About

Imported from personamanagmentlayer/pcl (stdlib/qa/jest-expert/SKILL.md). Install upstream with npx skills add personamanagmentlayer/pcl --skill jest-expert. Copyright stays with the author.

Jest Expert

Name: jest-expert
Author: personamanagmentlayer

Core Concepts

Jest Framework

Zero Config - Works out of the box
Snapshot Testing - UI component testing
Mocking - Functions, modules, and timers
Code Coverage - Built-in coverage reports
Watch Mode - Interactive test runner
Parallel Testing - Fast test execution

Test Structure

describe/it - Test organization
Matchers - Assertion library
Setup/Teardown - beforeEach, afterEach, beforeAll, afterAll
Mock Functions - jest.fn(), jest.spyOn()
Async Testing - Promises, async/await
Test Lifecycle - Hooks and execution order

Advanced Features

Custom Matchers - Extend expect
Module Mocking - Mock entire modules
Timer Mocks - Control time in tests
Manual Mocks - mocks directory
Configuration - jest.config.js
Transformers - Babel, TypeScript support

Best Practices

Test Organization

One test file per source file
Group related tests with describe
Use descriptive test names
Follow AAA pattern (Arrange, Act, Assert)
Keep tests simple and focused
Test behavior, not implementation

Mocking

Mock external dependencies
Use jest.fn() for function mocks
Clear mocks between tests
Mock at the right level
Avoid over-mocking
Document mock behavior

Assertions

Use specific matchers
One assertion per test when possible
Test edge cases
Verify error conditions
Use snapshots judiciously
Provide clear failure messages

Coverage

Aim for high coverage (80%+)
Don't chase 100% coverage
Focus on critical paths
Test edge cases and errors
Exclude generated code
Review coverage reports

Anti-Patterns

Common Mistakes

Testing implementation details
Too many assertions in one test
Sharing state between tests
Not cleaning up after tests
Ignoring async/await
Hard-coding test data

Mocking Issues

Over-mocking everything
Not resetting mocks
Mocking too deep
Inconsistent mock data
Forgetting to restore mocks
Mocking what you don't need

Test Design Problems

Tests depending on execution order
Flaky tests due to timing
No arrange-act-assert structure
Testing multiple things at once
Duplicate test code
Poor test names

Reference Documentation

Detailed material lives alongside this skill and is read on demand:

Implementation Examples — Basic Unit Tests, Testing Async Code, Mocking and Spies, Snapshot Testing, Testing React Components with React Testing Library, Timer Mocks, Custom Matchers, Configuration (jest.config.js)

Resources

Official Documentation

Jest Documentation - Complete guide
API Reference - API docs
Expect Matchers - Assertion reference
Mock Functions - Mocking guide

Learning Resources

Testing JavaScript - Kent C. Dodds course
Jest Crash Course - Video tutorial
React Testing Library - React testing
Jest Cheat Sheet - Quick reference

Tools & Extensions

VS Code Jest Extension - IDE integration
jest-watch-typeahead - Better watch mode
jest-extended - Additional matchers
snapshot-diff - Snapshot comparison

Community Resources

GitHub Jest - Source code and issues
Stack Overflow - Community help
Discord Jest - Community chat
Jest Blog - Updates and articles

Use it

Copy one of these into your project. Installing also returns the manifest and these snippets.

yaml

targets:
  - https://api.opensmartroute.ai/api/v1/registry/personamanagmentlayer-pcl-jest-expert/manifest   # or paste the manifest below

Manifest

An Open Capability Manifest: the router reads it to know what this does, what it costs and when to pick it.

personamanagmentlayer-pcl-jest-expert.ocm.jsonjson

{
  "ocm": "1",
  "id": "personamanagmentlayer-pcl-jest-expert",
  "kind": "skill",
  "name": "jest-expert",
  "description": "Expert in Jest unit testing framework, mocks, snapshots, coverage reports, watch mode, and custom matchers. Use when the user mentions testing, unit testing, JavaScript, QA, mocking, or snapshots, or when the task involves Jest Framework, Test Structure, Advanced Features, or Basic Unit Tests.",
  "publisher": "personamanagmentlayer",
  "version": "1.0.0",
  "capabilities": {
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    "tags": [
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      "jest-matchers"
    ],
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  "examples": [
    "Expert in Jest unit testing framework, mocks, snapshots, coverage reports, watch mode, and custom matchers. Use when the user mentions testing, unit testing, JavaScript, QA, mocking, or snapshots, or when the task involves Jest Framework, Test Structure, Advanced Features, or Basic Unit Tests."
  ],
  "primary": false,
  "metadata": {
    "source": {
      "provider": "skills.sh",
      "repository": "https://github.com/personamanagmentlayer/pcl",
      "path": "stdlib/qa/jest-expert/SKILL.md",
      "ref": "HEAD",
      "url": "https://github.com/personamanagmentlayer/pcl/blob/HEAD/stdlib/qa/jest-expert/SKILL.md",
      "key": "personamanagmentlayer/pcl/stdlib/qa/jest-expert/SKILL.md"
    },
    "allowed_tools": [
      "Read",
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  },
  "instructions": "# Jest Expert\n\n## Core Concepts\n\n### Jest Framework\n\n- **Zero Config** - Works out of the box\n- **Snapshot Testing** - UI component testing\n- **Mocking** - Functions, modules, and timers\n- **Code Coverage** - Built-in coverage reports\n- **Watch Mode** - Interactive test runner\n- **Parallel Testing** - Fast test execution\n\n### Test Structure\n\n- **describe/it** - Test organization\n- **Matchers** - Assertion library\n- **Setup/Teardown** - beforeEach, afterEach, beforeAll, afterAll\n- **Mock Functions** - jest.fn(), jest.spyOn()\n- **Async Testing** - Promises, async/await\n- **Test Lifecycle** - Hoo",
  "cost": {
    "context_tokens": 1013
  }
}

Fetch it by URL: GET /api/v1/registry/personamanagmentlayer-pcl-jest-expert/manifest?version=1.0.0

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