How to generate and configure code coverage reports in Jest? What are the coverage metrics?

Jest provides powerful code coverage reporting to help developers understand test coverage:

Generating Coverage Reports:

bash
# Run tests and generate coverage report
jest --coverage

# Or configure in package.json
{
  "scripts": {
    "test:coverage": "jest --coverage"
  }
}

Coverage Metrics:

• Statements: Percentage of executed code statements
• Branches: Percentage of executed conditional branches
• Functions: Percentage of called functions
• Lines: Percentage of executed code lines

Configuring Coverage:

javascript
// jest.config.js
module.exports = {
  collectCoverage: true,
  collectCoverageFrom: [
    'src/**/*.{js,jsx,ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/*.stories.{js,jsx,ts,tsx}',
    '!src/index.{js,jsx,ts,tsx}'
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80
    }
  },
  coverageReporters: ['text', 'lcov', 'html']
};

Coverage Report Formats:

• text: Command line text output
• lcov: Generates lcov.info file for CI/CD tools
• html: Generates HTML report for visual inspection
• json: JSON format report

Best Practices:

• Set reasonable coverage thresholds (usually 80%)
• Exclude files that don't need testing (config files, type definitions)
• Integrate coverage checks in CI/CD
• Regularly review uncovered code
• Focus on core business logic coverage