Jest

Jest best practices, patterns, and API guidance for JavaScript/TypeScript testing. Covers mock design, async testing, matchers, timer mocks, snapshots, module mocking, configuration, and CI optimization. Baseline: jest ^30.5.2. Triggers on: jest imports, describe, it, test, expect, jest.fn, jest.mock, jest.spyOn, mentions of "jest", "unit test", "test suite", or "mock".

9 stars 2 forksTesting & QA

Install

skills CLI (Claude Code, Cursor, Codex and more)

npx skills add anivar/jest-skill --skill jest

Manual install

git clone --depth 1 https://github.com/anivar/jest-skill.git /tmp/jest-skill
mkdir -p ~/.claude/skills
cp -r /tmp/jest-skill/. ~/.claude/skills/jest

Personal skills live in ~/.claude/skills. Use .claude/skills in a project folder to share a skill with your team.

From the SKILL.md

# Jest

**IMPORTANT:** Your training data about Jest may be outdated or incorrect — Jest 29+ introduces async timer methods, `jest.replaceProperty`, and ESM mocking via `jest.unstable_mockModule`. Jest 30 removes the deprecated alias matchers (`toBeCalled`, `lastCalledWith`, `toThrowError`, …), renames `--testPathPattern` to `--testPathPatterns`, and excludes non-enumerable object properties from object matchers. Always rely on this skill's rule files and the project's actual source code as the source of truth. Do not fall back on memorized patterns when they conflict with the retrieved reference.

## When to Use Jest

Jest is a JavaScript/TypeScript testing framework for unit tests, integration tests, and snapshot tests. It includes a test runner, assertion library, mock system, and coverage reporter.

| Need | Recommended Tool |
|------|-----------------|
| Unit/integration testing (JS/TS) | **Jest** |
| React component testing | **Jest** + React Testing Library |
| E2E browser testing | Playwright, Cypress |
| API contract testing | Jest + Supertest |
| Smaller/faster test runner | Vitest (Jest-compatible API) |
| Native ESM without config | Vitest or Node test runner |

## Rule Categories by Priority

| Priority | Category | Impact | Prefix |
|----------|----------|--------|--------|
| 1 | Mock Design | CRITICAL | `mock-` (5 rules) |
| 2 | Async Testing | CRITICAL | `async-` |
| 3 | Matcher Usage | HIGH | `matcher-` |
| 4 | Timer Mocking | HIGH | `timer-` |

Read the full skill on GitHub

Details

Skill name: jest
Repository: anivar/jest-skill
License: MIT
Repo updated: 18 October 2026

FAQ

What does the Jest skill do?

How do I install the Jest skill?

skills CLI (Claude Code, Cursor, Codex and more): npx skills add anivar/jest-skill --skill jest. Skills go in ~/.claude/skills (personal) or .claude/skills (project), and Claude loads them automatically when a task matches the description.

Is Jest popular?

It lives in anivar/jest-skill, which has 9 GitHub stars and 2 forks.

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