User request → What type of testing?
    │
    ├─ Unit Tests
    │   ├─ Runner?
    │   │   ├─ Vitest → Fast, Vite-native, ESM
    │   │   ├─ Jest → Mature, wide ecosystem
    │   │   └─ Bun test → Ultra-fast, built-in
    │   │
    │   └─ What to test?
    │       ├─ Functions → Pure logic, utilities
    │       ├─ Components → React Testing Library
    │       ├─ Hooks → @testing-library/react-hooks
    │       └─ API handlers → MSW for mocking
    │
    ├─ Integration Tests
    │   ├─ Database → Test containers, in-memory
    │   ├─ API → Supertest, actual endpoints
    │   └─ Services → Real dependencies
    │
    ├─ E2E Tests
    │   ├─ Playwright → Cross-browser, fast
    │   ├─ Cypress → Great DX, dashboard
    │   └─ What to test?
    │       ├─ User flows → Critical paths
    │       ├─ Visual regression → Screenshots
    │       └─ Accessibility → axe-core
    │
    └─ Test Strategy
        ├─ TDD → Red-green-refactor
        ├─ Coverage → Istanbul/c8
        └─ CI → GitHub Actions integration

# Install
pnpm add -D vitest @vitest/coverage-v8 @vitest/ui

# With React
pnpm add -D @testing-library/react @testing-library/jest-dom jsdom

// vitest.config.ts
import { defineConfig } from "vitest/config";
import react from "@vitejs/plugin-react";

export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./src/test/setup.ts"],
    include: ["src/**/*.{test,spec}.{ts,tsx}"],
    coverage: {
      provider: "v8",
      reporter: ["text", "json", "html"],
      exclude: ["node_modules/", "src/test/"],
    },
  },
});

# Install
pnpm add -D @playwright/test
npx playwright install

// playwright.config.ts
import { defineConfig, devices } from "@playwright/test";

export default defineConfig({
  testDir: "./e2e",
  fullyParallel: true,
  forbidOnly: !!process.env.CI,
  retries: process.env.CI ? 2 : 0,
  workers: process.env.CI ? 1 : undefined,
  reporter: "html",
  use: {
    baseURL: "http://localhost:3000",
    trace: "on-first-retry",
    screenshot: "only-on-failure",
  },
  projects: [
    { name: "chromium", use: { ...devices["Desktop Chrome"] } },
    { name: "firefox", use: { ...devices["Desktop Firefox"] } },
    { name: "webkit", use: { ...devices["Desktop Safari"] } },
  ],
  webServer: {
    command: "pnpm dev",
    url: "http://localhost:3000",
    reuseExistingServer: !process.env.CI,
  },
});

// utils/format.test.ts
import { describe, it, expect } from "vitest";
import { formatCurrency, formatDate } from "./format";

describe("formatCurrency", () => {
  it("formats USD correctly", () => {
    expect(formatCurrency(1234.56, "USD")).toBe("$1,234.56");
  });

  it("handles zero", () => {
    expect(formatCurrency(0, "USD")).toBe("$0.00");
  });

  it("handles negative values", () => {
    expect(formatCurrency(-50, "USD")).toBe("-$50.00");
  });
});

// components/Button.test.tsx
import { render, screen, fireEvent } from "@testing-library/react";
import { describe, it, expect, vi } from "vitest";
import { Button } from "./Button";

describe("Button", () => {
  it("renders with text", () => {
    render(<Button>Click me</Button>);
    expect(screen.getByRole("button")).toHaveTextContent("Click me");
  });

  it("calls onClick when clicked", () => {
    const handleClick = vi.fn();
    render(<Button onClick={handleClick}>Click</Button>);

    fireEvent.click(screen.getByRole("button"));
    expect(handleClick).toHaveBeenCalledTimes(1);
  });

  it("is disabled when loading", () => {
    render(<Button loading>Submit</Button>);
    expect(screen.getByRole("button")).toBeDisabled();
  });
});

// e2e/auth.spec.ts
import { test, expect } from "@playwright/test";

test.describe("Authentication", () => {
  test("user can login", async ({ page }) => {
    await page.goto("/login");

    await page.fill('[name="email"]', "user@example.com");
    await page.fill('[name="password"]', "password123");
    await page.click('button[type="submit"]');

    await expect(page).toHaveURL("/dashboard");
    await expect(page.locator("h1")).toContainText("Welcome");
  });

  test("shows error for invalid credentials", async ({ page }) => {
    await page.goto("/login");

    await page.fill('[name="email"]', "wrong@example.com");
    await page.fill('[name="password"]', "wrongpass");
    await page.click('button[type="submit"]');

    await expect(page.locator('[role="alert"]')).toContainText("Invalid");
  });
});