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Intermediate⏱ 7 min read

Unit Testing Guide

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Unit testing validates individual functions, methods, or components in isolation from the rest of the system. They are the fastest, cheapest, and most specific form of automated test, and form the foundation of a healthy test suite.

◈Key Points

  • A unit test should test one thing and have a single reason to fail.
  • Use the AAA pattern: Arrange (setup), Act (execute), Assert (verify).
  • Mock external dependencies (databases, APIs, services) to keep tests fast and isolated.
  • Aim for high unit test coverage of business logic, but don't worship the coverage number.
  • Unit tests double as living documentation of intended behaviour.
⚡ Expert Pro Tip

Write unit tests before fixing bugs — first write a failing test that reproduces the bug, then fix it. This prevents regression and proves your fix works.