# Testing

Verify behavior automatically so you can change code with confidence.

## Tools by Language

| Language | Recommended | Alternatives |
|---|---|---|
| Python | `pytest` | unittest, hypothesis |
| JavaScript/TS | `vitest` | jest, mocha |
| Go | `testing` (stdlib) | testify |
| C++ | `googletest` | catch2, doctest |

## Python — `pytest`

```bash
pytest                    # run all tests
pytest -x                 # stop on first failure
pytest -k "test_login"    # run matching tests
pytest --cov=src          # with coverage
```

Minimal test:

```python
def add(a: int, b: int) -> int:
    return a + b

def test_add():
    assert add(2, 3) == 5
```

Configure in `pyproject.toml`:

```toml
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-ra -q"
```

## JavaScript — `vitest`

```bash
vitest run          # run once
vitest              # watch mode
vitest --coverage
```

## Go

```bash
go test ./...           # run all tests
go test -race ./...     # with race detector
go test -cover ./...
```

## Key Principles

- Test behavior, not implementation — tests that break on refactors aren't useful
- Fast tests get run; slow tests get skipped
- Don't mock more than necessary — integration tests catch what unit tests miss
- Add a test when you fix a bug; the bug was a missing test