Test Isolation: Preventing State Leaks Between Tests

Tests pass individually but fail when run together. One test's side effects (global state, mocks, timers, DOM nodes) contaminate subsequent tests.

Reset all shared state after each test using framework hooks.

// Reset mocks after each test
afterEach(() => {
  jest.clearAllMocks();  // Clear calls and return values
  // jest.resetAllMocks() — also removes implementations
  // jest.restoreAllMocks() — restores spied-on originals
});

// Reset timers
beforeEach(() => { jest.useFakeTimers(); });
afterEach(() => { jest.useRealTimers(); });

// React: automatic DOM cleanup
import '@testing-library/react'; // auto-cleanup on import
// Or manually:
import { cleanup } from '@testing-library/react';
afterEach(cleanup);

// Database: use transactions rolled back after each test
beforeEach(async () => {
  await db.beginTransaction();
});
afterEach(async () => {
  await db.rollbackTransaction();
});

// Module-level singletons: reset in beforeEach
beforeEach(() => {
  jest.isolateModules(() => {
    // Fresh module imports for each test
  });
});

Test order should be irrelevant. Shared mutable state (module-level variables, global fetch mocks, DOM nodes) persist across tests unless explicitly cleaned up.