jest-configuration

# Jest Configuration

Master Jest configuration, setup files, module resolution, and project organization for optimal testing environments. This skill covers all aspects of configuring Jest for modern JavaScript and TypeScript projects, from basic setup to advanced multi-project configurations.

## Installation and Setup

### Basic Installation

```bash
# npm
npm install --save-dev jest

# yarn
yarn add --dev jest

# pnpm
pnpm add -D jest
```

### TypeScript Support

```bash
npm install --save-dev @types/jest ts-jest
```

### React Testing Libraries

```bash
npm install --save-dev @testing-library/react @testing-library/jest-dom
```

## Configuration Files

### jest.config.js (Recommended)

```javascript
/** @type {import('jest').Config} */
module.exports = {
  // Test environment
  testEnvironment: 'node', // or 'jsdom' for browser-like environment

  // Root directory for tests
  roots: ['<rootDir>/src'],

  // File extensions to consider
  moduleFileExtensions: ['js', 'jsx', 'ts', 'tsx', 'json'],

  // Test match patterns
  testMatch: [
    '**/__tests__/**/*.[jt]s?(x)',
    '**/?(*.)+(spec|test).[jt]s?(x)'
  ],

  // Transform files before testing
  transform: {
    '^.+\\.tsx?$': 'ts-jest',
    '^.+\\.jsx?$': 'babel-jest'
  },

  // Coverage configuration
  collectCoverageFrom: [
    'src/**/*.{js,jsx,ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/*.stories.{js,jsx,ts,tsx}',
    '!src/**/__tests__/**'
  ],

  // Coverage thresholds
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80
    }
  },

  // Setup files
  setupFilesAfterEnv: ['<rootDir>/jest.setup.js'],

  // Module name mapper for imports
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
    '\\.(css|less|scss|sass)$': 'identity-obj-proxy',
    '\\.(jpg|jpeg|png|gif|svg)$': '<rootDir>/__mocks__/fileMock.js'
  },

  // Clear mocks between tests
  clearMocks: true,

  // Restore mocks between tests
  restoreMocks: true,

  // Verbose o