What are best practices for writing good tests?

Question

Accepted Answer

Writing **good tests** — that are reliable, maintainable, and valuable — follows established best practices. Poor tests (brittle, slow, unclear) become a burden, while good tests provide lasting value. Understanding these practices is important for effective testing.

## Characteristics of good tests (FIRST)

```text
FAST → run quickly (so they're run often)
INDEPENDENT/ISOLATED → don't depend on each other or order; each sets up its own state
REPEATABLE → same result every time, any environment (no flakiness)
SELF-VALIDATING → clear pass/fail (assertions, not manual inspection)
TIMELY → written with (or before) the code
```

## Key practices

```text
✓ Test BEHAVIOR, not implementation → tests survive refactoring (don't test internals)
✓ ONE concept per test → focused tests; clear what failed when one breaks
✓ CLEAR NAMES — describe what's tested ("returns error when email is invalid")
✓ Arrange-Act-Assert structure → readable, organized
✓ Cover EDGE cases and error conditions (not just the happy path)
✓ Keep tests SIMPLE and readable (tests are documentation; avoid complex logic in tests)
✓ DETERMINISTIC — no reliance on time, randomness, external state, or order (avoid flaky)
✓ DRY but readable — share setup (fixtures), but keep each test clear on its own
```

## Avoid these (anti-patterns)

```text
✗ Testing implementation details → brittle tests breaking on every refactor
✗ FLAKY tests (intermittent failures) → erode trust; fix or remove them
✗ Over-mocking → tests coupled to internals
✗ Tests with no clear assertion; giant tests checking many things; interdependent tests
```

## Why it matters

Understanding best practices for writing good tests is important because **the quality of tests determines whether they provide value or become a burden**, so it's valuable knowledge for effective testing.

Tests are code that must be maintained, and **poor tests** (brittle, slow, flaky, unclear) become a costly burden — breaking constantly on refactoring, failing intermittently, and being hard to understand — while **good tests** provide lasting value (catching real bugs, enabling confident change, documenting behavior).

Understanding the **FIRST characteristics** (Fast, Independent, Repeatable, Self-validating, Timely) captures the qualities good tests should have.

Understanding the **key practices** — testing **behavior not implementation** (so tests survive refactoring, a crucial practice since implementation-coupled tests are brittle), **one concept per test** (focused tests with clear failures), **clear descriptive names** (so a failing test communicates what broke), AAA structure, covering **edge cases and errors** (not just happy paths, where many bugs hide), keeping tests simple and readable (since they're documentation), being **deterministic** (avoiding flakiness from time, randomness, or order dependence), and balancing DRY with readability — reflects writing tests that are reliable and maintainable.

Understanding the **anti-patterns to avoid** — testing implementation details (brittleness), **flaky tests** (which erode trust in the whole suite, a serious problem), over-mocking, and unfocused or interdependent tests — is equally important for not creating burdensome tests.

Since test quality determines whether tests help or hinder (good tests provide value; poor tests are a maintenance burden and erode trust), and since understanding the practices (behavior-focused, isolated, deterministic, clear, edge-case-covering) and anti-patterns (implementation-coupled, flaky) is necessary to write valuable tests, understanding test best practices is valuable, practically-important knowledge — essential for writing tests that provide lasting value rather than becoming a liability, a key skill that distinguishes effective testing from counterproductive testing.