Testing Fundamentals: Mastering Spies and Mocks

Writing reliable tests requires more than just checking return values; you need to know exactly how your code interacts with its dependencies. Spies and mocks let you peek inside your functions to verify behavior without executing real, slow, or unpredictable external services. This text-only course guides you through the core concepts of test doubles, focusing on how to use spies to track function executions, arguments, and call frequencies. You will transition from writing basic assertions to designing robust, isolated unit tests that verify complex logical workflows. What you'll learn: Understand the foundational differences between spies, mocks, stubs, and dummy objects; Configure spies to track function execution, call counts, and passed arguments; Mock external dependencies and module imports to isolate your unit tests; Verify that functions are triggered under specific conditions and in the correct order; Apply modern testing patterns using popular JavaScript test runners; Practice writing clean, maintainable test suites that prevent regression. You will start with key testing terminology and foundational definitions before moving on to practical written exercises and code-based scenarios. Each module provides clear, step-by-step explanations of spying techniques that you can immediately apply to your own projects. This course is designed for beginner developers who want to improve their testing skills; basic familiarity with programming concepts is helpful, but no prior testing experience is required. Start reading today to write cleaner, more reliable tests with confidence.