Unit Tests (One Piece in Isolation)

A unit test checks one small piece of code — usually a single function — all by itself, with nothing else attached. Think of testing one light bulb by screwing it into a lamp you know works, rather than flipping the whole house's breaker and guessing. Because a unit test touches only one function, when it fails you know precisely where the problem is. Unit tests are tiny, fast, and you write lots of them.

1. 1Pick one function that takes inputs and returns a result, with no hidden dependencies.
2. 2Write several cases: normal inputs, edge cases, and the boundaries where behavior changes.
3. 3For each case, call the function and compare its output to the expected value.
4. 4Count passes. A green run means that one unit behaves correctly across every case you thought of.

Use unit tests for pure logic: calculations, formatting, validation, parsing — anything that takes inputs and returns outputs without reaching out to a database or network. They're the broad base of most test suites because they're cheap and fast.

• ⚠Testing only one 'happy' input. The bugs hide at the edges — zero percent, one hundred percent, empty, negative — so cover those explicitly.
• ⚠Letting the unit secretly depend on the outside world (a clock, a database). If it does, it isn't really a unit test anymore, and it'll fail for reasons unrelated to your logic.