---

name: test-specialist description: Test-writing patterns for JS/TS, Python, Go, and Rust (unit, integration, E2E, visual regression). Use only when the user explicitly asks for tests or test coverage; for debugging without a test request, use debug-systematic.

Test Specialist

Test-writing patterns for JS/TS, Python, Go, and Rust, plus bug-analysis habits that apply when tests are requested.

When to Use

Use this skill only when the user explicitly asks for tests. Without that request, validate by reviewing the diff and the code it touches.

Use for (when tests are requested):

• Writing unit, integration, or E2E tests
• Improving test coverage
• Security and performance tests

Don't use when:

• Debugging without a test request → use debug-systematic
• 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

Browser automation (Playwright, Percy, Chromatic, npx browser installs) is not installed or run on the owner's Mac. Run it only when the user asks, through an environment they have approved for it.

Test Patterns

Unit Tests (AAA Pattern)

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)

// ✅ 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)

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"]', "test@example.com");
  await page.click('button:has-text("Place Order")');

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

Integration Tests

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

1. Reproduce - Document exact steps, expected vs actual
2. Isolate - Binary search, minimal reproduction
3. Root Cause - Trace execution, check assumptions, git blame
4. Fix - Fix the root cause (write a failing test first only when the user asked for tests)
5. Validate - Review the diff and the code paths it touches against the reproduction and 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
• Fix reviewed against the reproduction (failing test only if tests were requested)

Common Bug Patterns

Race Conditions

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

Null Safety

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

Boundary Values

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

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

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

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

Workflow Decision Tree

Situation	Action
Adding feature (tests requested)	Write test first (TDD)
Fixing bug (tests requested)	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

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="test@example.com")

class TestUserService:
    def test_create_user(self, user_service):
        user = user_service.create(name="John", email="john@example.com")
        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", [
        ("user@example.com", True),
        ("user@sub.domain.com", 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

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="test@example.com",
            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

# 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

// 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)

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

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)

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)

# 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

---

See Also

• Code Review Standards - Quality requirements
• Project CLAUDE.md - Testing rules