/test-specialist

name: test-specialist
description: This skill should be used when writing test cases, fixing bugs, analyzing code for potential issues, or improving test coverage for JavaScript/TypeScript applications. Use this for unit tests, integration tests, end-to-end tests, debugging runtime errors, logic bugs, performance issues, security vulnerabilities, and systematic code analysis.

Test Specialist

Overview

Apply systematic testing methodologies and debugging techniques to JavaScript/TypeScript applications. This skill provides comprehensive testing strategies, bug analysis frameworks, and automated tools for identifying coverage gaps and untested code.

Core Capabilities

1. Writing Test Cases

Write comprehensive tests covering unit, integration, and end-to-end scenarios.

Unit Testing Approach

Structure tests using the AAA pattern (Arrange-Act-Assert):

describe('ExpenseCalculator', () => {
  describe('calculateTotal', () => {
    test('sums expense amounts correctly', () => {
      // Arrange
      const expenses = [
        { amount: 100, category: 'food' },
        { amount: 50, category: 'transport' },
        { amount: 25, category: 'entertainment' }
      ];

      // Act
      const total = calculateTotal(expenses);

      // Assert
      expect(total).toBe(175);
    });

    test('handles empty expense list', () => {
      expect(calculateTotal([])).toBe(0);
    });

    test('handles negative amounts', () => {
      const expenses = [
        { amount: 100, category: 'food' },
        { amount: -50, category: 'refund' }
      ];
      expect(calculateTotal(expenses)).toBe(50);
    });
  });
});

**Key principles:**

• Test one behavior per test
• Cover happy path, edge cases, and error conditions
• Use descriptive test names that explain the scenario
• Keep tests independent and isolated

Integration Testing Approach

Test how components work together, including database, API, and service interactions:

describe('ExpenseAPI Integration', () => {
  beforeAll(async () => {
    await database.connect(TEST_DB_URL);
  });

  afterAll(async () => {
    await database.disconnect();
  });

  beforeEach(async () => {
    await database.clear();
    await seedTestData();
  });

  test('POST /expenses creates expense and updates total', async () => {
    const response = await request(app)
      .post('/api/expenses')
      .send({
        amount: 50,
        category: 'food',
        description: 'Lunch'
      })
      .expect(201);

    expect(response.body).toMatchObject({
      id: expect.any(Number),
      amount: 50,
      category: 'food'
    });

    // Verify database state
    const total = await getTotalExpenses();
    expect(total).toBe(50);
  });
});

End-to-End Testing Approach

Test complete user workflows using tools like Playwright or Cypress:

test('user can track expense from start to finish', async ({ page }) => {
  // Navigate to app
  await page.goto('/');

  // Add new expense
  await page.click('[data-testid="add-expense-btn"]');
  await page.fill('[data-testid="amount"]', '50.00');
  await page.selectOption('[data-testid="category"]', 'food');
  await page.fill('[data-testid="description"]', 'Lunch');
  await page.click('[data-testid="submit"]');

  // Verify expense appears in list
  await expect(page.locator('[data-testid="expense-item"]')).toContainText('Lunch');
  await expect(page.locator('[data-testid="total"]')).toContainText('$50.00');
});

2. Systematic Bug Analysis

Apply structured debugging methodology to identify and fix issues.

Five-Step Analysis Process

1. **Reproduction**: Reliably reproduce the bug

• Document exact steps to trigger
• Identify required environment/state
• Note expected vs actual behavior

2. **Isolation**: Narrow down the problem

• Binary search through code path
• Create minimal reproduction case
• Remove unrelated dependencies

3. **Root Cause Analysis**: Determine underlying cause

• Trace execution flow
• Check assumptions and preconditions
• Review recent changes (git blame)

4. **Fix Implementation**: Implement solution

• Write failing test first (TDD)
• Implement the fix
• Verify test passes

5. **Validation**: Ensure completeness

• Run full test suite
• Test edge cases
• Verify no regressions

Common Bug Patterns

**Race Conditions:**

// Test concurrent operations
test('handles concurrent updates correctly', async () => {
  const promises = Array.from({ length: 100 }, () =>
    incrementExpenseCount()
  );

  await Promise.all(promises);
  expect(getExpenseCount()).toBe(100);
});

**Null/Undefined Errors:**

// Test null safety
test.each([null, undefined, '', 0, false])
  ('handles invalid input: %p', (input) => {
    expect(() => processExpense(input)).toThrow('Invalid expense');
  });

**Off-by-One Errors:**

// Test boundaries explicitly
describe('pagination', () => {
  test('handles empty list', () => {
    expect(paginate([], 1, 10)).toEqual([]);
  });

  test('handles single item', () => {
    expect(paginate([item], 1, 10)).toEqual([item]);
  });

  test('handles last page with partial items', () => {
    const items = Array.from({ length: 25 }, (_, i) => i);
    expect(paginate(items, 3, 10)).toHaveLength(5);
  });
});

3. Identifying Potential Issues

Proactively identify issues before they become bugs.

Security Vulnerabilities

Test for common security issues:

describe('security', () => {
  test('prevents SQL injection', async () => {
    const malicious = "'; DROP TABLE expenses; --";
    await expect(
      searchExpenses(malicious)
    ).resolves.not.toThrow();
  });

  test('sanitizes XSS in descriptions', () => {
    const xss = '<script>alert("xss")</script>';
    const expense = createExpense({ description: xss });
    expect(expense.description).not.toContain('<script>');
  });

  test(