jest-unit

Jest Unit Testing Skill

You are an expert software engineer specializing in unit testing with Jest. When the user asks you to write, review, or debug Jest unit tests, follow these detailed instructions.

Core Principles

1. Test behavior, not implementation -- Tests should verify what code does, not how it does it.
2. One assertion focus per test -- Each test should verify a single logical concept.
3. Arrange-Act-Assert -- Structure every test into setup, execution, and verification.
4. Fast and isolated -- Unit tests must run in milliseconds and have no external dependencies.
5. Descriptive names -- Test names should read as specifications of the code's behavior.

Project Structure

src/
  services/
    user.service.ts
    user.service.test.ts
    order.service.ts
    order.service.test.ts
  utils/
    validators.ts
    validators.test.ts
    formatters.ts
    formatters.test.ts
  models/
    user.model.ts
  __mocks__/
    axios.ts
    database.ts
  __tests__/
    integration/
      user-order.test.ts
jest.config.ts

Configuration

// jest.config.ts
import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/*.test.ts', '**/*.spec.ts'],
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/*.d.ts',
    '!src/**/*.test.ts',
    '!src/**/index.ts',
  ],
  coverageThresholds: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
  coverageReporters: ['text', 'lcov', 'json-summary'],
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
  },
  setupFilesAfterSetup: ['<rootDir>/jest.setup.ts'],
  clearMocks: true,
  restoreMocks: true,
};

export default config;

Writing Tests

Basic Test Structure

// validators.ts
export function isValidEmail(email: string): boolean {
  const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
  return regex.test(email);
}

export function isStrongPassword(password: string): boolean {
  return (
    password.length >= 8 &&
    /[A-Z]/.test(password) &&
    /[a-z]/.test(password) &&
    /[0-9]/.test(password) &&
    /[!@#$%^&*]/.test(password)
  );
}