Testing Unit Test Generation Jest

Skill: Unit Test Generation (Jest)

Purpose

To create robust unit tests for individual functions, classes, or modules to ensure they behave as expected in isolation.

When to Use

• When writing new business logic or utility functions.
• When fixing a bug (write a failing test first).
• When refactoring code to ensure no regressions.

Procedure

1. Setup Jest

Ensure Jest is configured for your project (e.g., in package.json or jest.config.js).

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

2. Implementation: Basic Unit Test

Test a pure function.

// math.ts
export const add = (a: number, b: number) => a + b;

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

describe('add function', () => {
  it('should correctly add two positive numbers', () => {
    expect(add(2, 3)).toBe(5);
  });
});

3. Implementation: Mocking Dependencies

Use jest.mock() to isolate the unit under test.

// userService.test.ts
import { getUser } from './userService';
import { db } from './db';

jest.mock('./db'); // Mock the database module

describe('getUser', () => {
  it('should return a user if found in DB', async () => {
    (db.findUser as jest.Mock).mockResolvedValue({ id: 1, name: 'Alice' });
    
    const user = await getUser(1);
    expect(user.name).toBe('Alice');
    expect(db.findUser).toHaveBeenCalledWith(1);
  });
});

4. Testing Asynchronous Code

Use async/await in your test cases.

it('should throw error if API fails', async () => {
  api.getData.mockRejectedValue(new Error('Network error'));
  await expect(fetchData()).rejects.toThrow('Network error');
});

5. Running with Coverage

Verify how much of your code is tested.

npm test -- --coverage

Constraints

• Isolate: Never call real databases, file systems, or external APIs.
• Speed: Unit tests should be extremely fast (hundreds per second).
• Descriptive Names: Use clear describe and it blocks (e.g., it('should return 400 when email is invalid')).

Expected Output

A high-coverage test suite that gives developers confidence when refactoring or adding new features.

Source: Ditto190/crispy-nextjs-turborepo-monorepo — distributed by TomeVault.