How to test TypeScript projects in Jest? How to configure ts-jest?

Testing TypeScript projects in Jest requires proper configuration and type support:

1. Install Required Dependencies:

bash
npm install --save-dev jest @types/jest ts-jest @types/node

2. Configure Jest:

javascript
// jest.config.js
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/__tests__/**/*.ts', '**/?(*.)+(spec|test).ts'],
  transform: {
    '^.+\\.ts$': 'ts-jest',
  },
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/*.d.ts',
  ],
  moduleFileExtensions: ['ts', 'js', 'json'],
};

3. tsconfig.json Configuration:

json
{
  "compilerOptions": {
    "types": ["jest", "node"],
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true
  }
}

4. Writing TypeScript Tests:

typescript
// calculator.ts
export function add(a: number, b: number): number {
  return a + b;
}

// calculator.test.ts
import { add } from './calculator';

describe('Calculator', () => {
  test('adds two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });
});

5. Testing Types:

typescript
interface User {
  id: number;
  name: string;
}

function getUser(id: number): User {
  return { id, name: 'John' };
}

test('returns user with correct type', () => {
  const user: User = getUser(1);
  expect(user).toEqual({ id: 1, name: 'John' });
});

6. Mock TypeScript Modules:

typescript
// api.ts
export interface ApiResponse<T> {
  data: T;
  error: string | null;
}

export async function fetchData<T>(url: string): Promise<ApiResponse<T>> {
  const response = await fetch(url);
  const data = await response.json();
  return { data, error: null };
}

// api.test.ts
import { fetchData } from './api';

jest.mock('./api', () => ({
  fetchData: jest.fn(),
}));

test('fetches data', async () => {
  const mockData = { name: 'John' };
  (fetchData as jest.Mock).mockResolvedValue({
    data: mockData,
    error: null,
  });
  
  const result = await fetchData('/api/users');
  expect(result.data).toEqual(mockData);
});

Best Practices:

• Use ts-jest preset to simplify configuration
• Ensure @types/jest is installed
• Use TypeScript types to enhance test readability
• Explicitly declare types in tests
• Use type-safe mocks
• Configure proper path aliases