Migu Developer Financial Management Jest 30

Jest 30

Version

jest@30.3.0 (from pnpm catalog), ts-jest@29.4.9

Note: client/main uses jest@~29.7.0 directly (not the catalog version) for Expo compatibility.

Critical Patterns

• Use describe / it / expect structure for all tests
• Use jest.mock() at the top of the file for module mocking (London School TDD)
• Use beforeEach to reset mocks: jest.clearAllMocks() or jest.resetAllMocks()
• Use async/await for async tests -- never use done callbacks
• Default moduleFileExtensions now includes .mts and .cts
• Jest 30 uses unrs-resolver for faster module resolution (37% faster, 77% less memory)
• Jest 30 warns about uncleaned globals -- clean up in afterEach
• Use jest.fn<ReturnType, Args>() for typed mock functions
• Use jest.spyOn() for spying on methods without replacing them
• Test files go in tests/ or colocated as *.test.ts next to source

Must NOT Do

• NEVER use snapshot tests excessively -- prefer explicit assertions
• NEVER use done callback for async tests -- use async/await
• NEVER mock what you do not own without a wrapper (wrap third-party APIs)
• NEVER write tests that depend on execution order
• NEVER use jest.setTimeout() globally -- set per test if needed
• NEVER import from jest directly (globals are available)
• NEVER use SpyInstance type -- use jest.SpiedFunction (SpyInstance removed in Jest 30)
• NEVER leave unresolved promises or timers -- Jest 30 detects and warns

Migration from Jest 29 (client/main)

• jest.SpyInstance renamed to jest.SpiedFunction in Jest 30
• expect.addSnapshotSerializer moved to project config
• fakeTimers.legacyFakeTimers removed -- use modern fake timers only
• jest-environment-jsdom upgraded to jsdom 26

Examples

Basic test with mocks (London School)

import { UserService } from '../user.service';
import { UserRepository } from '../user.repository';

jest.mock('../user.repository');

describe('UserService', () => {
  let service: UserService;
  const mockRepo = jest.mocked(new UserRepository());

  beforeEach(() => {
    jest.clearAllMocks();
    service = new UserService(mockRepo);
  });

  it('should return user by id', async () => {
    const expected = { id: '1', name: 'Test' };
    mockRepo.findById.mockResolvedValue(expected);

    const result = await service.getUser('1');

    expect(result).toEqual(expected);
    expect(mockRepo.findById).toHaveBeenCalledWith('1');
  });

  it('should throw when user not found', async () => {
    mockRepo.findById.mockResolvedValue(null);

    await expect(service.getUser('999')).rejects.toThrow('User not found');
  });
});

Typed mock functions

const mockFetch = jest.fn<Promise<Response>, [string, RequestInit?]>();

jest.config.ts pattern (services)

import type { Config } from 'jest';

const config: Config = {
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/*.test.ts'],
  testPathIgnorePatterns: ['/node_modules/', '\\.integration\\.test\\.ts$'],
  transform: {
    '^.+\\.ts$': ['ts-jest', { useESM: false }],
  },
  moduleNameMapper: {
    '^@services/users/(.*)$': '<rootDir>/src/$1',
    '^@packages/models/(.*)$': '<rootDir>/node_modules/@packages/models/src/$1',
  },
  moduleFileExtensions: ['ts', 'js', 'json'],
};

export default config;

Testing async error handling

it('should handle network errors gracefully', async () => {
  mockClient.send.mockRejectedValue(new Error('Network timeout'));

  const result = await service.fetchData();

  expect(result).toEqual({ success: false, error: 'Network timeout' });
});

Source: migu-developer/financial-management — distributed by TomeVault.