# Jest > Jest unit testing conventions — configuration, describe/it structure, mocking, async tests, and coverage. Load when working with Jest test files or jest.config.*. Use when this capability is needed. - Skill: `tomevault-io/jest-4` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/jest-4` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/jest-4/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/jest-4 --- Full standards in [jest.md](jest.md). Always-on summary: **Structure:** - One `describe(` per module/class; one `describe(` per method inside - Test names complete the sentence: `it('returns null when user not found')` - Use `expect(` for all assertions — Arrange-Act-Assert with blank lines between sections **Setup / teardown:** - `beforeEach(` to reset state before each test; call `jest.clearAllMocks(` in `beforeEach` to prevent cross-test pollution **Mocking:** - `jest.mock(` calls go at the top of the file, before imports take effect - Use `mockReturnValue(` or `mockResolvedValue(` to control return values - Mock at the system boundary (HTTP, DB, filesystem) — not internal functions - `jest.spyOn()` for partial mocks where you want the real implementation for some calls - Always restore mocks: use `jest.restoreAllMocks()` in `afterEach` or `restoreMocks: true` in config **Async:** - Always `return` or `await` promises — an unawaited assertion can silently pass - Use `jest.useFakeTimers()` for `setTimeout`/`setInterval` tests; always call `jest.useRealTimers()` in `afterEach` **Coverage:** - Set thresholds in `jest.config.ts` — don't chase 100%; target branches and functions - `/* istanbul ignore next */` — use sparingly with a comment explaining why **Never:** - `it.only()` or `describe.only()` merged to main — use `--testNamePattern` in dev - Test implementation details — test behaviour observable from the outside - Assertions inside loops — extract to helpers or use `test.each` **Related skills:** Your frontend or backend layer (framework-specific setup), `mocking/msw` (HTTP mocking for integration tests) --- > Source: [manikumarkv/devrunway-claude-plugin](https://github.com/manikumarkv/devrunway-claude-plugin) — distributed by [TomeVault](https://tomevault.io).