Test Isolation

Overview

Each test must be independent. No shared state. No dependencies between tests.

Tests that depend on each other are brittle, hard to debug, and can't run in parallel. Every test should set up its own state and clean up after itself.

When to Use

• Writing any test that uses shared data
• Tests that must run in a specific order
• Tests that fail randomly or when run alone
• Test suites that can't run in parallel

The Iron Rule