jest

> Base Jest configuration for jmlweb projects. TypeScript support, coverage settings, and sensible defaults out of the box.

✨ Features

• 🔧 TypeScript Support: Configured for TypeScript projects with ts-jest
• 📊 Coverage Configuration: Pre-configured with 80% coverage thresholds
• 🎯 Sensible Defaults: Node.js environment, optimized test execution, clear mocks
• ⚡ Performance Optimized: Efficient test execution with proper module isolation
• 🚀 Easy Extension: Factory function for easy customization
• 📦 Zero Config: Works out of the box with minimal setup

📦 Installation

pnpm add -D @jmlweb/jest-config jest ts-jest @types/jest

> 💡 Upgrading from a previous version? See the Migration Guide for breaking changes and upgrade instructions.

🚀 Quick Start

Create a jest.config.ts file in your project root:

import jestConfig from '@jmlweb/jest-config';

export default jestConfig;

Or use the factory function for customization:

import { createJestConfig } from '@jmlweb/jest-config';

export default createJestConfig({
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
});

💡 Examples

Basic Setup

jest.config.ts

import jestConfig from '@jmlweb/jest-config';

export default jestConfig;

With Project-Specific Overrides

jest.config.ts

import { createJestConfig } from '@jmlweb/jest-config';

export default createJestConfig({
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
  testTimeout: 10000,
  coverageThreshold: {
    global: {
      lines: 90,
      functions: 90,
      branches: 85,
      statements: 90,
    },
  },
});

React/JSX Project Example

jest.config.ts

import { createJestConfig } from '@jmlweb/jest-config';

export default createJestConfig({
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
  },
});

Lower Coverage Thresholds

jest.config.ts

import { createJestConfig } from '@jmlweb/jest-config';

export default createJestConfig({
  coverageThreshold: {
    global: {
      lines: 60,
      functions: 60,
      branches: 60,
      statements: 60,
    },
  },
});

Custom Test Patterns

jest.config.ts

import { createJestConfig } from '@jmlweb/jest-config';

export default createJestConfig({
  testMatch: ['**/__tests__/**/*.ts', '**/*.test.ts'],
  collectCoverageFrom: ['src/**/*.ts', '!src/**/*.d.ts'],
});

🤔 Why Use This?

> Philosophy: Tests should run reliably with meaningful coverage metrics that enforce code quality without being overly strict.

This package provides a Jest configuration optimized for TypeScript projects with sensible coverage thresholds and test isolation. It balances strictness with practicality, enforcing coverage standards while keeping tests fast and maintainable.

Design Decisions

80% Coverage Thresholds: Requires 80% coverage across all metrics

• Why: 80% coverage strikes a good balance - it’s high enough to ensure most code paths are tested but not so high that you waste time testing trivial code or struggle to reach unrealistic goals. It catches most bugs while allowing flexibility for edge cases that are hard to test
• Trade-off: Some projects may want higher coverage (90%+) or lower (60%). This is easily adjustable per project
• When to override: Increase for critical systems (financial, healthcare), decrease for experimental projects or when initially adding tests to legacy code

Automatic Mock Clearing (clearMocks: true, restoreMocks: true): Resets mocks between tests

• Why: Test isolation is crucial for reliable tests. Automatically clearing and restoring mocks prevents test interdependence and the dreaded “works in isolation but fails in suite” problem. This eliminates an entire class of flaky tests
• Trade-off: Can’t rely on mock state persisting between tests. But tests shouldn’t share state anyway
• When to override: Rarely needed - test isolation is a best practice

Node Environment by Default (testEnvironment: 'node'): Server-side testing environment

• Why: Most TypeScript projects tested with Jest are Node.js backends or libraries. The Node environment is lighter and faster than jsdom. For React/browser code, developers can override to jsdom per project
• Trade-off: Need to explicitly set jsdom for browser/React testing
• When to override: For React components or browser-specific code - set testEnvironment: 'jsdom' in your project config

5 Second Test Timeout: Reasonable default for most tests

• Why: Most unit tests should complete in milliseconds. 5 seconds is generous for integration tests while preventing truly hung tests from blocking CI indefinitely. It catches accidentally synchronous code or missing mocks
• Trade-off: Some integration tests with real databases might need longer timeouts
• When to override: For slow integration tests - increase per test suite or project-wide

📋 Configuration Details

Default Settings

Setting	Value	Description
testEnvironment	node	Node.js environment by default
moduleFileExtensions	['ts', 'tsx', 'js', 'jsx', 'json']	Supported file extensions
testMatch	['**/*.{test,spec}.{ts,tsx,js,jsx}']	Test file patterns
testTimeout	5000	Test timeout in milliseconds (5 seconds)
coverageThreshold.global.lines	80	Minimum line coverage
coverageThreshold.global.functions	80	Minimum function coverage
coverageThreshold.global.branches	80	Minimum branch coverage
coverageThreshold.global.statements	80	Minimum statement coverage
verbose	true	Detailed test output
clearMocks	true	Clear mocks between tests
restoreMocks	true	Restore mocks after each test

Coverage Exclusions

The following patterns are excluded from coverage by default:

• **/*.d.ts - TypeScript declaration files
• **/*.config.{ts,js} - Configuration files
• **/dist/** - Build output
• **/build/** - Build directories
• **/node_modules/** - Dependencies
• **/coverage/** - Coverage reports
• **/*.test.{ts,tsx,js,jsx} - Test files
• **/*.spec.{ts,tsx,js,jsx} - Spec files

Test File Patterns

Tests are automatically discovered from:

• **/*.test.{ts,tsx,js,jsx}
• **/*.spec.{ts,tsx,js,jsx}

Coverage Reporters

• text - Text summary in terminal
• json - JSON format for CI/CD integration
• html - HTML coverage report (generated in coverage/ directory)

🎯 When to Use

Use this configuration when you want:

• ✅ Consistent testing setup across projects using Jest
• ✅ TypeScript support out of the box
• ✅ Coverage thresholds enforced
• ✅ Sensible defaults for Node.js and browser projects
• ✅ Easy customization and extension

For projects using Vitest, use @jmlweb/vitest-config instead.

🔧 Extending the Configuration

You can extend or override the configuration for your specific needs:

import { createJestConfig } from '@jmlweb/jest-config';

export default createJestConfig({
  // Add custom setup files
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],

  // Custom test patterns
  testMatch: ['**/*.test.ts', '**/__tests__/**/*.ts'],

  // Custom environment
  testEnvironment: 'jsdom',

  // Path aliases
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
    '^~/(.*)$': '<rootDir>/src/$1',
  },
});

📝 Usage with Scripts

Add test scripts to your package.json:

{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage",
    "test:ci": "jest --ci --coverage --maxWorkers=2"
  }
}

Then run:

pnpm test           # Run tests once
pnpm test:watch     # Run tests in watch mode
pnpm test:coverage  # Run tests with coverage
pnpm test:ci        # Run tests in CI mode

TypeScript Configuration

The configuration uses ts-jest for TypeScript support. The default transform configuration includes:

• esModuleInterop: true - Allows default imports from CommonJS modules
• allowSyntheticDefaultImports: true - Allows default imports from modules with no default export

These settings ensure compatibility with modern TypeScript projects.

📋 Requirements

• Node.js >= 20.11.0
• Jest >= 29.0.0
• ts-jest >= 29.0.0
• @types/jest >= 29.0.0
• TypeScript project (recommended, but not required)

📦 Peer Dependencies

This package requires the following peer dependencies:

• jest (>=29.0.0)
• ts-jest (>=29.0.0)
• @types/jest (>=29.0.0)

🔗 Related Packages

Internal Packages

• @jmlweb/vitest-config - Vitest configuration for modern testing
• @jmlweb/eslint-config-base - ESLint config for TypeScript projects
• @jmlweb/prettier-config-base - Prettier config for consistent formatting
• @jmlweb/tsconfig-base - TypeScript configuration

External Tools

• Jest - Delightful JavaScript testing framework
• ts-jest - TypeScript preprocessor for Jest
• @testing-library/jest-dom - Custom Jest matchers for the DOM
• @testing-library/react - React testing utilities (for React projects)

🔄 Migration Guide

Upgrading to a New Version

> Note: If no breaking changes were introduced in a version, it’s safe to upgrade without additional steps.

No breaking changes have been introduced yet. This package follows semantic versioning. When breaking changes are introduced, detailed migration instructions will be provided here.

For version history, see the Changelog.

Need Help? If you encounter issues during migration, please open an issue.

📜 Changelog

See CHANGELOG.md for version history and release notes.

📄 License

MIT