Unit testing JavaScript code requires changing how code is written to enable automated testing, validation, and refactoring. Key aspects include:
1. Tests should run quickly in real browsers and output code coverage data to integrate with continuous integration systems.
2. Popular tools like Karma and Jasmine allow defining test suites with descriptions, asynchronous behavior, and assertions to validate code works as expected across browsers.
3. Jasmine defines tests using describe, it, expect functions and matches actual vs expected output to validate functionality. Well-written tests provide reliable, fast execution and protection during refactoring.