test-coverage-improver

name	test-coverage-improver
description	Analyze test coverage gaps and generate tests to improve coverage. Use when improving test coverage, finding untested code, or writing missing tests.

Test Coverage Improver

Instructions

When improving test coverage:

1. Run coverage report to identify gaps
2. Prioritize critical/complex code paths
3. Write tests for uncovered code
4. Verify coverage improved

Generate Coverage Report

# Jest
npx jest --coverage

# Vitest
npx vitest --coverage

# NYC (Istanbul) for any test runner
npx nyc npm test

# View HTML report
open coverage/lcov-report/index.html

Coverage Targets

Type	Minimum	Good	Excellent
Lines	70%	80%	90%+
Branches	60%	75%	85%+
Functions	70%	80%	90%+
Statements	70%	80%	90%+

Test Templates

Unit Test (Jest/Vitest)

import { describe, it, expect, vi, beforeEach } from 'vitest';
import { calculateTotal, formatCurrency } from './utils';

describe('calculateTotal', () => {
  it('should sum all item prices', () => {
    const items = [
      { price: 10, quantity: 2 },
      { price: 5, quantity: 1 },
    ];
    expect(calculateTotal(items)).toBe(25);
  });

  it('should return 0 for empty array', () => {
    expect(calculateTotal([])).toBe(0);
  });

  it('should handle decimal prices', () => {
    const items = [{ price: 10.99, quantity: 1 }];
    expect(calculateTotal(items)).toBeCloseTo(10.99);
  });
});

Testing Async Functions

describe('fetchUser', () => {
  it('should return user data', async () => {
    const user = await fetchUser(1);
    expect(user).toEqual({
      id: 1,
      name: expect.any(String),
      email: expect.stringContaining('@'),
    });
  });

  it('should throw for non-existent user', async () => {
    await expect(fetchUser(999)).rejects.toThrow('User not found');
  });
});

Mocking Dependencies

import { vi } from 'vitest';
import { sendEmail } from './email';
import { createUser } from './user';

vi.mock('./email', () => ({
  sendEmail: vi.fn().mockResolvedValue({ success: true }),
}));

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

  it('should send welcome email after creating user', async () => {
    await createUser({ name: 'John', email: '[e-mail]' });

    expect(sendEmail).toHaveBeenCalledWith({
      to: '[e-mail]',
      template: 'welcome',
    });
  });
});

React Component Testing

import { render, screen, fireEvent } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { Button } from './Button';

describe('Button', () => {
  it('should render children', () => {
    render(<Button>Click me</Button>);
    expect(screen.getByText('Click me')).toBeInTheDocument();
  });

  it('should call onClick when clicked', async () => {
    const handleClick = vi.fn();
    render(<Button onClick={handleClick}>Click</Button>);

    await userEvent.click(screen.getByRole('button'));
    expect(handleClick).toHaveBeenCalledTimes(1);
  });

  it('should be disabled when loading', () => {
    render(< >Submit);
    (screen.()).();
  });
});

API Route Testing

import { createMocks } from 'node-mocks-http';
import handler from './api/users';

describe('GET /api/users', () => {
  it('should return users list', async () => {
    const { req, res } = createMocks({ method: 'GET' });

    await handler(req, res);

    expect(res._getStatusCode()).toBe(200);
    expect(JSON.parse(res._getData())).toHaveProperty('users');
  });
});

Branch Coverage Checklist

Ensure tests cover:

• If/else branches
• Ternary operators
• Switch cases (including default)
• Try/catch blocks
• Early returns
• Nullish coalescing (??)
• Optional chaining results (?.)
• Loop conditions (0, 1, many iterations)

Coverage Configuration

// vitest.config.ts
export default {
  test: {
    coverage: {
      provider: 'v8',
      reporter: ['text', 'html', 'lcov'],
      exclude: [
        'node_modules/',
        '**/*.d.ts',
        '**/*.test.ts',
        '**/types/',
      ],
      thresholds: {
        lines: 80,
        branches: 75,
        functions: 80,
        statements: 80,
      },
    },
  },
};

Priority Order for Testing

1. Critical paths: Auth, payments, data mutations
2. Complex logic: Algorithms, state machines, calculations
3. Error handlers: Catch blocks, error boundaries
4. Edge cases: Empty arrays, null values, boundaries
5. Integration points: API calls, database queries