What is unit testing?
Unit testing checks the smallest pieces of code, such as functions or classes, in isolation.
Unit tests are written by developers alongside the code. They run in milliseconds, pinpoint exactly which function broke, and form the base of the test pyramid.
Their limitation is isolation. A product can have thousands of passing unit tests and still fail for users, because unit tests do not check how pieces work together or how the product behaves in a real browser.
Example
A unit test checks that a tax function returns 20% of the net amount and rounds to two decimal places.
Best practices
- Test business logic and edge cases at this level
- Keep tests fast and independent
- Mock external dependencies
- Pair with integration and end-to-end tests for user-level confidence
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