Jest Config Skill

Jest Config Skill

Purpose

Generate the jest.config.cjs file for unit testing configuration with Jest.

⚠️ CONDITIONAL EXECUTION

This skill should ONLY be used if the project uses Jest instead of Vitest.

Test Framework Detection:

1. Check package.json for test framework dependency:
   • If "vitest" found → SKIP THIS SKILL (use vitest-config skill instead)
   • If "jest" found → EXECUTE THIS SKILL
   • If neither found → PROMPT USER: "Which test framework do you want to use: Jest or Vitest?"

Note: Vitest is the officially recommended test framework for Vue 3 projects as of 2024+. Jest is considered legacy but still widely used.

🚨 MANDATORY FILE COUNT

This skill MUST create exactly 1 file:

• jest.config.cjs (preferred format - CommonJS)

🔍 BEFORE GENERATING - CRITICAL RESEARCH REQUIRED

⚠️ HIGH PRIORITY: Verify current Jest ecosystem to prevent outdated configuration

Required Research Steps:

1. Jest Version: Check current Jest version and configuration format
   • Verify Jest is still maintained and supported
   • Check if jest.config.cjs format is still recommended
2. Vue 3 Jest Transformer: CRITICAL - Check if @vue/​vue3-jest is still maintained
   • Run: npm view @vue/​vue3-jest time
   • Check last publish date and maintenance status
   • If deprecated or unmaintained:
     • STOP and PROMPT USER: "@vue/​vue3-jest appears unmaintained. Vitest is now recommended for Vue 3. Should we continue with Jest or switch to Vitest?"
     • Wait for user decision before proceeding
   • If maintained: Verify current version and compatibility with Jest 29+
3. Coverage Provider: Verify v8 vs babel coverage provider
   • Check if v8 is still recommended over babel
   • Verify compatibility with current Jest version
   • Check performance and accuracy differences
4. TypeScript Support: Verify ts-jest transformer
   • Check if ts-jest is still maintained
   • Verify compatibility with TypeScript and Jest versions
5. ESM Package Detection: Identify packages requiring transformation
   • Check package.json for ESM-only packages (axios, date-fns, etc.)
   • Research which packages need to be in transformIgnorePatterns
   • Common ESM packages: axios, lodash-es, uuid (verify current list)
6. Test Environment: Verify jsdom is still recommended
   • Check if jsdom or node environment is appropriate
   • Verify jsdom version compatibility
7. File Format Options: Check if format is deprecated
   • Verify .cjs format is still preferred
   • Check .js, .mjs, .json, package.json field alternatives

Output

Create the file: jest.config.cjs

Supported Format:

• jest.config.cjs (strict preferred format - CommonJS, most compatible)

Alternative Formats (only if .cjs deprecated):

• jest.config.js (JavaScript, follows package.json type)
• jest.config.mjs (ES Module format)
• jest.config.json (JSON format, limited functionality)

Example File

See: examples.md in this directory for complete examples and detailed explanations.

⚠️ IMPORTANT: The examples.md file contains December 2025 configurations. Always verify current Jest ecosystem before using.

Execution Checklist

• Detect test framework from package.json (Jest vs Vitest)
• If Vitest detected, skip this skill entirely
• If Jest detected, proceed with research
• Research current Jest version and configuration format
• CRITICAL: Verify @vue/​vue3-jest maintenance status
• If @vue/​vue3-jest unmaintained, prompt user for decision
• Verify ts-jest transformer compatibility
• Research coverage provider (v8 vs babel)
• Detect ESM packages in package.json needing transformation
• Verify jsdom test environment compatibility
• Create jest.config.cjs with current standards
• Verify file format is still recommended (not deprecated)

🛑 BLOCKING VALIDATION CHECKPOINT

STOP! Before proceeding to the next skill, verify:

Automated Verification

Run this command to verify the file exists:

if [ -f "jest.config.cjs" ] || [ -f "jest.config.js" ] || [ -f "jest.config.mjs" ] || grep -q "\"jest\":" package.json 2>/​dev/​null; then
  echo "✓ Jest configuration found"
  if [ -f "jest.config.cjs" ]; then
    echo "✓ Using jest.config.cjs (preferred format)"
  fi
  
  # Check for Vitest conflict
  if grep -q "\"vitest\"" package.json 2>/​dev/​null; then
    echo "⚠ WARNING: Both Jest and Vitest detected in package.json"
    echo "  Consider using only one test framework"
  fi
else
  echo "✗ Jest configuration missing"
  exit 1
fi

Manual Verification

1. ✓ jest.config.cjs exists at project root
2. ✓ File uses CommonJS format (module.exports)
3. ✓ File includes preset: 'ts-jest' for TypeScript support
4. ✓ File includes testEnvironment: 'jsdom' for DOM testing
5. ✓ File includes Vue transformer (@vue/​vue3-jest - if maintained)
6. ✓ transformIgnorePatterns includes detected ESM packages
7. ✓ Coverage provider is set (verify v8 or babel)
8. ✓ Module name mapper includes path aliases (@/ → src/)
9. ✓ No Vitest configuration conflict in package.json

If Validation Fails

DO NOT PROCEED to the next skill. Create or fix the missing/​incorrect file immediately.

Notes

• Conditional Execution: Only use if project uses Jest (not Vitest)
• Vitest Preferred: For new Vue 3 projects, Vitest is officially recommended
• Coverage Provider: v8 is faster and more accurate than babel (verify current recommendation)
• Vue Transformer: @vue/​vue3-jest transforms Vue SFC files (check maintenance status)
• TypeScript Support: ts-jest transforms TypeScript files
• ESM Handling: transformIgnorePatterns allows testing ESM packages
• Coverage Exclusions: Excludes infrastructure files (main.ts, router, services, etc.)
• Module Name Mapper: Handles @/ path aliases and asset imports
• Test Environment: jsdom provides DOM APIs for component testing
• Setup Files: Can configure test environment and globals
• Always verify current ecosystem - Jest/​Vue testing landscape evolves
• CommonJS Format: .cjs ensures compatibility with all module systems
• Legacy Status: Jest is legacy for Vue 3; consider migrating to Vitest