Test Generator

Overview

Automatically generate comprehensive tests for existing source code. This skill analyzes functions, classes, and modules to produce well-structured test suites with meaningful assertions, edge case coverage, and proper mocking. It supports multiple frameworks and test types (unit, integration, e2e).

Instructions

When a user asks you to generate tests for their code, follow these steps:

Step 1: Analyze the target code

Read the source file(s) and identify:

• Functions/methods to test, their signatures, parameters, and return types
• Dependencies that need mocking (database calls, API requests, file I/O)
• Edge cases — null inputs, empty arrays, boundary values, error paths
• Side effects — state mutations, event emissions, DOM changes

Step 2: Detect the testing framework

Check the project for existing test setup:

# Check package.json for JS/TS projects
cat package.json | grep -E "jest|vitest|playwright|cypress|mocha|testing-library"

# Check for Python test frameworks
cat requirements.txt pyproject.toml setup.cfg 2>/dev/null | grep -E "pytest|unittest|hypothesis"

# Check for existing test config
ls jest.config* vitest.config* pytest.ini conftest.py .pytest.ini setup.cfg 2>/dev/null

If no framework is detected, recommend based on the project:

• React/Next.js: Vitest + React Testing Library
• Node.js/Express: Jest or Vitest
• Python: Pytest
• E2E/browser: Playwright

Step 3: Generate the test file

Create tests following the framework conventions:

Naming: Place test files next to source files or in a __tests__/tests directory matching the project convention. Use *.test.ts, *.spec.ts, test_*.py, or *_test.py.

Structure each test with:

1. Arrange — set up inputs, mocks, and preconditions
2. Act — call the function or perform the action
3. Assert — verify the output, side effects, or state changes

For each function, generate tests covering:

• Happy path with typical inputs
• Edge cases (empty input, null, zero, max values)
• Error handling (invalid input, thrown exceptions)
• Boundary conditions

Step 4: Set up mocks and fixtures

Mock external dependencies to isolate the unit under test:

JavaScript/TypeScript (Jest/Vitest):

import { describe, it, expect, vi, beforeEach } from 'vitest';
import { processOrder } from './order-service';
import { db } from './database';
import { sendEmail } from './email-service';

vi.mock('./database');
vi.mock('./email-service');

describe('processOrder', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('should save the order and send confirmation email', async () => {
    const mockOrder = { id: 'ord_123', items: [{ sku: 'WIDGET-A', qty: 2 }], total: 49.98 };
    vi.mocked(db.orders.create).mockResolvedValue(mockOrder);
    vi.mocked(sendEmail).mockResolvedValue({ delivered: true });

    const result = await processOrder({ items: [{ sku: 'WIDGET-A', qty: 2 }] });

    expect(db.orders.create).toHaveBeenCalledOnce();
    expect(sendEmail).toHaveBeenCalledWith(expect.objectContaining({ orderId: 'ord_123' }));
    expect(result.id).toBe('ord_123');
  });
});

Python (Pytest):

import pytest
from unittest.mock import patch, MagicMock
from order_service import process_order

@pytest.fixture
def mock_db():
    with patch('order_service.db') as mock:
        mock.orders.create.return_value = {"id": "ord_123", "total": 49.98}
        yield mock

def test_process_order_saves_and_sends_email(mock_db):
    result = process_order(items=[{"sku": "WIDGET-A", "qty": 2}])
    mock_db.orders.create.assert_called_once()
    assert result["id"] == "ord_123"

def test_process_order_rejects_empty_items():
    with pytest.raises(ValueError, match="at least one item"):
        process_order(items=[])

Step 5: Add integration and e2e tests if requested

Integration tests — test multiple modules together with real (or test) databases:

• Use test containers or in-memory databases
• Test API routes end-to-end through the HTTP layer
• Verify database state after operations

E2E tests (Playwright):

import { test, expect } from '@playwright/test';

test('user can add item to cart and checkout', async ({ page }) => {
  await page.goto('/products');
  await page.click('[data-testid="product-widget-a"]');
  await page.click('button:has-text("Add to Cart")');

  await page.goto('/cart');
  await expect(page.locator('.cart-item')).toHaveCount(1);
  await expect(page.locator('.cart-total')).toContainText('$24.99');

  await page.click('button:has-text("Checkout")');
  await expect(page).toHaveURL(/\/checkout/);
});

Step 6: Verify and report coverage

Run the tests and report results:

# JavaScript
npx vitest run --coverage
# Python
pytest --cov=src --cov-report=term-missing

Report: total tests, pass/fail, and coverage percentage. Suggest additional tests if coverage is below 80%.

Examples

Example 1: Generate unit tests for a utility module

User request: "Write tests for src/utils/string-helpers.ts"

Source code analyzed:

export function slugify(text: string): string {
  return text.toLowerCase().replace(/\s+/g, '-').replace(/[^a-z0-9-]/g, '');
}
export function truncate(text: string, maxLength: number): string {
  if (text.length <= maxLength) return text;
  return text.slice(0, maxLength - 3) + '...';
}

Generated test file (src/utils/string-helpers.test.ts):

import { describe, it, expect } from 'vitest';
import { slugify, truncate } from './string-helpers';

describe('slugify', () => {
  it('converts spaces to hyphens', () => {
    expect(slugify('Hello World')).toBe('hello-world');
  });
  it('removes special characters', () => {
    expect(slugify('Price: $99.99!')).toBe('price-9999');
  });
  it('handles empty string', () => {
    expect(slugify('')).toBe('');
  });
  it('collapses multiple spaces', () => {
    expect(slugify('too   many   spaces')).toBe('too-many-spaces');
  });
});

describe('truncate', () => {
  it('returns original if within limit', () => {
    expect(truncate('short', 10)).toBe('short');
  });
  it('truncates and adds ellipsis', () => {
    expect(truncate('This is a very long sentence', 15)).toBe('This is a ve...');
  });
  it('handles exact length', () => {
    expect(truncate('exact', 5)).toBe('exact');
  });
});

Result: 7 tests generated, all passing. Coverage: 100% for string-helpers.ts.

Example 2: Generate Pytest tests for a Flask API endpoint

User request: "Add tests for the /api/users endpoint in app.py"

Source code analyzed:

@app.route('/api/users', methods=['POST'])
def create_user():
    data = request.get_json()
    if not data.get('email'):
        return jsonify({"error": "Email is required"}), 400
    user = User(email=data['email'], name=data.get('name', ''))
    db.session.add(user)
    db.session.commit()
    return jsonify({"id": user.id, "email": user.email}), 201

Generated test file (tests/test_users_api.py):

import pytest
from app import app, db

@pytest.fixture
def client():
    app.config['TESTING'] = True
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
    with app.test_client() as client:
        with app.app_context():
            db.create_all()
        yield client

def test_create_user_success(client):
    response = client.post('/api/users', json={"email": "alice@example.com", "name": "Alice"})
    assert response.status_code == 201
    data = response.get_json()
    assert data["email"] == "alice@example.com"
    assert "id" in data

def test_create_user_missing_email(client):
    response = client.post('/api/users', json={"name": "Bob"})
    assert response.status_code == 400
    assert "Email is required" in response.get_json()["error"]

def test_create_user_empty_body(client):
    response = client.post('/api/users', json={})
    assert response.status_code == 400

Result: 3 tests generated, all passing. Covers success, validation error, and empty input.

Guidelines

• Always match the existing test framework and style in the project. Do not introduce Jest if the project uses Vitest.
• Place test files following the project's existing convention (co-located vs. tests/ directory).
• Use descriptive test names that explain the expected behavior, not the implementation.
• Mock external dependencies (databases, APIs, file system) but avoid over-mocking — test real logic.
• For each function, aim for at least: one happy path, one error case, one edge case.
• Generate beforeEach/afterEach setup when tests share common fixtures.
• Do not generate snapshot tests unless explicitly requested — they are brittle and rarely useful.
• When generating e2e tests, use stable selectors (data-testid) over CSS classes.
• If the project has no testing setup, install the framework and create the config file before writing tests.
• Report test count and coverage after generation so the user knows the current state.