Test-Driven Development

Test-driven development is the practice of writing a failing test before the code that satisfies it, then writing the minimum implementation to pass, then refactoring with the test as protection. The cycle is usually named red, green, refactor.

The red step is the one that carries the value, and it is the one most often skipped. A test written after the implementation and added to a passing suite has never been observed to fail, so it might be asserting nothing at all — see Falsifiability. Writing it first makes the failure a precondition rather than a hope.

The secondary benefit is design pressure. Code that is hard to test from the outside is usually code with tangled dependencies or unclear boundaries, and the friction shows up before the design has calcified.

The main misuse is treating it as a mandate to test every function in isolation. Tests bound to internal structure make refactoring expensive — the suite breaks when the implementation changes even though behaviour did not, which trains people to distrust it. Favouring tests at the seams that outlast implementations, and integration over unit tests on private helpers, keeps the suite an asset rather than a tax. See Integration Test.