TypeScript Testing

Guidance for writing effective tests in TypeScript projects using Vitest or Jest.

Rules

• Arrange-Act-Assert: Structure tests with clear setup, execution, and verification phases
• Single Assertion Focus: Each test should verify one specific behavior
• Descriptive Names: Use describe blocks for grouping and it/test with clear descriptions
• Test Isolation: Tests must not depend on execution order or shared mutable state
• Mock External Dependencies: Always mock I/O, network calls, and system interactions
• Coverage Targets: Aim for 80%+ line coverage, 70%+ branch coverage
• No Logic in Tests: Avoid conditionals and loops in test code
• Clean Up: Use beforeEach/afterEach for setup/teardown, never leave test artifacts

Patterns

Test Structure