Jest Skill

Jest Skill

An AI agent skill for writing, debugging, and reviewing Jest tests with modern best practices.

Created by Anivar Aravind

The Problem

AI agents frequently generate outdated or incorrect Jest patterns — wrong mock cleanup semantics (clearAllMocks vs restoreAllMocks), missing await on async assertions, incorrect timer mock usage, factory hoisting bugs, and anti-patterns that cause tests to silently pass without actually asserting anything. These produce test suites that compile and pass but don't test what they claim to test.

This Solution

28 rules with incorrect→correct code examples that teach agents Jest's actual API behavior, mock lifecycle, async patterns, timer control, module mocking, snapshot testing, configuration, and CI optimization. Each rule targets a specific mistake and shows exactly how to fix it.

Install

npx skills add anivar/jest-skill -g

Or with full URL:

npx skills add https://github.com/anivar/jest-skill

Baseline

• jest ^29.0.0 / ^30.0.0
• JavaScript / TypeScript

What's Inside

28 Rules Across 9 Categories

| Priority | Category | Rules | Impact | |----------|----------|-------|--------| | 1 | Mock Design | 5 | CRITICAL | | 2 | Async Testing | 3 | CRITICAL | | 3 | Matcher Usage | 3 | HIGH | | 4 | Timer Mocking | 3 | HIGH | | 5 | Test Structure | 3 | HIGH | | 6 | Module Mocking | 3 | MEDIUM | | 7 | Snapshot Testing | 3 | MEDIUM | | 8 | Configuration | 3 | MEDIUM | | 9 | Performance & CI | 2 | MEDIUM |

Each rule file contains:

• Why it matters
• Incorrect code with explanation
• Correct code with explanation
• Decision tables and additional context

9 Deep-Dive References

| Reference | Covers | |-----------|--------| | matchers.md | All matchers: equality, truthiness, numbers, strings, arrays, objects, asymmetric, custom | | mock-functions.md | jest.fn, jest.spyOn, .mock property, return values, implementations, mock matchers | | jest-object.md | jest.mock, jest.useFakeTimers, jest.setTimeout, jest.retryTimes, jest.replaceProperty | | async-patterns.md | Promises, async/await, done callbacks, .resolves/.rejects, expect.assertions | | configuration.md | Key config options: testMatch, transform, moduleNameMapper, coverage, environments | | snapshot-testing.md | toMatchSnapshot, inline snapshots, property matchers, serializers, updating | | module-mocking.md | Manual mocks, __mocks__ directory, ESM mocking, partial mocking, jest.requireActual | | anti-patterns.md | 15 common mistakes with BAD/GOOD examples | | ci-and-debugging.md | CI optimization, --runInBand, --shard, debugging with inspector, troubleshooting |

Structure

├── SKILL.md                          # Entry point for AI agents
├── AGENTS.md                         # Compiled guide with all rules expanded
├── rules/                            # Individual rules (Incorrect→Correct)
│   ├── mock-*                        # Mock design (CRITICAL)
│   ├── async-*                       # Async testing (CRITICAL)
│   ├── matcher-*                     # Matcher usage (HIGH)
│   ├── timer-*                       # Timer mocking (HIGH)
│   ├── structure-*                   # Test structure (HIGH)
│   ├── module-*                      # Module mocking (MEDIUM)
│   ├── snapshot-*                    # Snapshot testing (MEDIUM)
│   ├── config-*                      # Configuration (MEDIUM)
│   └── perf-*                        # Performance & CI (MEDIUM)
└── references/                       # Deep-dive reference docs
    ├── matchers.md
    ├── mock-functions.md
    ├── jest-object.md
    ├── async-patterns.md
    ├── configuration.md
    ├── snapshot-testing.md
    ├── module-mocking.md
    ├── anti-patterns.md
    └── ci-and-debugging.md

Ecosystem — Skills by Anivar Aravind

This Jest skill covers Jest's own API surface — the foundation layer. For framework-specific testing patterns built on top of Jest, use companion skills:

Testing Skills (Jest Foundation)

| Need | Skill | Install | |------|-------|---------| | API mocking (network-level) | msw-skill — MSW 2.0 handlers, server lifecycle, per-test overrides | npx skills add anivar/msw-skill -g | | React Native components | react-native-testing — RNTL v13/v14 queries, userEvent, async render | npx skills add anivar/react-native-testing -g | | Zod schema validation | zod-testing — safeParse testing, mock data, property-based testing | npx skills add anivar/zod-testing -g | | Redux-Saga side effects | redux-saga-testing — expectSaga, testSaga, providers | npx skills add anivar/redux-saga-testing -g |

Library & Framework Skills

| Skill | What it covers | Install | |-------|---------------|---------| | zod-skill | Zod v4 schema validation, parsing, error handling, type inference | npx skills add anivar/zod-skill -g | | redux-saga-skill | Redux-Saga effects, fork model, channels, RTK integration | npx skills add anivar/redux-saga-skill -g | | msw-skill | MSW 2.0 API mocking — handlers, responses, GraphQL, v1→v2 migration | npx skills add anivar/msw-skill -g |

Engineering Analysis Skills

| Skill | What it covers | Install | |-------|---------------|---------| | contributor-codebase-analyzer | Deep-dive code analysis, annual reviews, promotion readiness | npx skills add anivar/contributor-codebase-analyzer -g |

Author

Anivar Aravind — Building AI agent skills for modern JavaScript/TypeScript development.

License

MIT

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: anivar
• Source: anivar/jest-skill
• License: MIT
• Homepage: https://skills.sh/anivar/jest-skill

Install and usage instructions live in the source repository linked above.