How to optimize Jest test performance? What methods can improve test speed?

Optimizing Jest test performance can improve development efficiency and CI/CD speed:

1. Parallel Test Execution: Jest runs tests in parallel by default, significantly improving speed. Ensure tests are independent.

2. Selective Test Running:

bash
# Run tests only for changed files
jest --onlyChanged

# Run tests matching pattern
jest --testPathPattern=auth

# Use .only to temporarily run specific tests
test.only('specific test', () => { /* ... */ });

3. Optimize Test Environment:

javascript
// jest.config.js
module.exports = {
  testEnvironment: 'node', // Faster than jsdom
  maxWorkers: '50%', // Limit worker processes
  testTimeout: 5000, // Set reasonable timeout
};

4. Use Mocks to Reduce External Dependencies:

javascript
jest.mock('./api', () => ({
  fetchData: jest.fn(() => Promise.resolve('mocked data'))
}));

5. Optimize Coverage Collection:

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

6. Cache Optimization:

• Jest automatically caches transformation results
• Use --no-cache to disable cache (for debugging)
• Clear cache: jest --clearCache

7. Reduce Unnecessary Tests:

• Avoid over-testing
• Skip slow tests: test.skip('slow test', () => { /* ... */ });
• Use describe.skip to skip entire test suites

8. Watch Mode Optimization:

bash
# Use --watchAll to watch all files
jest --watchAll

# Use --watch to watch only related files
jest --watch

Best Practices:

• Keep tests fast and independent
• Use mocks to isolate external dependencies
• Regularly review and optimize slow tests
• Use --maxWorkers in CI to control resource usage