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):

```typescript
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:

```typescript
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:

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

**Off-by-One Errors:**
```typescript
// 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:

```typescript
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('requires authentication for expense operations', async () => {
    await request(app)
      .post('/api/expenses')
      .send({ amount: 50 })
      .expect(401);
  });
});
```

#### Performance Issues

Test for performance problems:

```typescript
test('processes large expense list efficiently', () => {
  const largeList = Array.from({ length: 10000 }, (_, i) => ({
    amount: i,
    category: 'test'
  }));

  const start = performance.now();
  const total = calculateTotal(largeList);
  const duration = performance.now() - start;

  expect(duration).toBeLessThan(100); // Should complete in <100ms
  expect(total).toBe(49995000);
});
```

#### Logic Errors

Use parameterized tests to catch edge cases:

```typescript
test.each([
  // [input, expected, description]
  [[10, 20, 30], 60, 'normal positive values'],
  [[0, 0, 0], 0, 'all zeros'],
  [[-10, 20, -5], 5, 'mixed positive and negative'],
  [[0.1, 0.2], 0.3, 'decimal precision'],
  [[Number.MAX_SAFE_INTEGER], Number.MAX_SAFE_INTEGER, 'large numbers'],
])('calculateTotal(%p) = %p (%s)', (amounts, expected, description) => {
  const expenses = amounts.map(amount => ({ amount, category: 'test' }));
  expect(calculateTotal(expenses)).toBeCloseTo(expected);
});
```

### 4. Test Coverage Analysis

Use automated tools to identify gaps in test coverage.

#### Finding Untested Code

Run the provided script to identify source files without tests:

```bash
python3 scripts/find_untested_code.py src
```

The script will:
- Scan source directory for all code files
- Identify which files lack corresponding test files
- Categorize untested files by type (components, services, utils, etc.)
- Prioritize files that need testing most

**Interpretation:**
- **API/Services**: High priority - test business logic and data operations
- **Models**: High priority - test data validation and transformations
- **Hooks**: Medium priority - test stateful behavior
- **Components**: Medium priority - test complex UI logic
- **Utils**: Low priori