Frontend Testing

Complete testing strategies for React and frontend applications.

Testing Pyramid

      /\
     /  \     E2E Tests (Playwright/Cypress)
    /----\    - Critical user flows
   /      \   - Cross-browser testing
  /--------\  Integration Tests
 /          \ - Component interactions
/------------\ Unit Tests (Jest/Vitest)
              - Functions, hooks, utilities

Unit Testing with Jest/Vitest

Setup

# Jest
npm install -D jest @types/jest ts-jest @testing-library/react @testing-library/jest-dom

# Vitest (faster, ESM-native)
npm install -D vitest @testing-library/react @testing-library/jest-dom jsdom

Jest Configuration

// jest.config.js
module.exports = {
  testEnvironment: "jsdom",
  setupFilesAfterEnv: ["<rootDir>/jest.setup.ts"],
  moduleNameMapper: {
    "^@/(.*)$": "<rootDir>/src/$1",
    "\\.(css|less|scss)$": "identity-obj-proxy",
  },
  transform: {
    "^.+\\.(ts|tsx)$": "ts-jest",
  },
  collectCoverageFrom: [
    "src/**/*.{ts,tsx}",
    "!src/**/*.d.ts",
    "!src/**/*.stories.tsx",
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
};

Testing Utilities

// src/utils/format.ts
export function formatCurrency(amount: number, currency = "USD"): string {
  return new Intl.NumberFormat("en-US", {
    style: "currency",
    currency,
  }).format(amount);
}

export function formatDate(date: Date | string): string {
  return new Intl.DateTimeFormat("en-US", {
    year: "numeric",
    month: "long",
    day: "numeric",
  }).format(new Date(date));
}

// src/utils/format.test.ts
import { formatCurrency, formatDate } from "./format";

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

  it("formats EUR correctly", () => {
    expect(formatCurrency(1234.56, "EUR")).toBe("€1,234.56");
  });

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

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

describe("formatDate", () => {
  it("formats Date object", () => {
    const date = new Date("2024-03-15");
    expect(formatDate(date)).toBe("March 15, 2024");
  });

  it("formats ISO string", () => {
    expect(formatDate("2024-03-15")).toBe("March 15, 2024");
  });
});

Testing Custom Hooks

// src/hooks/useCounter.ts
import { useState, useCallback } from "react";

export function useCounter(initialValue = 0) {
  const [count, setCount] = useState(initialValue);

  const increment = useCallback(() => setCount((c) => c + 1), []);
  const decrement = useCallback(() => setCount((c) => c - 1), []);
  const reset = useCallback(() => setCount(initialValue), [initialValue]);

  return { count, increment, decrement, reset };
}

// src/hooks/useCounter.test.ts
import { renderHook, act } from "@testing-library/react";
import { useCounter } from "./useCounter";

describe("useCounter", () => {
  it("initializes with default value", () => {
    const { result } = renderHook(() => useCounter());
    expect(result.current.count).toBe(0);
  });

  it("initializes with custom value", () => {
    const { result } = renderHook(() => useCounter(10));
    expect(result.current.count).toBe(10);
  });

  it("increments count", () => {
    const { result } = renderHook(() => useCounter());

    act(() => {
      result.current.increment();
    });

    expect(result.current.count).toBe(1);
  });

  it("decrements count", () => {
    const { result } = renderHook(() => useCounter(5));

    act(() => {
      result.current.decrement();
    });

    expect(result.current.count).toBe(4);
  });

  it("resets to initial value", () => {
    const { result } = renderHook(() => useCounter(10));

    act(() => {
      result.current.increment();
      result.current.increment();
      result.current.reset();
    });

    expect(result.current.count).toBe(10);
  });
});

Component Testing with React Testing Library

Basic Component Testing

// src/components/Button.tsx
interface ButtonProps {
  children: React.ReactNode;
  onClick?: () => void;
  disabled?: boolean;
  variant?: "primary" | "secondary" | "danger";
  loading?: boolean;
}

export function Button({
  children,
  onClick,
  disabled = false,
  variant = "primary",
  loading = false,
}: ButtonProps) {
  return (
    <button
      onClick={onClick}
      disabled={disabled || loading}
      className={`btn btn-${variant}`}
      aria-busy={loading}
    >
      {loading ? "Loading..." : children}
    </button>
  );
}

// src/components/Button.test.tsx
import { render, screen, fireEvent } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { Button } from "./Button";

describe("Button", () => {
  it("renders children", () => {
    render(<Button>Click me</Button>);
    expect(
      screen.getByRole("button", { name: /click me/i }),
    ).toBeInTheDocument();
  });

  it("calls onClick when clicked", async () => {
    const handleClick = jest.fn();
    const user = userEvent.setup();

    render(<Button onClick={handleClick}>Click me</Button>);
    await user.click(screen.getByRole("button"));

    expect(handleClick).toHaveBeenCalledTimes(1);
  });

  it("is disabled when disabled prop is true", () => {
    render(<Button disabled>Click me</Button>);
    expect(screen.getByRole("button")).toBeDisabled();
  });

  it("shows loading state", () => {
    render(<Button loading>Submit</Button>);

    expect(screen.getByRole("button")).toHaveTextContent("Loading...");
    expect(screen.getByRole("button")).toBeDisabled();
    expect(screen.getByRole("button")).toHaveAttribute("aria-busy", "true");
  });

  it("applies variant class", () => {
    render(<Button variant="danger">Delete</Button>);
    expect(screen.getByRole("button")).toHaveClass("btn-danger");
  });
});

Testing Forms

// src/components/LoginForm.tsx
import { useState } from "react";

interface LoginFormProps {
  onSubmit: (email: string, password: string) => Promise<void>;
}

export function LoginForm({ onSubmit }: LoginFormProps) {
  const [email, setEmail] = useState("");
  const [password, setPassword] = useState("");
  const [error, setError] = useState("");
  const [loading, setLoading] = useState(false);

  const handleSubmit = async (e: React.FormEvent) => {
    e.preventDefault();
    setError("");
    setLoading(true);

    try {
      await onSubmit(email, password);
    } catch (err) {
      setError(err instanceof Error ? err.message : "Login failed");
    } finally {
      setLoading(false);
    }
  };

  return (
    <form onSubmit={handleSubmit} aria-label="Login form">
      {error && (
        <div role="alert" className="error">
          {error}
        </div>
      )}

      <label htmlFor="email">Email</label>
      <input
        id="email"
        type="email"
        value={email}
        onChange={(e) => setEmail(e.target.value)}
        required
        aria-describedby={error ? "error-message" : undefined}
      />

      <label htmlFor="password">Password</label>
      <input
        id="password"
        type="password"
        value={password}
        onChange={(e) => setPassword(e.target.value)}
        required
        minLength={8}
      />

      <button type="submit" disabled={loading}>
        {loading ? "Signing in..." : "Sign In"}
      </button>
    </form>
  );
}

// src/components/LoginForm.test.tsx
import { render, screen, waitFor } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { LoginForm } from "./LoginForm";

describe("LoginForm", () => {
  const mockOnSubmit = jest.fn();

  beforeEach(() => {
    mockOnSubmit.mockReset();
  });

  it("renders all form fields", () => {
    render(<LoginForm onSubmit={mockOnSubmit} />);

    expect(screen.getByLabelText(/email/i)).toBeInTheDocument();
    expect(screen.getByLabelText(/password/i)).toBeInTheDocument();
    expect(
      screen.getByRole("button", { name: /sign in/i }),
    ).toBeInTheDocument();
  });

  it("submits form with valid data", async () => {
    const user = userEvent.setup();
    mockOnSubmit.mockResolvedValueOnce(undefined);

    render(<LoginForm onSubmit={mockOnSubmit} />);

    await user.type(screen.getByLabelText(/email/i), "test@example.com");
    await user.type(screen.getByLabelText(/password/i), "password123");
    await user.click(screen.getByRole("button", { name: /sign in/i }));

    await waitFor(() => {
      expect(mockOnSubmit).toHaveBeenCalledWith(
        "test@example.com",
        "password123",
      );
    });
  });

  it("shows loading state during submission", async () => {
    const user = userEvent.setup();
    mockOnSubmit.mockImplementation(() => new Promise(() => {})); // Never resolves

    render(<LoginForm onSubmit={mockOnSubmit} />);

    await user.type(screen.getByLabelText(/email/i), "test@example.com");
    await user.type(screen.getByLabelText(/password/i), "password123");
    await user.click(screen.getByRole("button", { name: /sign in/i }));

    expect(screen.getByRole("button")).toHaveTextContent("Signing in...");
    expect(screen.getByRole("button")).toBeDisabled();
  });

  it("displays error message on failure", async () => {
    const user = userEvent.setup();
    mockOnSubmit.mockRejectedValueOnce(new Error("Invalid credentials"));

    render(<LoginForm onSubmit={mockOnSubmit} />);

    await user.type(screen.getByLabelText(/email/i), "test@example.com");
    await user.type(screen.getByLabelText(/password/i), "wrongpassword");
    await user.click(screen.getByRole("button", { name: /sign in/i }));

    await waitFor(() => {
      expect(screen.getByRole("alert")).toHaveTextContent(
        "Invalid credentials",
      );
    });
  });

  it("requires email field", async () => {
    render(<LoginForm onSubmit={mockOnSubmit} />);
    expect(screen.getByLabelText(/email/i)).toBeRequired();
  });

  it("requires password field", async () => {
    render(<LoginForm onSubmit={mockOnSubmit} />);
    expect(screen.getByLabelText(/password/i)).toBeRequired();
  });
});

Testing Async Components

// src/components/UserProfile.tsx
import { useState, useEffect } from "react";

interface User {
  id: string;
  name: string;
  email: string;
}

export function UserProfile({ userId }: { userId: string }) {
  const [user, setUser] = useState<User | null>(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState<string | null>(null);

  useEffect(() => {
    async function fetchUser() {
      try {
        const res = await fetch(`/api/users/${userId}`);
        if (!res.ok) throw new Error("User not found");
        const data = await res.json();
        setUser(data);
      } catch (err) {
        setError(err instanceof Error ? err.message : "Failed to load");
      } finally {
        setLoading(false);
      }
    }
    fetchUser();
  }, [userId]);

  if (loading) return <div aria-label="Loading">Loading...</div>;
  if (error) return <div role="alert">{error}</div>;
  if (!user) return null;

  return (
    <div>
      <h1>{user.name}</h1>
      <p>{user.email}</p>
    </div>
  );
}

// src/components/UserProfile.test.tsx
import { render, screen, waitFor } from "@testing-library/react";
import { UserProfile } from "./UserProfile";

// Mock fetch globally
global.fetch = jest.fn();

describe("UserProfile", () => {
  beforeEach(() => {
    (fetch as jest.Mock).mockReset();
  });

  it("shows loading state initially", () => {
    (fetch as jest.Mock).mockImplementation(() => new Promise(() => {}));

    render(<UserProfile userId="1" />);

    expect(screen.getByLabelText("Loading")).toBeInTheDocument();
  });

  it("displays user data on success", async () => {
    (fetch as jest.Mock).mockResolvedValueOnce({
      ok: true,
      json: () =>
        Promise.resolve({
          id: "1",
          name: "John Doe",
          email: "john@example.com",
        }),
    });

    render(<UserProfile userId="1" />);

    await waitFor(() => {
      expect(screen.getByText("John Doe")).toBeInTheDocument();
      expect(screen.getByText("john@example.com")).toBeInTheDocument();
    });
  });

  it("displays error on failure", async () => {
    (fetch as jest.Mock).mockResolvedValueOnce({
      ok: false,
    });

    render(<UserProfile userId="invalid" />);

    await waitFor(() => {
      expect(screen.getByRole("alert")).toHaveTextContent("User not found");
    });
  });
});

E2E Testing with Playwright

Setup

npm init playwright@latest

Configuration

// 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"], ["junit", { outputFile: "test-results/junit.xml" }]],
  use: {
    baseURL: "http://localhost:3000",
    trace: "on-first-retry",
    screenshot: "only-on-failure",
    video: "retain-on-failure",
  },
  projects: [
    {
      name: "chromium",
      use: { ...devices["Desktop Chrome"] },
    },
    {
      name: "firefox",
      use: { ...devices["Desktop Firefox"] },
    },
    {
      name: "webkit",
      use: { ...devices["Desktop Safari"] },
    },
    {
      name: "mobile-chrome",
      use: { ...devices["Pixel 5"] },
    },
    {
      name: "mobile-safari",
      use: { ...devices["iPhone 12"] },
    },
  ],
  webServer: {
    command: "npm run dev",
    url: "http://localhost:3000",
    reuseExistingServer: !process.env.CI,
  },
});

Page Objects Pattern

// e2e/pages/LoginPage.ts
import { Page, Locator } from "@playwright/test";

export class LoginPage {
  readonly page: Page;
  readonly emailInput: Locator;
  readonly passwordInput: Locator;
  readonly submitButton: Locator;
  readonly errorMessage: Locator;

  constructor(page: Page) {
    this.page = page;
    this.emailInput = page.getByLabel("Email");
    this.passwordInput = page.getByLabel("Password");
    this.submitButton = page.getByRole("button", { name: "Sign In" });
    this.errorMessage = page.getByRole("alert");
  }

  async goto() {
    await this.page.goto("/login");
  }

  async login(email: string, password: string) {
    await this.emailInput.fill(email);
    await this.passwordInput.fill(password);
    await this.submitButton.click();
  }

  async expectError(message: string) {
    await expect(this.errorMessage).toContainText(message);
  }
}

E2E Test Examples

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

test.describe("Authentication", () => {
  test("successful login redirects to dashboard", async ({ page }) => {
    const loginPage = new LoginPage(page);
    await loginPage.goto();

    await loginPage.login("user@example.com", "validpassword");

    await expect(page).toHaveURL("/dashboard");
    await expect(page.getByText("Welcome back")).toBeVisible();
  });

  test("invalid credentials show error", async ({ page }) => {
    const loginPage = new LoginPage(page);
    await loginPage.goto();

    await loginPage.login("user@example.com", "wrongpassword");

    await loginPage.expectError("Invalid credentials");
    await expect(page).toHaveURL("/login");
  });

  test("logout returns to home page", async ({ page }) => {
    // Login first
    const loginPage = new LoginPage(page);
    await loginPage.goto();
    await loginPage.login("user@example.com", "validpassword");

    // Now logout
    await page.getByRole("button", { name: "Logout" }).click();

    await expect(page).toHaveURL("/");
  });
});

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

test.describe("Dashboard", () => {
  test.beforeEach(async ({ page }) => {
    // Login before each test
    await page.goto("/login");
    await page.getByLabel("Email").fill("user@example.com");
    await page.getByLabel("Password").fill("validpassword");
    await page.getByRole("button", { name: "Sign In" }).click();
    await page.waitForURL("/dashboard");
  });

  test("displays user stats", async ({ page }) => {
    await expect(page.getByTestId("total-users")).toBeVisible();
    await expect(page.getByTestId("revenue")).toBeVisible();
    await expect(page.getByTestId("orders")).toBeVisible();
  });

  test("filters data by date range", async ({ page }) => {
    await page.getByRole("button", { name: "Date Range" }).click();
    await page.getByRole("option", { name: "Last 7 days" }).click();

    // Wait for data to reload
    await page.waitForResponse("**/api/stats*");

    // Verify filter is applied
    await expect(page.getByText("Last 7 days")).toBeVisible();
  });

  test("exports data to CSV", async ({ page }) => {
    const downloadPromise = page.waitForEvent("download");
    await page.getByRole("button", { name: "Export CSV" }).click();

    const download = await downloadPromise;
    expect(download.suggestedFilename()).toContain(".csv");
  });
});

Visual Regression Testing

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

test.describe("Visual Regression", () => {
  test("homepage matches snapshot", async ({ page }) => {
    await page.goto("/");
    await expect(page).toHaveScreenshot("homepage.png", {
      fullPage: true,
      maxDiffPixels: 100,
    });
  });

  test("dashboard matches snapshot", async ({ page }) => {
    // Login first
    await page.goto("/login");
    await page.getByLabel("Email").fill("user@example.com");
    await page.getByLabel("Password").fill("password");
    await page.getByRole("button", { name: "Sign In" }).click();
    await page.waitForURL("/dashboard");

    // Wait for all data to load
    await page.waitForLoadState("networkidle");

    await expect(page).toHaveScreenshot("dashboard.png", {
      maxDiffPixels: 200,
    });
  });

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

    // Screenshot specific component
    const button = page.getByTestId("primary-button");
    await expect(button).toHaveScreenshot("primary-button.png");

    const card = page.getByTestId("user-card");
    await expect(card).toHaveScreenshot("user-card.png");
  });
});

Accessibility Testing

With jest-axe

// src/components/Form.test.tsx
import { render } from "@testing-library/react";
import { axe, toHaveNoViolations } from "jest-axe";
import { ContactForm } from "./ContactForm";

expect.extend(toHaveNoViolations);

describe("ContactForm accessibility", () => {
  it("has no accessibility violations", async () => {
    const { container } = render(<ContactForm />);
    const results = await axe(container);

    expect(results).toHaveNoViolations();
  });
});

With Playwright

// e2e/accessibility.spec.ts
import { test, expect } from "@playwright/test";
import AxeBuilder from "@axe-core/playwright";

test.describe("Accessibility", () => {
  test("homepage has no violations", async ({ page }) => {
    await page.goto("/");

    const accessibilityScanResults = await new AxeBuilder({ page })
      .withTags(["wcag2a", "wcag2aa", "wcag21a", "wcag21aa"])
      .analyze();

    expect(accessibilityScanResults.violations).toEqual([]);
  });

  test("form has no violations", async ({ page }) => {
    await page.goto("/contact");

    const accessibilityScanResults = await new AxeBuilder({ page })
      .include("#contact-form")
      .analyze();

    expect(accessibilityScanResults.violations).toEqual([]);
  });

  test("keyboard navigation works", async ({ page }) => {
    await page.goto("/");

    // Tab through interactive elements
    await page.keyboard.press("Tab");
    await expect(page.getByRole("link", { name: "Home" })).toBeFocused();

    await page.keyboard.press("Tab");
    await expect(page.getByRole("link", { name: "About" })).toBeFocused();

    // Press Enter to activate
    await page.keyboard.press("Enter");
    await expect(page).toHaveURL("/about");
  });
});

CI/CD Integration

GitHub Actions

# .github/workflows/test.yml
name: Tests

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  unit-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-          cache: "npm"
      - run: npm ci
      - run: npm run test:coverage
      - uses: codecov/codecov-action@v3
        with:
          files: ./coverage/lcov.info

  e2e-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-          cache: "npm"
      - run: npm ci
      - run: npx playwright install --with-deps
      - run: npm run test:e2e
      - uses: actions/upload-artifact@v4
        if: failure()
        with:
          name: playwright-report
          path: playwright-report/
          retention-days: 7

Best Practices

1. Test Behavior, Not Implementation - Focus on what users see and do
2. Use Accessible Queries - getByRole, getByLabelText, getByText
3. Avoid Test IDs When Possible - Use semantic queries first
4. Mock External Dependencies - API calls, timers, etc.
5. Keep Tests Isolated - Each test should be independent
6. Test Edge Cases - Error states, empty states, loading
7. Run Tests in CI - Catch regressions early
8. Maintain Test Coverage - Aim for 80%+ on critical paths

When to Use

• Unit Tests: Functions, hooks, utilities, pure logic
• Component Tests: UI components, forms, interactions
• Integration Tests: Component combinations, API integration
• E2E Tests: Critical user flows, cross-browser testing
• Visual Tests: Design consistency, responsive layouts
• Accessibility Tests: WCAG compliance, keyboard navigation