Unit Testing with Mocha and Chai: JS Guide

What is unit testing and why does it matter? A unit test verifies that a single, isolated piece of your code -- a function, a method, a class -- does what it is supposed to do. You give it known inputs and check that the outputs match your expectations. If someone changes that code later and breaks its behavior, the test catches it before the bug reaches production. The value of unit testing is not theoretical. Teams with strong test suites ship faster because they refactor with confidence, catch regressions early, and spend less time debugging in production. Teams without tests move cautiously, fear changes to core logic, and rely on manual QA that misses edge cases. Unit testing is the foundation of the testing pyramid. Integration tests verify that components work together. End-to-end tests verify entire user workflows. But unit tests are the fastest, cheapest, and most numerous tests in a healthy codebase. They run in milliseconds, pinpoint exactly which function broke, and give yo