Tests that matter

Writes tests for what can actually break, instead of chasing a coverage number.

Cuándo se usa

When wrapping up a feature, or right after fixing a bug so it cannot come back.

SKILL.md

# Tests that matter

Coverage measures executed lines, not prevented failures. Write the tests that
would have caught the last bug that slipped through.

## What to test

- **The edges.** Empty, one, many, negative, null, very long text, emoji.
- **The error paths.** The network fails, the file is missing, the response comes
  back in another shape.
- **The business rules.** What matters to whoever uses the app, not to whoever
  wrote it.
- **Every fixed bug.** Before the fix, a test that fails.

## What not to test

- That the third-party library does what it promises.
- Getters and setters with no logic.
- The internals of a private function: test what is visible from outside, or any
  refactor will break your suite for no reason.

## How to write them

A name that states the case, not the function: "returns an empty list when there
are no results". Arrange, act, assert, with a blank line between the three parts.
One assertion per concept. No logic inside the test: if a test needs an if, the
test needs splitting in two.

Before trusting a test, break it on purpose and check that it fails. A test that
always passes is testing nothing.