback-based asynchronous code.
• Run setup and teardown logic before and after tests.
• Use fakes, dummies, stubs, spies, and mocks to isolate code from its dependencies.