Agent skill

test-specialist

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.

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SKILL.md

Test Specialist

Systematic testing methodologies and debugging techniques for JS/TS applications.

When to Use

Use for:

Writing unit, integration, or E2E tests
Fixing bugs and debugging
Improving test coverage
Analyzing code for potential issues
Security and performance testing

Don't use when:

Code review → use generic-code-reviewer
Technical debt → use tech-debt-analyzer
Feature development → use generic-feature-developer

Testing Stack by Project

Project Type	Unit Tests	Component	E2E
React/Next.js	Vitest/Jest	Testing Library	Playwright
Node.js	Vitest/Jest	Supertest	Playwright
Static	Jest	-	Playwright

Test Patterns

Unit Tests (AAA Pattern)

typescript

describe("calculateTotal", () => {
  test("sums amounts correctly", () => {
    // Arrange
    const items = [{ amount: 100 }, { amount: 50 }];
    // Act
    const total = calculateTotal(items);
    // Assert
    expect(total).toBe(150);
  });

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

Component Tests (User Behavior)

typescript

// ✅ Test user behavior, not implementation
it('creates item when user clicks Add', async () => {
  const user = userEvent.setup();
  render(<ItemList />);

  await user.click(screen.getByRole('button', { name: /add/i }));
  await user.type(screen.getByLabelText(/title/i), 'New item');
  await user.click(screen.getByRole('button', { name: /save/i }));

  expect(screen.getByText('New item')).toBeInTheDocument();
});

E2E Tests (Playwright)

typescript

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

test("user can complete checkout", async ({ page }) => {
  await page.goto("/products");

  // Add to cart
  await page.click('button:has-text("Add to Cart")');
  await page.click('a:has-text("Cart")');

  // Checkout
  await page.click('button:has-text("Checkout")');
  await page.fill('[name="email"]', "[email protected]");
  await page.click('button:has-text("Place Order")');

  // Verify
  await expect(page.locator("h1")).toContainText("Order Confirmed");
});

Integration Tests

typescript

test("POST /items creates item", async () => {
  const response = await request(app)
    .post("/api/items")
    .send({ name: "Test" })
    .expect(201);

  expect(response.body).toMatchObject({ id: expect.any(Number) });
});

Bug Analysis Process

Reproduce - Document exact steps, expected vs actual
Isolate - Binary search, minimal reproduction
Root Cause - Trace execution, check assumptions, git blame
Fix - Write failing test first, implement fix
Validate - Run full suite, test edge cases

Debugging Checklist

When debugging an issue:

Can reproduce consistently
Minimal reproduction created
Console/network logs checked
State at failure point inspected
Git blame checked for recent changes
Failing test written before fix

Common Bug Patterns

Race Conditions

typescript

test("handles concurrent updates", async () => {
  const promises = Array.from({ length: 100 }, () => increment());
  await Promise.all(promises);
  expect(getCount()).toBe(100);
});

Null Safety

typescript

test.each([null, undefined, "", 0])("handles invalid input: %p", (input) => {
  expect(() => process(input)).toThrow("Invalid");
});

Boundary Values

typescript

test("handles edge cases", () => {
  expect(paginate([], 1, 10)).toEqual([]); // empty
  expect(paginate([item], 1, 10)).toEqual([item]); // single
  expect(paginate(items25, 3, 10)).toHaveLength(5); // partial last page
});

Security Tests

typescript

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

test("sanitizes XSS", () => {
  const xss = '<script>alert("xss")</script>';
  expect(sanitize(xss)).not.toContain("<script>");
});

test("requires auth", async () => {
  await request(app).post("/api/items").expect(401);
});

Performance Tests

typescript

test("handles large datasets efficiently", () => {
  const largeList = Array.from({ length: 10000 }, (_, i) => ({ value: i }));
  const start = performance.now();
  process(largeList);
  expect(performance.now() - start).toBeLessThan(100);
});

Coverage Targets

Code Type	Target
Critical paths	90%+
Business logic	85%+
UI components	75%+
Utilities	70%+

Test Quality Principles

One behavior per test
Descriptive names - test names explain scenario
Independent tests - no shared state
Cover edge cases - null, empty, boundaries, errors
Mock external deps - tests should be fast
Test behavior - not implementation details

Workflow Decision Tree

Situation	Action
Adding feature	Write test first (TDD)
Fixing bug	Write failing test, then fix
Improving coverage	Find gaps, prioritize critical paths
Code review	Check edge cases, error handling

Python Testing (pytest)

Fixtures and Parametrize

python

import pytest
from myapp.services import UserService

@pytest.fixture
def user_service(db_session):
    """Provide a UserService with test database."""
    return UserService(session=db_session)

@pytest.fixture
def sample_user(user_service):
    """Create and return a sample user."""
    return user_service.create(name="Test User", email="[email protected]")

class TestUserService:
    def test_create_user(self, user_service):
        user = user_service.create(name="John", email="[email protected]")
        assert user.name == "John"
        assert user.id is not None

    def test_get_user_not_found(self, user_service):
        with pytest.raises(UserNotFoundError, match="User 999 not found"):
            user_service.get(999)

    @pytest.mark.parametrize("email,is_valid", [
        ("[email protected]", True),
        ("[email protected]", True),
        ("invalid", False),
        ("@example.com", False),
        ("", False),
    ])
    def test_validate_email(self, user_service, email: str, is_valid: bool):
        assert user_service.validate_email(email) == is_valid

Mocking

python

from unittest.mock import Mock, patch, AsyncMock

def test_send_notification(user_service):
    with patch("myapp.services.email_client") as mock_email:
        mock_email.send = Mock(return_value=True)
        user_service.notify(user_id=1, message="Hello")
        mock_email.send.assert_called_once_with(
            to="[email protected]",
            body="Hello",
        )

# Async mocking
@pytest.mark.asyncio
async def test_fetch_data():
    with patch("myapp.client.fetch", new_callable=AsyncMock) as mock_fetch:
        mock_fetch.return_value = {"status": "ok"}
        result = await process_data()
        assert result["status"] == "ok"

conftest.py Patterns

python

# tests/conftest.py
import pytest
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

@pytest.fixture(scope="session")
def engine():
    return create_engine("sqlite:///:memory:")

@pytest.fixture(scope="function")
def db_session(engine):
    Base.metadata.create_all(engine)
    Session = sessionmaker(bind=engine)
    session = Session()
    yield session
    session.rollback()
    session.close()
    Base.metadata.drop_all(engine)

Go Testing

Table-Driven Tests

func TestCalculateDiscount(t *testing.T) {
    tests := []struct {
        name     string
        amount   float64
        code     string
        want     float64
        wantErr  bool
    }{
        {
            name:   "valid 10% discount",
            amount: 100.0,
            code:   "SAVE10",
            want:   90.0,
        },
        {
            name:   "no discount",
            amount: 100.0,
            code:   "",
            want:   100.0,
        },
        {
            name:    "invalid code",
            amount:  100.0,
            code:    "INVALID",
            wantErr: true,
        },
        {
            name:   "zero amount",
            amount: 0,
            code:   "SAVE10",
            want:   0,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got, err := CalculateDiscount(tt.amount, tt.code)
            if (err != nil) != tt.wantErr {
                t.Errorf("CalculateDiscount() error = %v, wantErr %v", err, tt.wantErr)
                return
            }
            if got != tt.want {
                t.Errorf("CalculateDiscount() = %v, want %v", got, tt.want)
            }
        })
    }
}

Testify Assertions and Mocks

import (
    "testing"
    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/mock"
    "github.com/stretchr/testify/require"
)

// Mock
type MockUserRepo struct {
    mock.Mock
}

func (m *MockUserRepo) FindByID(id string) (*User, error) {
    args := m.Called(id)
    if args.Get(0) == nil {
        return nil, args.Error(1)
    }
    return args.Get(0).(*User), args.Error(1)
}

func TestGetUser(t *testing.T) {
    repo := new(MockUserRepo)
    repo.On("FindByID", "123").Return(&User{ID: "123", Name: "John"}, nil)

    service := NewUserService(repo)
    user, err := service.GetUser("123")

    require.NoError(t, err)
    assert.Equal(t, "John", user.Name)
    repo.AssertExpectations(t)
}

// HTTP handler testing
func TestGetUserHandler(t *testing.T) {
    req := httptest.NewRequest("GET", "/users/123", nil)
    w := httptest.NewRecorder()

    handler := NewHandler(mockService)
    handler.GetUser(w, req)

    assert.Equal(t, http.StatusOK, w.Code)

    var user User
    err := json.NewDecoder(w.Body).Decode(&user)
    require.NoError(t, err)
    assert.Equal(t, "123", user.ID)
}

Rust Testing

Unit and Integration Tests

rust

// src/lib.rs - Unit tests (same file)
pub fn calculate_discount(amount: f64, percentage: f64) -> Result<f64, DiscountError> {
    if percentage < 0.0 || percentage > 100.0 {
        return Err(DiscountError::InvalidPercentage(percentage));
    }
    Ok(amount * (1.0 - percentage / 100.0))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_valid_discount() {
        let result = calculate_discount(100.0, 10.0).unwrap();
        assert!((result - 90.0).abs() < f64::EPSILON);
    }

    #[test]
    fn test_zero_discount() {
        assert_eq!(calculate_discount(100.0, 0.0).unwrap(), 100.0);
    }

    #[test]
    fn test_invalid_percentage() {
        assert!(matches!(
            calculate_discount(100.0, 150.0),
            Err(DiscountError::InvalidPercentage(_))
        ));
    }

    #[test]
    fn test_negative_percentage() {
        assert!(calculate_discount(100.0, -10.0).is_err());
    }
}

// tests/integration_test.rs - Integration tests (separate file)
use my_crate::calculate_discount;

#[test]
fn test_full_workflow() {
    let original = 200.0;
    let discounted = calculate_discount(original, 25.0).unwrap();
    assert_eq!(discounted, 150.0);
}

Property-Based Testing (proptest)

rust

use proptest::prelude::*;

proptest! {
    #[test]
    fn discount_never_exceeds_original(amount in 0.0f64..10000.0, pct in 0.0f64..100.0) {
        let result = calculate_discount(amount, pct).unwrap();
        prop_assert!(result <= amount);
        prop_assert!(result >= 0.0);
    }

    #[test]
    fn roundtrip_serialization(name in "[a-zA-Z]{1,50}", age in 0u32..150) {
        let user = User { name: name.clone(), age };
        let json = serde_json::to_string(&user).unwrap();
        let deserialized: User = serde_json::from_str(&json).unwrap();
        prop_assert_eq!(user, deserialized);
    }
}

Visual Regression Testing

Playwright Screenshots

typescript

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

test("homepage visual regression", async ({ page }) => {
  await page.goto("/");
  await expect(page).toHaveScreenshot("homepage.png", {
    maxDiffPixelRatio: 0.01,
  });
});

test("component states", async ({ page }) => {
  await page.goto("/components");

  // Default state
  await expect(page.locator(".card")).toHaveScreenshot("card-default.png");

  // Hover state
  await page.locator(".card").hover();
  await expect(page.locator(".card")).toHaveScreenshot("card-hover.png");

  // Full page with specific viewport
  await page.setViewportSize({ width: 375, height: 812 }); // iPhone
  await expect(page).toHaveScreenshot("homepage-mobile.png");
});

// Update snapshots: npx playwright test --update-snapshots

Percy (Cloud Visual Testing)

typescript

import percySnapshot from "@percy/playwright";

test("visual test with Percy", async ({ page }) => {
  await page.goto("/dashboard");
  await percySnapshot(page, "Dashboard");
  // Percy compares across browsers and viewport sizes
});

Chromatic (Storybook Visual Testing)

bash

# Run Chromatic on Storybook stories
npx chromatic --project-token=<token>

# CI integration
# Chromatic captures every story as a visual snapshot
# and detects pixel-level changes across PRs

Tool	Approach	Best For
Playwright	Local screenshot comparison	E2E visual tests, free
Percy	Cloud cross-browser comparison	Multi-browser, team review
Chromatic	Storybook story snapshots	Component library visual QA

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SKILL.md

Test Specialist

When to Use

Testing Stack by Project

Test Patterns

Unit Tests (AAA Pattern)

Component Tests (User Behavior)

E2E Tests (Playwright)

Integration Tests

Bug Analysis Process

Debugging Checklist

Common Bug Patterns

Race Conditions

Null Safety

Boundary Values

Security Tests

Performance Tests

Coverage Targets

Test Quality Principles

Workflow Decision Tree

Python Testing (pytest)

Fixtures and Parametrize

Mocking

conftest.py Patterns

Go Testing

Table-Driven Tests

Testify Assertions and Mocks

Rust Testing

Unit and Integration Tests

Property-Based Testing (proptest)

Visual Regression Testing

Playwright Screenshots

Percy (Cloud Visual Testing)

Chromatic (Storybook Visual Testing)

See Also

Recommended Agent Skills

agent-ops-spec

agent-ops-state

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agent-ops-testing

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