Most teams write tests the way most people exercise: they know they should, they feel guilty when they don't, and when they finally do it, they push too hard on day one and quit by Friday. The codebase ends up with a scattering of shallow snapshot tests that break every time someone changes a CSS class, a few integration tests that nobody trusts, and a coverage badge in the README that lies.

I've been on both sides. I've shipped projects with zero tests and sweated through every deploy. I've also been on teams that chased 100% coverage and spent more time maintaining tests than writing features. Neither works. What works is writing the right tests, in the right places, with the right tools.

Vitest changed how I think about testing in JavaScript. Not because it invented new concepts — the fundamentals haven't changed since Kent Beck wrote about them decades ago. But because it removed enough friction that writing tests stopped feeling like a chore and started feeling like part of the development loop. When your test runner is as fast as your dev server and uses the same config, the excuses evaporate.

This post is everything I know about testing with Vitest, from initial setup to the philosophy that makes it all worthwhile.

Why Vitest Over Jest#

If you've used Jest, you already know most of Vitest's API. That's by design — Vitest is Jest-compatible at the API level. describe, it, expect, beforeEach, vi.fn() — it all works. So why switch?

Native ESM Support#

Jest was built for CommonJS. It can handle ESM, but it requires configuration, experimental flags, and occasional prayer. If you're using import/export syntax (which is everything modern), you've probably fought with Jest's transform pipeline.

Vitest runs on Vite. Vite understands ESM natively. There's no transform step for your source code — it just works. This matters more than it sounds. Half the Jest issues I've debugged over the years trace back to module resolution: SyntaxError: Cannot use import statement outside a module, or mocks not working because the module was already cached in a different format.

Same Config as Your Dev Server#

If your project uses Vite (and if you're building a React, Vue, or Svelte app in 2026, it probably does), Vitest reads your vite.config.ts automatically. Your path aliases, plugins, and environment variables work in tests without any additional configuration. With Jest, you maintain a parallel configuration that has to stay in sync with your bundler setup. Every time you add a path alias in vite.config.ts, you have to remember to add the corresponding moduleNameMapper in jest.config.ts. It's a small thing, but small things compound.

Speed#

Vitest is fast. Meaningfully fast. Not "saves you two seconds" fast — "changes how you work" fast. It uses Vite's module graph to understand which tests are affected by a file change and only runs those. Its watch mode uses the same HMR infrastructure that makes Vite's dev server feel instant.

On a project with 400+ tests, switching from Jest to Vitest cut our watch-mode feedback loop from ~4 seconds to under 500ms. That's the difference between "I'll wait for the test to pass" and "I'll glance at the terminal while my fingers are still on the keyboard."

Benchmarking Built In#

Vitest includes bench() out of the box for performance testing. No separate library needed:

typescript

import { bench, describe } from "vitest";
 
describe("string concatenation", () => {
  bench("template literals", () => {
    const name = "world";
    const _result = `hello ${name}`;
  });
 
  bench("string concat", () => {
    const name = "world";
    const _result = "hello " + name;
  });
});

Run with vitest bench. It's not the main event, but it's nice to have performance testing in the same toolchain without installing benchmark.js and wiring up a separate runner.

Setup#

Installation#

bash

npm install -D vitest @testing-library/react @testing-library/jest-dom @testing-library/user-event jsdom

Configuration#

Create vitest.config.ts at your project root:

typescript

import { defineConfig } from "vitest/config";
import react from "@vitejs/plugin-react";
import path from "path";
 
export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./src/test/setup.ts"],
    include: ["src/**/*.{test,spec}.{ts,tsx}"],
    alias: {
      "@": path.resolve(__dirname, "./src"),
    },
    css: false,
  },
});

Or, if you already have a vite.config.ts, you can extend it:

typescript

/// <reference types="vitest/config" />
import { defineConfig } from "vite";
import react from "@vitejs/plugin-react";
 
export default defineConfig({
  plugins: [react()],
  resolve: {
    alias: {
      "@": "/src",
    },
  },
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./src/test/setup.ts"],
  },
});

The `globals: true` Decision#

When globals is true, you don't need to import describe, it, expect, beforeEach, etc. — they're available everywhere, just like in Jest. When false, you import them explicitly:

typescript

// globals: false
import { describe, it, expect } from "vitest";
 
describe("math", () => {
  it("adds numbers", () => {
    expect(1 + 1).toBe(2);
  });
});

I use globals: true because it reduces visual noise and matches what most developers expect. If you're on a team that values explicit imports, set it to false — there's no wrong answer here.

If you use globals: true, add Vitest's types to your tsconfig.json so TypeScript recognizes them:

json

{
  "compilerOptions": {
    "types": ["vitest/globals"]
  }
}

Environment: jsdom vs happy-dom vs node#

Vitest lets you pick the DOM implementation per test or globally:

node — No DOM. For pure logic, utilities, API routes, and anything that doesn't touch the browser.
jsdom — The standard. Full DOM implementation. Heavier but more complete.
happy-dom — Lighter and faster than jsdom but less complete. Some edge cases (like Range, Selection, or IntersectionObserver) may not work.

I default to jsdom globally and override per-file when I need node:

typescript

// src/lib/utils.test.ts
// @vitest-environment node
 
import { formatDate, slugify } from "./utils";
 
describe("slugify", () => {
  it("converts spaces to hyphens", () => {
    expect(slugify("hello world")).toBe("hello-world");
  });
});

Setup Files#

The setup file runs before every test file. This is where you configure Testing Library matchers and global mocks:

typescript

// src/test/setup.ts
import "@testing-library/jest-dom/vitest";
 
// Mock IntersectionObserver — jsdom doesn't implement it
class MockIntersectionObserver {
  observe = vi.fn();
  unobserve = vi.fn();
  disconnect = vi.fn();
}
 
Object.defineProperty(window, "IntersectionObserver", {
  writable: true,
  value: MockIntersectionObserver,
});
 
// Mock window.matchMedia — needed for responsive components
Object.defineProperty(window, "matchMedia", {
  writable: true,
  value: vi.fn().mockImplementation((query: string) => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: vi.fn(),
    removeListener: vi.fn(),
    addEventListener: vi.fn(),
    removeEventListener: vi.fn(),
    dispatchEvent: vi.fn(),
  })),
});

The @testing-library/jest-dom/vitest import gives you matchers like toBeInTheDocument(), toHaveClass(), toBeVisible(), and many others. These make assertions on DOM elements readable and expressive.

Writing Good Tests#

The AAA Pattern#

Every test follows the same structure: Arrange, Act, Assert. Even when you don't write explicit comments, the structure should be visible:

typescript

it("calculates total with tax", () => {
  // Arrange
  const items = [
    { name: "Widget", price: 10 },
    { name: "Gadget", price: 20 },
  ];
  const taxRate = 0.08;
 
  // Act
  const total = calculateTotal(items, taxRate);
 
  // Assert
  expect(total).toBe(32.4);
});

When I see a test that mixes arrangement, action, and assertion into a single chain of method calls, I know it's going to be hard to understand when it fails. Keep the three phases visually distinct even if you don't add the comments.

Test Naming#

There are two schools: it('should calculate total with tax') and it('calculates total with tax'). The "should" prefix is verbose without adding information. When the test fails, you'll see:

FAIL  ✕ calculates total with tax

That's already a complete sentence. Adding "should" just adds noise. I prefer the direct form: it('renders loading state'), it('rejects invalid email'), it('returns empty array when no matches found').

For describe blocks, use the name of the unit under test:

typescript

describe("calculateTotal", () => {
  it("sums item prices", () => { /* ... */ });
  it("applies tax rate", () => { /* ... */ });
  it("returns 0 for empty array", () => { /* ... */ });
  it("handles negative prices", () => { /* ... */ });
});

Read it out loud: "calculateTotal sums item prices." "calculateTotal applies tax rate." If the sentence works, the naming works.

One Assertion Per Test vs Practical Grouping#

The purist rule says one assertion per test. The practical rule says: one concept per test. These are different.

typescript

// This is fine — one concept, multiple assertions
it("formats user display name", () => {
  const user = { firstName: "John", lastName: "Doe", title: "Dr." };
  const result = formatDisplayName(user);
 
  expect(result).toContain("John");
  expect(result).toContain("Doe");
  expect(result).toStartWith("Dr.");
});
 
// This is not fine — multiple concepts in one test
it("handles user operations", () => {
  const user = createUser("John");
  expect(user.id).toBeDefined();
 
  const updated = updateUser(user.id, { name: "Jane" });
  expect(updated.name).toBe("Jane");
 
  deleteUser(user.id);
  expect(getUser(user.id)).toBeNull();
});

The first test has three assertions but they all verify one thing: the format of a display name. If any assertion fails, you know exactly what's broken. The second test is three separate tests crammed together. If the second assertion fails, you don't know whether creation or updating is broken, and the third assertion never runs.

Test Descriptions as Documentation#

Good test suites serve as living documentation. Someone unfamiliar with the code should be able to read the test descriptions and understand the feature's behavior:

typescript

describe("PasswordValidator", () => {
  describe("minimum length", () => {
    it("rejects passwords shorter than 8 characters", () => { /* ... */ });
    it("accepts passwords with exactly 8 characters", () => { /* ... */ });
  });
 
  describe("character requirements", () => {
    it("requires at least one uppercase letter", () => { /* ... */ });
    it("requires at least one number", () => { /* ... */ });
    it("requires at least one special character", () => { /* ... */ });
  });
 
  describe("common password check", () => {
    it("rejects passwords in the common passwords list", () => { /* ... */ });
    it("performs case-insensitive comparison", () => { /* ... */ });
  });
});

When this test suite runs, the output reads like a specification. That's the goal.

Mocking#

Mocking is the most powerful and most dangerous tool in your testing toolkit. Used well, it isolates the unit under test and makes tests fast and deterministic. Used poorly, it creates tests that pass no matter what the code does.

vi.fn() — Creating Mock Functions#

The simplest mock is a function that records its calls:

typescript

const mockCallback = vi.fn();
 
// Call it
mockCallback("hello", 42);
mockCallback("world");
 
// Assert on calls
expect(mockCallback).toHaveBeenCalledTimes(2);
expect(mockCallback).toHaveBeenCalledWith("hello", 42);
expect(mockCallback).toHaveBeenLastCalledWith("world");

You can give it a return value:

typescript

const mockFetch = vi.fn().mockResolvedValue({
  ok: true,
  json: () => Promise.resolve({ id: 1, name: "Test" }),
});

Or make it return different values on successive calls:

typescript

const mockRandom = vi.fn()
  .mockReturnValueOnce(0.1)
  .mockReturnValueOnce(0.5)
  .mockReturnValueOnce(0.9);

vi.spyOn() — Watching Real Methods#

When you want to observe a method without replacing its behavior:

typescript

const consoleSpy = vi.spyOn(console, "warn");
 
validateInput("");
 
expect(consoleSpy).toHaveBeenCalledWith("Input cannot be empty");
consoleSpy.mockRestore();

spyOn keeps the original implementation by default. You can override it with .mockImplementation() when needed but restore the original afterward with .mockRestore().

vi.mock() — Module-Level Mocking#

This is the big one. vi.mock() replaces an entire module:

typescript

// Mock the entire module
vi.mock("@/lib/api", () => ({
  fetchUsers: vi.fn().mockResolvedValue([
    { id: 1, name: "Alice" },
    { id: 2, name: "Bob" },
  ]),
  fetchUser: vi.fn().mockResolvedValue({ id: 1, name: "Alice" }),
}));
 
// The import now uses the mocked version
import { fetchUsers } from "@/lib/api";
 
describe("UserList", () => {
  it("displays users from API", async () => {
    const users = await fetchUsers();
    expect(users).toHaveLength(2);
  });
});

Vitest hoists vi.mock() calls to the top of the file automatically. This means the mock is in place before any imports run. You don't need to worry about import order.

Automatic Mocking#

If you just want every export replaced with a vi.fn():

typescript

vi.mock("@/lib/analytics");
 
import { trackEvent, trackPageView } from "@/lib/analytics";
 
it("tracks form submission", () => {
  submitForm();
  expect(trackEvent).toHaveBeenCalledWith("form_submit", expect.any(Object));
});

Without the factory function, Vitest auto-mocks all exports. Each exported function becomes a vi.fn() that returns undefined. This is useful for modules you want to silence (like analytics or logging) without specifying every function.

Clearing vs Resetting vs Restoring#

This trips up everyone at some point:

typescript

const mockFn = vi.fn().mockReturnValue(42);
mockFn();
 
// mockClear — resets call history, keeps implementation
mockFn.mockClear();
expect(mockFn).not.toHaveBeenCalled(); // true
expect(mockFn()).toBe(42); // still returns 42
 
// mockReset — resets call history AND implementation
mockFn.mockReset();
expect(mockFn()).toBeUndefined(); // no longer returns 42
 
// mockRestore — for spies, restores original implementation
const spy = vi.spyOn(Math, "random").mockReturnValue(0.5);
spy.mockRestore();
// Math.random() now works normally again

In practice, use vi.clearAllMocks() in beforeEach to reset call history between tests. Use vi.restoreAllMocks() if you're using spyOn and want originals back:

typescript

beforeEach(() => {
  vi.clearAllMocks();
});
 
afterEach(() => {
  vi.restoreAllMocks();
});

The Danger of Over-Mocking#

This is the most important mocking advice I can give: every mock is a lie you're telling your test. When you mock a dependency, you're saying "I trust that this thing works correctly, so I'll replace it with a simplified version." If your assumption is wrong, the test passes but the feature is broken.

typescript

// Over-mocked — tests nothing useful
vi.mock("@/lib/validator");
vi.mock("@/lib/formatter");
vi.mock("@/lib/api");
vi.mock("@/lib/cache");
 
it("processes user input", () => {
  processInput("hello");
  expect(validator.validate).toHaveBeenCalledWith("hello");
  expect(formatter.format).toHaveBeenCalledWith("hello");
});

This test verifies that processInput calls validate and format. But what if processInput calls them in the wrong order? What if it ignores their return values? What if the validation is supposed to prevent the format step from running? The test doesn't know. You've mocked away all the interesting behavior.

The rule of thumb: mock at the boundaries, not in the middle. Mock network requests, file system access, and third-party services. Don't mock your own utility functions unless there's a compelling reason (like they're expensive to run or have side effects).

Testing React Components#

The Basics with Testing Library#

Testing Library enforces a philosophy: test components the way users interact with them. No checking internal state, no inspecting component instances, no shallow rendering. You render a component and interact with it through the DOM, just like a user would.

typescript

import { render, screen } from "@testing-library/react";
import { Button } from "@/components/ui/Button";
 
describe("Button", () => {
  it("renders with label text", () => {
    render(<Button>Click me</Button>);
    expect(screen.getByRole("button", { name: "Click me" })).toBeInTheDocument();
  });
 
  it("applies variant classes", () => {
    render(<Button variant="primary">Submit</Button>);
    const button = screen.getByRole("button");
    expect(button).toHaveClass("bg-primary");
  });
 
  it("is disabled when disabled prop is true", () => {
    render(<Button disabled>Submit</Button>);
    expect(screen.getByRole("button")).toBeDisabled();
  });
});

Queries: getBy vs queryBy vs findBy#

This is where beginners get confused. There are three query variants and each has a specific use case:

getBy* — Returns the element or throws if not found. Use when you expect the element to exist:

typescript

// Throws if no button found — test fails with a helpful error
const button = screen.getByRole("button", { name: "Submit" });

queryBy* — Returns the element or null if not found. Use when you're asserting something is NOT present:

typescript

// Returns null — doesn't throw
expect(screen.queryByText("Error message")).not.toBeInTheDocument();

findBy* — Returns a Promise. Use for elements that appear asynchronously:

typescript

// Waits up to 1000ms for the element to appear
const successMessage = await screen.findByText("Saved successfully");
expect(successMessage).toBeInTheDocument();

Accessibility-First Queries#

Testing Library provides these queries in a deliberate priority order:

getByRole — The best query. Uses ARIA roles. If your component isn't findable by role, it might have an accessibility problem.
getByLabelText — For form elements. If your input doesn't have a label, fix that first.
getByPlaceholderText — Acceptable but weaker. Placeholders disappear when the user types.
getByText — For non-interactive elements. Finds by visible text content.
getByTestId — Last resort. Use when no semantic query works.

typescript

// Prefer this
screen.getByRole("textbox", { name: "Email address" });
 
// Over this
screen.getByPlaceholderText("Enter your email");
 
// And definitely over this
screen.getByTestId("email-input");

The ranking isn't arbitrary. It matches how assistive technology navigates the page. If you can find an element by its role and accessible name, screen readers can too. If you can only find it by a test ID, you might have an accessibility gap.

User Events#

Don't use fireEvent. Use @testing-library/user-event. The difference matters:

typescript

import userEvent from "@testing-library/user-event";
 
describe("SearchInput", () => {
  it("filters results as user types", async () => {
    const user = userEvent.setup();
    const onSearch = vi.fn();
 
    render(<SearchInput onSearch={onSearch} />);
 
    const input = screen.getByRole("searchbox");
    await user.type(input, "vitest");
 
    // user.type fires keydown, keypress, input, keyup for EACH character
    // fireEvent.change just sets the value — skipping realistic event flow
    expect(onSearch).toHaveBeenLastCalledWith("vitest");
  });
 
  it("clears input on escape key", async () => {
    const user = userEvent.setup();
 
    render(<SearchInput onSearch={vi.fn()} />);
 
    const input = screen.getByRole("searchbox");
    await user.type(input, "hello");
    await user.keyboard("{Escape}");
 
    expect(input).toHaveValue("");
  });
});

userEvent simulates the full event chain that a real browser would fire. fireEvent.change is a single synthetic event. If your component listens to onKeyDown or uses onInput instead of onChange, fireEvent.change won't trigger those handlers but userEvent.type will.

Always call userEvent.setup() at the beginning and use the returned user instance. This ensures proper event ordering and state tracking.

Testing Component Interactions#

A realistic component test looks like this:

typescript

import { render, screen, within } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { TodoList } from "@/components/TodoList";
 
describe("TodoList", () => {
  it("adds a new todo item", async () => {
    const user = userEvent.setup();
    render(<TodoList />);
 
    const input = screen.getByRole("textbox", { name: /new todo/i });
    const addButton = screen.getByRole("button", { name: /add/i });
 
    await user.type(input, "Write tests");
    await user.click(addButton);
 
    expect(screen.getByText("Write tests")).toBeInTheDocument();
    expect(input).toHaveValue("");
  });
 
  it("marks a todo as completed", async () => {
    const user = userEvent.setup();
    render(<TodoList initialItems={[{ id: "1", text: "Buy groceries", done: false }]} />);
 
    const checkbox = screen.getByRole("checkbox", { name: /buy groceries/i });
    await user.click(checkbox);
 
    expect(checkbox).toBeChecked();
  });
 
  it("removes completed items when clear button is clicked", async () => {
    const user = userEvent.setup();
    render(
      <TodoList
        initialItems={[
          { id: "1", text: "Done task", done: true },
          { id: "2", text: "Pending task", done: false },
        ]}
      />
    );
 
    await user.click(screen.getByRole("button", { name: /clear completed/i }));
 
    expect(screen.queryByText("Done task")).not.toBeInTheDocument();
    expect(screen.getByText("Pending task")).toBeInTheDocument();
  });
});

Notice: no internal state inspection, no component.setState(), no checking implementation details. We render, we interact, we assert on what the user would see. If the component refactors its internal state management from useState to useReducer, these tests still pass. That's the point.

Testing Async Code#

waitFor#

When a component updates asynchronously, waitFor polls until the assertion passes:

typescript

import { render, screen, waitFor } from "@testing-library/react";
 
it("loads and displays user profile", async () => {
  render(<UserProfile userId="123" />);
 
  // Initially shows loading
  expect(screen.getByText("Loading...")).toBeInTheDocument();
 
  // Wait for content to appear
  await waitFor(() => {
    expect(screen.getByText("John Doe")).toBeInTheDocument();
  });
 
  // Loading indicator should be gone
  expect(screen.queryByText("Loading...")).not.toBeInTheDocument();
});

waitFor retries the callback every 50ms (by default) until it passes or times out (1000ms by default). You can customize both:

typescript

await waitFor(
  () => expect(screen.getByText("Done")).toBeInTheDocument(),
  { timeout: 3000, interval: 100 }
);

Fake Timers#

When testing code that uses setTimeout, setInterval, or Date, fake timers let you control time:

typescript

describe("Debounce", () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });
 
  afterEach(() => {
    vi.useRealTimers();
  });
 
  it("delays execution by the specified time", () => {
    const callback = vi.fn();
    const debounced = debounce(callback, 300);
 
    debounced();
    expect(callback).not.toHaveBeenCalled();
 
    vi.advanceTimersByTime(200);
    expect(callback).not.toHaveBeenCalled();
 
    vi.advanceTimersByTime(100);
    expect(callback).toHaveBeenCalledOnce();
  });
 
  it("resets timer on subsequent calls", () => {
    const callback = vi.fn();
    const debounced = debounce(callback, 300);
 
    debounced();
    vi.advanceTimersByTime(200);
 
    debounced(); // reset the timer
    vi.advanceTimersByTime(200);
    expect(callback).not.toHaveBeenCalled();
 
    vi.advanceTimersByTime(100);
    expect(callback).toHaveBeenCalledOnce();
  });
});

Important: always call vi.useRealTimers() in afterEach. Fake timers that leak between tests cause the most confusing failures you'll ever debug.

Testing with Fake Timers and Async Rendering#

Combining fake timers with React component testing requires care. React's internal scheduling uses real timers, so you often need to advance timers AND flush React updates together:

typescript

import { render, screen, act } from "@testing-library/react";
 
describe("Notification", () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });
 
  afterEach(() => {
    vi.useRealTimers();
  });
 
  it("auto-dismisses after 5 seconds", async () => {
    render(<Notification message="Saved!" autoDismiss={5000} />);
 
    expect(screen.getByText("Saved!")).toBeInTheDocument();
 
    // Advance timers inside act() to flush React updates
    await act(async () => {
      vi.advanceTimersByTime(5000);
    });
 
    expect(screen.queryByText("Saved!")).not.toBeInTheDocument();
  });
});

API Mocking with MSW#

For testing data fetching, Mock Service Worker (MSW) intercepts network requests at the network level. This means your component's fetch/axios code runs exactly as it would in production — MSW just replaces the network response:

typescript

import { http, HttpResponse } from "msw";
import { setupServer } from "msw/node";
import { render, screen } from "@testing-library/react";
 
const server = setupServer(
  http.get("/api/users", () => {
    return HttpResponse.json([
      { id: 1, name: "Alice", email: "alice@example.com" },
      { id: 2, name: "Bob", email: "bob@example.com" },
    ]);
  }),
  http.get("/api/users/:id", ({ params }) => {
    return HttpResponse.json({
      id: Number(params.id),
      name: "Alice",
      email: "alice@example.com",
    });
  })
);
 
beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());
 
describe("UserList", () => {
  it("displays users from API", async () => {
    render(<UserList />);
 
    expect(await screen.findByText("Alice")).toBeInTheDocument();
    expect(await screen.findByText("Bob")).toBeInTheDocument();
  });
 
  it("shows error state when API fails", async () => {
    // Override the default handler for this one test
    server.use(
      http.get("/api/users", () => {
        return new HttpResponse(null, { status: 500 });
      })
    );
 
    render(<UserList />);
 
    expect(await screen.findByText(/failed to load/i)).toBeInTheDocument();
  });
});

MSW is better than mocking fetch or axios directly because:

Your component's actual data fetching code runs — you test the real integration.
You can test error handling by overriding handlers per test.
The same handlers work in both tests and browser dev mode (Storybook, for instance).

Integration Tests#

Unit Tests vs Integration Tests#

A unit test isolates a single function or component and mocks everything else. An integration test lets multiple units work together and only mocks external boundaries (network, file system, databases).

The truth is: most of the bugs I've seen in production happen at the boundaries between units, not inside them. A function works perfectly in isolation but fails because the caller passes data in a slightly different format. A component renders fine with mock data but breaks when the actual API response has an extra nesting level.

Integration tests catch these bugs. They're slower than unit tests and harder to debug when they fail, but they give more confidence per test.

Testing Multiple Components Together#

typescript

import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { ShoppingCart } from "@/components/ShoppingCart";
import { CartProvider } from "@/contexts/CartContext";
 
// Only mock the API layer — everything else is real
vi.mock("@/lib/api", () => ({
  checkout: vi.fn().mockResolvedValue({ orderId: "ORD-123" }),
}));
 
describe("Shopping Cart Flow", () => {
  const renderCart = (initialItems = []) => {
    return render(
      <CartProvider initialItems={initialItems}>
        <ShoppingCart />
      </CartProvider>
    );
  };
 
  it("displays item count and total", () => {
    renderCart([
      { id: "1", name: "Keyboard", price: 79.99, quantity: 1 },
      { id: "2", name: "Mouse", price: 49.99, quantity: 2 },
    ]);
 
    expect(screen.getByText("3 items")).toBeInTheDocument();
    expect(screen.getByText("$179.97")).toBeInTheDocument();
  });
 
  it("updates quantity and recalculates total", async () => {
    const user = userEvent.setup();
 
    renderCart([
      { id: "1", name: "Keyboard", price: 79.99, quantity: 1 },
    ]);
 
    const incrementButton = screen.getByRole("button", { name: /increase quantity/i });
    await user.click(incrementButton);
 
    expect(screen.getByText("$159.98")).toBeInTheDocument();
  });
 
  it("completes checkout flow", async () => {
    const user = userEvent.setup();
    const { checkout } = await import("@/lib/api");
 
    renderCart([
      { id: "1", name: "Keyboard", price: 79.99, quantity: 1 },
    ]);
 
    await user.click(screen.getByRole("button", { name: /checkout/i }));
 
    expect(checkout).toHaveBeenCalledWith({
      items: [{ id: "1", quantity: 1 }],
    });
 
    expect(await screen.findByText(/order confirmed/i)).toBeInTheDocument();
    expect(screen.getByText("ORD-123")).toBeInTheDocument();
  });
});

In this test, ShoppingCart and CartProvider and their internal components (item rows, quantity selectors, totals display) all work together with real code. The only mock is the API call, because we don't want to make real network requests in tests.

When to Use Integration vs Unit Tests#

Use unit tests when:

The logic is complex and has many edge cases (a date parser, a state machine, a calculation).
You need fast feedback on a specific function's behavior.
The unit is relatively isolated and doesn't depend heavily on other units.

Use integration tests when:

Multiple components need to work together correctly (a form with validation and submission).
Data flows through several layers (context → component → child component).
You're testing a user workflow, not a function's return value.

In practice, a healthy test suite is heavy on integration tests for features and has unit tests for complex utilities. The components themselves are tested through integration tests — you don't need a separate unit test for every tiny component if the integration test exercises it.

Coverage#

Running Coverage#

bash

vitest run --coverage

You'll need a coverage provider. Vitest supports two:

bash

# V8 — faster, uses V8's built-in coverage
npm install -D @vitest/coverage-v8
 
# Istanbul — more mature, more configuration options
npm install -D @vitest/coverage-istanbul

Configure it in your Vitest config:

typescript

export default defineConfig({
  test: {
    coverage: {
      provider: "v8",
      reporter: ["text", "html", "lcov"],
      include: ["src/**/*.{ts,tsx}"],
      exclude: [
        "src/**/*.test.{ts,tsx}",
        "src/**/*.spec.{ts,tsx}",
        "src/test/**",
        "src/**/*.d.ts",
        "src/**/types.ts",
      ],
      thresholds: {
        statements: 80,
        branches: 75,
        functions: 80,
        lines: 80,
      },
    },
  },
});

Istanbul vs V8#

V8 coverage uses the V8 engine's built-in instrumentation. It's faster because there's no code transformation step. But it can be less accurate for some edge cases, especially around ES module boundaries.

Istanbul instruments your source code with counters before running tests. It's slower but more battle-tested and gives more accurate branch coverage. If you're enforcing coverage thresholds in CI, Istanbul's accuracy might matter.

I use V8 for local development (faster feedback) and Istanbul in CI (more accurate enforcement). You can configure different providers per environment if needed.

What Coverage Actually Means#

Coverage tells you which lines of code were executed during tests. That's it. It does not tell you whether those lines were tested correctly. Consider this:

typescript

function divide(a: number, b: number): number {
  return a / b;
}
 
it("divides numbers", () => {
  divide(10, 2);
  // No assertion!
});

This test gives 100% coverage of the divide function. It also tests absolutely nothing. The test would pass if divide returned null, threw an error, or launched missiles.

Coverage is a useful negative indicator: low coverage means there are definitely untested paths. But high coverage doesn't mean your code is well-tested. It just means every line ran during some test.

Lines vs Branches#

Line coverage is the most common metric but branch coverage is more valuable:

typescript

function getDiscount(user: User): number {
  if (user.isPremium) {
    return user.yearsActive > 5 ? 0.2 : 0.1;
  }
  return 0;
}

A test with getDiscount({ isPremium: true, yearsActive: 10 }) hits every line (100% line coverage) but only tests two of the three branches. The isPremium: false path and the yearsActive <= 5 path are untested.

Branch coverage catches this. It tracks every possible path through conditional logic. If you're going to enforce a coverage threshold, use branch coverage.

Ignoring Generated Code#

Some code shouldn't be counted in coverage. Generated files, type definitions, configuration — these inflate your metrics without adding value:

typescript

// vitest.config.ts
coverage: {
  exclude: [
    "src/**/*.d.ts",
    "src/**/types.ts",
    "src/**/*.stories.tsx",
    "src/generated/**",
    ".velite/**",
  ],
}

You can also ignore specific lines or blocks in your source code:

typescript

/* v8 ignore start */
if (process.env.NODE_ENV === "development") {
  console.log("Debug info:", data);
}
/* v8 ignore stop */
 
// Or for Istanbul
/* istanbul ignore next */
function devOnlyHelper() { /* ... */ }

Use this sparingly. If you find yourself ignoring large chunks of code, either those chunks need tests or they shouldn't be in the coverage report to begin with.

Testing Next.js#

Mocking next/navigation#

Next.js components that use useRouter, usePathname, or useSearchParams need mocks:

typescript

import { vi } from "vitest";
 
vi.mock("next/navigation", () => ({
  useRouter: () => ({
    push: vi.fn(),
    replace: vi.fn(),
    back: vi.fn(),
    prefetch: vi.fn(),
    refresh: vi.fn(),
  }),
  usePathname: () => "/en/blog",
  useSearchParams: () => new URLSearchParams("?page=1"),
  useParams: () => ({ locale: "en" }),
}));

For tests that need to verify navigation was called:

typescript

import { useRouter } from "next/navigation";
 
vi.mock("next/navigation", () => ({
  useRouter: vi.fn(),
}));
 
describe("LogoutButton", () => {
  it("redirects to home after logout", async () => {
    const mockPush = vi.fn();
    vi.mocked(useRouter).mockReturnValue({
      push: mockPush,
      replace: vi.fn(),
      back: vi.fn(),
      prefetch: vi.fn(),
      refresh: vi.fn(),
      forward: vi.fn(),
    });
 
    const user = userEvent.setup();
    render(<LogoutButton />);
 
    await user.click(screen.getByRole("button", { name: /log out/i }));
 
    expect(mockPush).toHaveBeenCalledWith("/");
  });
});

Mocking next-intl#

For internationalized components using next-intl:

typescript

vi.mock("next-intl", () => ({
  useTranslations: () => (key: string) => key,
  useLocale: () => "en",
}));

This is the simplest approach — translations return the key itself, so t("hero.title") returns "hero.title". In assertions, you check for the translation key rather than the actual translated string. This makes tests language-independent.

If you need actual translations in a specific test:

typescript

vi.mock("next-intl", () => ({
  useTranslations: () => {
    const translations: Record<string, string> = {
      "hero.title": "Welcome to My Site",
      "hero.subtitle": "Building things for the web",
    };
    return (key: string) => translations[key] ?? key;
  },
}));

Testing Route Handlers#

Next.js Route Handlers are regular functions that take a Request and return a Response. They're straightforward to test:

typescript

import { GET, POST } from "@/app/api/users/route";
import { NextRequest } from "next/server";
 
describe("GET /api/users", () => {
  it("returns users list", async () => {
    const request = new NextRequest("http://localhost:3000/api/users");
    const response = await GET(request);
    const data = await response.json();
 
    expect(response.status).toBe(200);
    expect(data).toEqual(
      expect.arrayContaining([
        expect.objectContaining({ id: expect.any(Number), name: expect.any(String) }),
      ])
    );
  });
 
  it("supports pagination via search params", async () => {
    const request = new NextRequest("http://localhost:3000/api/users?page=2&limit=10");
    const response = await GET(request);
    const data = await response.json();
 
    expect(data.page).toBe(2);
    expect(data.items).toHaveLength(10);
  });
});
 
describe("POST /api/users", () => {
  it("creates a new user", async () => {
    const request = new NextRequest("http://localhost:3000/api/users", {
      method: "POST",
      body: JSON.stringify({ name: "Alice", email: "alice@test.com" }),
    });
 
    const response = await POST(request);
    const data = await response.json();
 
    expect(response.status).toBe(201);
    expect(data.name).toBe("Alice");
  });
 
  it("returns 400 for invalid body", async () => {
    const request = new NextRequest("http://localhost:3000/api/users", {
      method: "POST",
      body: JSON.stringify({ name: "" }),
    });
 
    const response = await POST(request);
    expect(response.status).toBe(400);
  });
});

Testing Middleware#

Next.js middleware runs at the edge and processes every request. Test it as a function:

typescript

import { middleware } from "@/middleware";
import { NextRequest } from "next/server";
 
function createRequest(path: string, headers: Record<string, string> = {}): NextRequest {
  const url = new URL(path, "http://localhost:3000");
  return new NextRequest(url, { headers });
}
 
describe("middleware", () => {
  it("redirects unauthenticated users from protected routes", async () => {
    const request = createRequest("/dashboard");
    const response = await middleware(request);
 
    expect(response.status).toBe(307);
    expect(response.headers.get("location")).toContain("/login");
  });
 
  it("allows authenticated users through", async () => {
    const request = createRequest("/dashboard", {
      cookie: "session=valid-token",
    });
    const response = await middleware(request);
 
    expect(response.status).toBe(200);
  });
 
  it("adds security headers", async () => {
    const request = createRequest("/");
    const response = await middleware(request);
 
    expect(response.headers.get("x-frame-options")).toBe("DENY");
    expect(response.headers.get("x-content-type-options")).toBe("nosniff");
  });
 
  it("handles locale detection", async () => {
    const request = createRequest("/", {
      "accept-language": "tr-TR,tr;q=0.9,en;q=0.8",
    });
    const response = await middleware(request);
 
    expect(response.headers.get("location")).toContain("/tr");
  });
});

Testing Server Actions#

Server Actions are async functions that run on the server. Since they're just functions, you can test them directly — but you may need to mock server-only dependencies:

typescript

vi.mock("@/lib/db", () => ({
  db: {
    user: {
      update: vi.fn().mockResolvedValue({ id: "1", name: "Updated" }),
      findUnique: vi.fn().mockResolvedValue({ id: "1", name: "Original" }),
    },
  },
}));
 
vi.mock("next/cache", () => ({
  revalidatePath: vi.fn(),
  revalidateTag: vi.fn(),
}));
 
import { updateProfile } from "@/app/actions/profile";
import { revalidatePath } from "next/cache";
 
describe("updateProfile", () => {
  it("updates user and revalidates profile page", async () => {
    const formData = new FormData();
    formData.set("name", "New Name");
    formData.set("bio", "New bio text");
 
    const result = await updateProfile(formData);
 
    expect(result.success).toBe(true);
    expect(revalidatePath).toHaveBeenCalledWith("/profile");
  });
 
  it("returns error for invalid data", async () => {
    const formData = new FormData();
    // Missing required fields
 
    const result = await updateProfile(formData);
 
    expect(result.success).toBe(false);
    expect(result.error).toBeDefined();
  });
});

Practical Patterns#

Custom Render Function#

Most projects need the same providers wrapped around every component. Create a custom render:

typescript

// src/test/utils.tsx
import { render, type RenderOptions } from "@testing-library/react";
import { ReactElement } from "react";
import { ThemeProvider } from "@/contexts/ThemeContext";
import { CartProvider } from "@/contexts/CartContext";
 
interface CustomRenderOptions extends RenderOptions {
  theme?: "light" | "dark";
  initialCartItems?: CartItem[];
}
 
function AllProviders({ children, theme = "light", initialCartItems = [] }: {
  children: React.ReactNode;
  theme?: "light" | "dark";
  initialCartItems?: CartItem[];
}) {
  return (
    <ThemeProvider defaultTheme={theme}>
      <CartProvider initialItems={initialCartItems}>
        {children}
      </CartProvider>
    </ThemeProvider>
  );
}
 
export function renderWithProviders(
  ui: ReactElement,
  options: CustomRenderOptions = {}
) {
  const { theme, initialCartItems, ...renderOptions } = options;
 
  return render(ui, {
    wrapper: ({ children }) => (
      <AllProviders theme={theme} initialCartItems={initialCartItems}>
        {children}
      </AllProviders>
    ),
    ...renderOptions,
  });
}
 
// Re-export everything from testing library
export * from "@testing-library/react";
export { renderWithProviders as render };

Now every test file imports from your custom utils instead of @testing-library/react:

typescript

import { render, screen } from "@/test/utils";
 
it("renders in dark mode", () => {
  render(<Header />, { theme: "dark" });
  // Header and all its children have access to ThemeProvider and CartProvider
});

Testing Custom Hooks#

Vitest works with @testing-library/react's renderHook:

typescript

import { renderHook, act } from "@testing-library/react";
import { useCounter } from "@/hooks/useCounter";
 
describe("useCounter", () => {
  it("starts at initial value", () => {
    const { result } = renderHook(() => useCounter(10));
    expect(result.current.count).toBe(10);
  });
 
  it("increments", () => {
    const { result } = renderHook(() => useCounter(0));
 
    act(() => {
      result.current.increment();
    });
 
    expect(result.current.count).toBe(1);
  });
 
  it("decrements with floor", () => {
    const { result } = renderHook(() => useCounter(0, { min: 0 }));
 
    act(() => {
      result.current.decrement();
    });
 
    expect(result.current.count).toBe(0); // doesn't go below min
  });
});

Testing Error Boundaries#

typescript

import { render, screen } from "@testing-library/react";
import { ErrorBoundary } from "@/components/ErrorBoundary";
 
const ThrowingComponent = () => {
  throw new Error("Test explosion");
};
 
describe("ErrorBoundary", () => {
  // Suppress console.error for expected errors
  beforeEach(() => {
    vi.spyOn(console, "error").mockImplementation(() => {});
  });
 
  afterEach(() => {
    vi.restoreAllMocks();
  });
 
  it("displays fallback UI when child throws", () => {
    render(
      <ErrorBoundary fallback={<div>Something went wrong</div>}>
        <ThrowingComponent />
      </ErrorBoundary>
    );
 
    expect(screen.getByText("Something went wrong")).toBeInTheDocument();
  });
 
  it("renders children when no error", () => {
    render(
      <ErrorBoundary fallback={<div>Error</div>}>
        <div>All good</div>
      </ErrorBoundary>
    );
 
    expect(screen.getByText("All good")).toBeInTheDocument();
    expect(screen.queryByText("Error")).not.toBeInTheDocument();
  });
});

Snapshot Testing (Carefully)#

Snapshot tests have a bad reputation because people use them as a substitute for real assertions. A snapshot of an entire component's HTML output is a maintenance burden — it breaks on every CSS class change and nobody reviews the diff carefully.

But targeted snapshots can be useful:

typescript

import { render } from "@testing-library/react";
import { formatCurrency } from "@/lib/format";
 
// Good — small, targeted snapshot of a pure function's output
it("formats various currency values consistently", () => {
  expect(formatCurrency(0)).toMatchInlineSnapshot('"$0.00"');
  expect(formatCurrency(1234.5)).toMatchInlineSnapshot('"$1,234.50"');
  expect(formatCurrency(-99.99)).toMatchInlineSnapshot('"-$99.99"');
  expect(formatCurrency(1000000)).toMatchInlineSnapshot('"$1,000,000.00"');
});
 
// Bad — giant snapshot nobody will review
it("renders the dashboard", () => {
  const { container } = render(<Dashboard />);
  expect(container).toMatchSnapshot(); // Don't do this
});

Inline snapshots (toMatchInlineSnapshot) are better than file snapshots because the expected value is visible right in the test. You can see at a glance what the function returns without opening a separate .snap file.

The Testing Philosophy#

Test Behavior, Not Implementation#

This principle is so important that it's worth a dedicated section. Consider two tests for the same feature:

typescript

// Implementation test — brittle, breaks on refactors
it("calls setState with new count", () => {
  const setStateSpy = vi.spyOn(React, "useState");
  render(<Counter />);
  fireEvent.click(screen.getByText("+"));
  expect(setStateSpy).toHaveBeenCalledWith(expect.any(Function));
});
 
// Behavior test — resilient, tests what the user sees
it("increments the displayed count when plus button is clicked", async () => {
  const user = userEvent.setup();
  render(<Counter />);
  expect(screen.getByText("Count: 0")).toBeInTheDocument();
 
  await user.click(screen.getByRole("button", { name: "+" }));
 
  expect(screen.getByText("Count: 1")).toBeInTheDocument();
});

The first test breaks if you switch from useState to useReducer, even though the component works exactly the same. The second test only breaks if the component's behavior actually changes. It doesn't care how the count is managed internally — only that clicking "+" makes the number go up.

The litmus test is simple: can you refactor the implementation without changing the test? If yes, you're testing behavior. If no, you're testing implementation.

The Testing Trophy#

Kent C. Dodds proposed the "Testing Trophy" as an alternative to the traditional testing pyramid:

    ╭─────────╮
    │  E2E    │   Few — expensive, slow, high confidence
    ├─────────┤
    │         │
    │ Integr. │   Most — good confidence-to-cost ratio
    │         │
    ├─────────┤
    │  Unit   │   Some — fast, focused, low-cost
    ├─────────┤
    │ Static  │   Always — TypeScript, ESLint
    ╰─────────╯

The traditional pyramid puts unit tests at the bottom (lots of them) and integration tests in the middle (fewer). The trophy inverts this: integration tests are the sweet spot. Here's why:

Static analysis (TypeScript, ESLint) catches typos, wrong types, and simple logical errors for free. You don't even have to run anything.
Unit tests are great for complex pure logic but don't tell you if the pieces work together.
Integration tests verify that components, hooks, and contexts work together. They give the most confidence per test written.
End-to-end tests verify the entire system but are slow, flaky, and expensive to maintain. You need a few for critical paths but not hundreds.

I follow this distribution in practice: TypeScript catches most of my type errors, I write unit tests for complex utilities and algorithms, integration tests for features and user flows, and a handful of E2E tests for the critical path (sign up, purchase, core workflow).

What Gives Confidence vs What Wastes Time#

Tests exist to give you confidence to ship. Not confidence that every line of code runs — confidence that the application works for users. These are different things.

High confidence, high value:

Integration test of a checkout flow — covers form validation, API calls, state updates, and success/error UI.
Unit test of a price calculation function with edge cases — floating point, rounding, discounts, zero/negative values.
Test that protected routes redirect unauthenticated users.

Low confidence, time wasters:

Snapshot test of a static marketing page — breaks every time copy changes, catches nothing meaningful.
Unit test that a component passes a prop to a child — testing React itself, not your code.
Test that useState is called — testing the framework, not behavior.
100% coverage of a configuration file — it's static data, TypeScript already validates its shape.

The question to ask before writing a test: "If this test didn't exist, what bug could slip into production?" If the answer is "none that TypeScript wouldn't catch" or "none that anyone would notice," the test probably isn't worth writing.

Testing as Design Feedback#

Hard-to-test code is usually poorly designed code. If you need to mock five things to test one function, that function has too many dependencies. If you can't render a component without setting up elaborate context providers, the component is too coupled to its environment.

Tests are a user of your code. If your tests struggle to use your API, other developers will too. When you find yourself fighting the test setup, take it as a signal to refactor the code under test, not to add more mocks.

typescript

// Hard to test — function does too much
async function processOrder(orderId: string) {
  const order = await db.orders.findById(orderId);
  const user = await db.users.findById(order.userId);
  const inventory = await checkInventory(order.items);
  if (!inventory.available) {
    await sendEmail(user.email, "out-of-stock", { items: inventory.unavailable });
    return { success: false, reason: "out-of-stock" };
  }
  const payment = await chargeCard(user.paymentMethod, order.total);
  if (!payment.success) {
    await sendEmail(user.email, "payment-failed", { error: payment.error });
    return { success: false, reason: "payment-failed" };
  }
  await db.orders.update(orderId, { status: "confirmed" });
  await sendEmail(user.email, "order-confirmed", { orderId });
  return { success: true };
}
 
// Easier to test — separated concerns
function determineOrderAction(
  inventory: InventoryResult,
  payment: PaymentResult
): OrderAction {
  if (!inventory.available) return { type: "out-of-stock", items: inventory.unavailable };
  if (!payment.success) return { type: "payment-failed", error: payment.error };
  return { type: "confirmed" };
}

The second version is a pure function. You can test every combination of inventory and payment results without mocking a database, payment provider, or email service. The orchestration logic (fetching data, sending emails) lives in a thin layer that's tested at the integration level.

This is the real value of testing: not catching bugs after they're written, but preventing bad designs before they're committed. The discipline of writing tests pushes you toward smaller functions, clearer interfaces, and more modular architecture. The tests are the side effect. The design improvement is the main event.

Write Tests You Trust#

The worst thing that can happen to a test suite isn't that it has gaps. It's that people stop trusting it. A test suite with a few flaky tests that randomly fail on CI teaches the team to ignore red builds. Once that happens, the test suite is worse than useless — it actively provides false security.

If a test fails intermittently, fix it or delete it. If a test is slow, speed it up or move it to a separate slow-test suite. If a test breaks on every unrelated change, rewrite it to test behavior instead of implementation.

The goal is a test suite where every failure means something real is broken. When developers trust the tests, they run them before every commit. When they don't trust the tests, they bypass them with --no-verify and deploy with crossed fingers.

Build a test suite you'd bet your weekend on. Nothing less is worth maintaining.

This post is everything I know about testing with Vitest, from initial setup to the philosophy that makes it all worthwhile.

Why Vitest Over Jest#

Native ESM Support#

Same Config as Your Dev Server#

Speed#

Benchmarking Built In#

Vitest includes bench() out of the box for performance testing. No separate library needed:

typescript

import { bench, describe } from "vitest";
 
describe("string concatenation", () => {
  bench("template literals", () => {
    const name = "world";
    const _result = `hello ${name}`;
  });
 
  bench("string concat", () => {
    const name = "world";
    const _result = "hello " + name;
  });
});

Run with vitest bench. It's not the main event, but it's nice to have performance testing in the same toolchain without installing benchmark.js and wiring up a separate runner.

Setup#

Installation#

bash

npm install -D vitest @testing-library/react @testing-library/jest-dom @testing-library/user-event jsdom

Configuration#

Create vitest.config.ts at your project root:

typescript

import { defineConfig } from "vitest/config";
import react from "@vitejs/plugin-react";
import path from "path";
 
export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./src/test/setup.ts"],
    include: ["src/**/*.{test,spec}.{ts,tsx}"],
    alias: {
      "@": path.resolve(__dirname, "./src"),
    },
    css: false,
  },
});

Or, if you already have a vite.config.ts, you can extend it:

typescript

/// <reference types="vitest/config" />
import { defineConfig } from "vite";
import react from "@vitejs/plugin-react";
 
export default defineConfig({
  plugins: [react()],
  resolve: {
    alias: {
      "@": "/src",
    },
  },
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./src/test/setup.ts"],
  },
});

The `globals: true` Decision#

When globals is true, you don't need to import describe, it, expect, beforeEach, etc. — they're available everywhere, just like in Jest. When false, you import them explicitly:

typescript

// globals: false
import { describe, it, expect } from "vitest";
 
describe("math", () => {
  it("adds numbers", () => {
    expect(1 + 1).toBe(2);
  });
});

I use globals: true because it reduces visual noise and matches what most developers expect. If you're on a team that values explicit imports, set it to false — there's no wrong answer here.

If you use globals: true, add Vitest's types to your tsconfig.json so TypeScript recognizes them:

json

{
  "compilerOptions": {
    "types": ["vitest/globals"]
  }
}

Environment: jsdom vs happy-dom vs node#

Vitest lets you pick the DOM implementation per test or globally:

node — No DOM. For pure logic, utilities, API routes, and anything that doesn't touch the browser.
jsdom — The standard. Full DOM implementation. Heavier but more complete.
happy-dom — Lighter and faster than jsdom but less complete. Some edge cases (like Range, Selection, or IntersectionObserver) may not work.

I default to jsdom globally and override per-file when I need node:

typescript

// src/lib/utils.test.ts
// @vitest-environment node
 
import { formatDate, slugify } from "./utils";
 
describe("slugify", () => {
  it("converts spaces to hyphens", () => {
    expect(slugify("hello world")).toBe("hello-world");
  });
});

Setup Files#

The setup file runs before every test file. This is where you configure Testing Library matchers and global mocks:

typescript

// src/test/setup.ts
import "@testing-library/jest-dom/vitest";
 
// Mock IntersectionObserver — jsdom doesn't implement it
class MockIntersectionObserver {
  observe = vi.fn();
  unobserve = vi.fn();
  disconnect = vi.fn();
}
 
Object.defineProperty(window, "IntersectionObserver", {
  writable: true,
  value: MockIntersectionObserver,
});
 
// Mock window.matchMedia — needed for responsive components
Object.defineProperty(window, "matchMedia", {
  writable: true,
  value: vi.fn().mockImplementation((query: string) => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: vi.fn(),
    removeListener: vi.fn(),
    addEventListener: vi.fn(),
    removeEventListener: vi.fn(),
    dispatchEvent: vi.fn(),
  })),
});

Writing Good Tests#

The AAA Pattern#

Every test follows the same structure: Arrange, Act, Assert. Even when you don't write explicit comments, the structure should be visible:

typescript

it("calculates total with tax", () => {
  // Arrange
  const items = [
    { name: "Widget", price: 10 },
    { name: "Gadget", price: 20 },
  ];
  const taxRate = 0.08;
 
  // Act
  const total = calculateTotal(items, taxRate);
 
  // Assert
  expect(total).toBe(32.4);
});

Test Naming#

There are two schools: it('should calculate total with tax') and it('calculates total with tax'). The "should" prefix is verbose without adding information. When the test fails, you'll see:

FAIL  ✕ calculates total with tax

For describe blocks, use the name of the unit under test:

typescript

describe("calculateTotal", () => {
  it("sums item prices", () => { /* ... */ });
  it("applies tax rate", () => { /* ... */ });
  it("returns 0 for empty array", () => { /* ... */ });
  it("handles negative prices", () => { /* ... */ });
});

Read it out loud: "calculateTotal sums item prices." "calculateTotal applies tax rate." If the sentence works, the naming works.

One Assertion Per Test vs Practical Grouping#

The purist rule says one assertion per test. The practical rule says: one concept per test. These are different.

typescript

// This is fine — one concept, multiple assertions
it("formats user display name", () => {
  const user = { firstName: "John", lastName: "Doe", title: "Dr." };
  const result = formatDisplayName(user);
 
  expect(result).toContain("John");
  expect(result).toContain("Doe");
  expect(result).toStartWith("Dr.");
});
 
// This is not fine — multiple concepts in one test
it("handles user operations", () => {
  const user = createUser("John");
  expect(user.id).toBeDefined();
 
  const updated = updateUser(user.id, { name: "Jane" });
  expect(updated.name).toBe("Jane");
 
  deleteUser(user.id);
  expect(getUser(user.id)).toBeNull();
});

Test Descriptions as Documentation#

Good test suites serve as living documentation. Someone unfamiliar with the code should be able to read the test descriptions and understand the feature's behavior:

typescript

describe("PasswordValidator", () => {
  describe("minimum length", () => {
    it("rejects passwords shorter than 8 characters", () => { /* ... */ });
    it("accepts passwords with exactly 8 characters", () => { /* ... */ });
  });
 
  describe("character requirements", () => {
    it("requires at least one uppercase letter", () => { /* ... */ });
    it("requires at least one number", () => { /* ... */ });
    it("requires at least one special character", () => { /* ... */ });
  });
 
  describe("common password check", () => {
    it("rejects passwords in the common passwords list", () => { /* ... */ });
    it("performs case-insensitive comparison", () => { /* ... */ });
  });
});

When this test suite runs, the output reads like a specification. That's the goal.

Mocking#

vi.fn() — Creating Mock Functions#

The simplest mock is a function that records its calls:

typescript

const mockCallback = vi.fn();
 
// Call it
mockCallback("hello", 42);
mockCallback("world");
 
// Assert on calls
expect(mockCallback).toHaveBeenCalledTimes(2);
expect(mockCallback).toHaveBeenCalledWith("hello", 42);
expect(mockCallback).toHaveBeenLastCalledWith("world");

You can give it a return value:

typescript

const mockFetch = vi.fn().mockResolvedValue({
  ok: true,
  json: () => Promise.resolve({ id: 1, name: "Test" }),
});

Or make it return different values on successive calls:

typescript

const mockRandom = vi.fn()
  .mockReturnValueOnce(0.1)
  .mockReturnValueOnce(0.5)
  .mockReturnValueOnce(0.9);

vi.spyOn() — Watching Real Methods#

When you want to observe a method without replacing its behavior:

typescript

const consoleSpy = vi.spyOn(console, "warn");
 
validateInput("");
 
expect(consoleSpy).toHaveBeenCalledWith("Input cannot be empty");
consoleSpy.mockRestore();

spyOn keeps the original implementation by default. You can override it with .mockImplementation() when needed but restore the original afterward with .mockRestore().

vi.mock() — Module-Level Mocking#

This is the big one. vi.mock() replaces an entire module:

typescript

// Mock the entire module
vi.mock("@/lib/api", () => ({
  fetchUsers: vi.fn().mockResolvedValue([
    { id: 1, name: "Alice" },
    { id: 2, name: "Bob" },
  ]),
  fetchUser: vi.fn().mockResolvedValue({ id: 1, name: "Alice" }),
}));
 
// The import now uses the mocked version
import { fetchUsers } from "@/lib/api";
 
describe("UserList", () => {
  it("displays users from API", async () => {
    const users = await fetchUsers();
    expect(users).toHaveLength(2);
  });
});

Vitest hoists vi.mock() calls to the top of the file automatically. This means the mock is in place before any imports run. You don't need to worry about import order.

Automatic Mocking#

If you just want every export replaced with a vi.fn():

typescript

vi.mock("@/lib/analytics");
 
import { trackEvent, trackPageView } from "@/lib/analytics";
 
it("tracks form submission", () => {
  submitForm();
  expect(trackEvent).toHaveBeenCalledWith("form_submit", expect.any(Object));
});

Clearing vs Resetting vs Restoring#

This trips up everyone at some point:

typescript

const mockFn = vi.fn().mockReturnValue(42);
mockFn();
 
// mockClear — resets call history, keeps implementation
mockFn.mockClear();
expect(mockFn).not.toHaveBeenCalled(); // true
expect(mockFn()).toBe(42); // still returns 42
 
// mockReset — resets call history AND implementation
mockFn.mockReset();
expect(mockFn()).toBeUndefined(); // no longer returns 42
 
// mockRestore — for spies, restores original implementation
const spy = vi.spyOn(Math, "random").mockReturnValue(0.5);
spy.mockRestore();
// Math.random() now works normally again

In practice, use vi.clearAllMocks() in beforeEach to reset call history between tests. Use vi.restoreAllMocks() if you're using spyOn and want originals back:

typescript

beforeEach(() => {
  vi.clearAllMocks();
});
 
afterEach(() => {
  vi.restoreAllMocks();
});

The Danger of Over-Mocking#

typescript

// Over-mocked — tests nothing useful
vi.mock("@/lib/validator");
vi.mock("@/lib/formatter");
vi.mock("@/lib/api");
vi.mock("@/lib/cache");
 
it("processes user input", () => {
  processInput("hello");
  expect(validator.validate).toHaveBeenCalledWith("hello");
  expect(formatter.format).toHaveBeenCalledWith("hello");
});

Testing React Components#

The Basics with Testing Library#

typescript

import { render, screen } from "@testing-library/react";
import { Button } from "@/components/ui/Button";
 
describe("Button", () => {
  it("renders with label text", () => {
    render(<Button>Click me</Button>);
    expect(screen.getByRole("button", { name: "Click me" })).toBeInTheDocument();
  });
 
  it("applies variant classes", () => {
    render(<Button variant="primary">Submit</Button>);
    const button = screen.getByRole("button");
    expect(button).toHaveClass("bg-primary");
  });
 
  it("is disabled when disabled prop is true", () => {
    render(<Button disabled>Submit</Button>);
    expect(screen.getByRole("button")).toBeDisabled();
  });
});

Queries: getBy vs queryBy vs findBy#

This is where beginners get confused. There are three query variants and each has a specific use case:

getBy* — Returns the element or throws if not found. Use when you expect the element to exist:

typescript

// Throws if no button found — test fails with a helpful error
const button = screen.getByRole("button", { name: "Submit" });

queryBy* — Returns the element or null if not found. Use when you're asserting something is NOT present:

typescript

// Returns null — doesn't throw
expect(screen.queryByText("Error message")).not.toBeInTheDocument();

findBy* — Returns a Promise. Use for elements that appear asynchronously:

typescript

// Waits up to 1000ms for the element to appear
const successMessage = await screen.findByText("Saved successfully");
expect(successMessage).toBeInTheDocument();

Accessibility-First Queries#

Testing Library provides these queries in a deliberate priority order:

getByRole — The best query. Uses ARIA roles. If your component isn't findable by role, it might have an accessibility problem.
getByLabelText — For form elements. If your input doesn't have a label, fix that first.
getByPlaceholderText — Acceptable but weaker. Placeholders disappear when the user types.
getByText — For non-interactive elements. Finds by visible text content.
getByTestId — Last resort. Use when no semantic query works.

typescript

// Prefer this
screen.getByRole("textbox", { name: "Email address" });
 
// Over this
screen.getByPlaceholderText("Enter your email");
 
// And definitely over this
screen.getByTestId("email-input");

User Events#

Don't use fireEvent. Use @testing-library/user-event. The difference matters:

typescript

import userEvent from "@testing-library/user-event";
 
describe("SearchInput", () => {
  it("filters results as user types", async () => {
    const user = userEvent.setup();
    const onSearch = vi.fn();
 
    render(<SearchInput onSearch={onSearch} />);
 
    const input = screen.getByRole("searchbox");
    await user.type(input, "vitest");
 
    // user.type fires keydown, keypress, input, keyup for EACH character
    // fireEvent.change just sets the value — skipping realistic event flow
    expect(onSearch).toHaveBeenLastCalledWith("vitest");
  });
 
  it("clears input on escape key", async () => {
    const user = userEvent.setup();
 
    render(<SearchInput onSearch={vi.fn()} />);
 
    const input = screen.getByRole("searchbox");
    await user.type(input, "hello");
    await user.keyboard("{Escape}");
 
    expect(input).toHaveValue("");
  });
});

Always call userEvent.setup() at the beginning and use the returned user instance. This ensures proper event ordering and state tracking.

Testing Component Interactions#

A realistic component test looks like this:

typescript

import { render, screen, within } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { TodoList } from "@/components/TodoList";
 
describe("TodoList", () => {
  it("adds a new todo item", async () => {
    const user = userEvent.setup();
    render(<TodoList />);
 
    const input = screen.getByRole("textbox", { name: /new todo/i });
    const addButton = screen.getByRole("button", { name: /add/i });
 
    await user.type(input, "Write tests");
    await user.click(addButton);
 
    expect(screen.getByText("Write tests")).toBeInTheDocument();
    expect(input).toHaveValue("");
  });
 
  it("marks a todo as completed", async () => {
    const user = userEvent.setup();
    render(<TodoList initialItems={[{ id: "1", text: "Buy groceries", done: false }]} />);
 
    const checkbox = screen.getByRole("checkbox", { name: /buy groceries/i });
    await user.click(checkbox);
 
    expect(checkbox).toBeChecked();
  });
 
  it("removes completed items when clear button is clicked", async () => {
    const user = userEvent.setup();
    render(
      <TodoList
        initialItems={[
          { id: "1", text: "Done task", done: true },
          { id: "2", text: "Pending task", done: false },
        ]}
      />
    );
 
    await user.click(screen.getByRole("button", { name: /clear completed/i }));
 
    expect(screen.queryByText("Done task")).not.toBeInTheDocument();
    expect(screen.getByText("Pending task")).toBeInTheDocument();
  });
});

Testing Async Code#

waitFor#

When a component updates asynchronously, waitFor polls until the assertion passes:

typescript

import { render, screen, waitFor } from "@testing-library/react";
 
it("loads and displays user profile", async () => {
  render(<UserProfile userId="123" />);
 
  // Initially shows loading
  expect(screen.getByText("Loading...")).toBeInTheDocument();
 
  // Wait for content to appear
  await waitFor(() => {
    expect(screen.getByText("John Doe")).toBeInTheDocument();
  });
 
  // Loading indicator should be gone
  expect(screen.queryByText("Loading...")).not.toBeInTheDocument();
});

waitFor retries the callback every 50ms (by default) until it passes or times out (1000ms by default). You can customize both:

typescript

await waitFor(
  () => expect(screen.getByText("Done")).toBeInTheDocument(),
  { timeout: 3000, interval: 100 }
);

Fake Timers#

When testing code that uses setTimeout, setInterval, or Date, fake timers let you control time:

typescript

describe("Debounce", () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });
 
  afterEach(() => {
    vi.useRealTimers();
  });
 
  it("delays execution by the specified time", () => {
    const callback = vi.fn();
    const debounced = debounce(callback, 300);
 
    debounced();
    expect(callback).not.toHaveBeenCalled();
 
    vi.advanceTimersByTime(200);
    expect(callback).not.toHaveBeenCalled();
 
    vi.advanceTimersByTime(100);
    expect(callback).toHaveBeenCalledOnce();
  });
 
  it("resets timer on subsequent calls", () => {
    const callback = vi.fn();
    const debounced = debounce(callback, 300);
 
    debounced();
    vi.advanceTimersByTime(200);
 
    debounced(); // reset the timer
    vi.advanceTimersByTime(200);
    expect(callback).not.toHaveBeenCalled();
 
    vi.advanceTimersByTime(100);
    expect(callback).toHaveBeenCalledOnce();
  });
});

Important: always call vi.useRealTimers() in afterEach. Fake timers that leak between tests cause the most confusing failures you'll ever debug.

Testing with Fake Timers and Async Rendering#

Combining fake timers with React component testing requires care. React's internal scheduling uses real timers, so you often need to advance timers AND flush React updates together:

typescript

import { render, screen, act } from "@testing-library/react";
 
describe("Notification", () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });
 
  afterEach(() => {
    vi.useRealTimers();
  });
 
  it("auto-dismisses after 5 seconds", async () => {
    render(<Notification message="Saved!" autoDismiss={5000} />);
 
    expect(screen.getByText("Saved!")).toBeInTheDocument();
 
    // Advance timers inside act() to flush React updates
    await act(async () => {
      vi.advanceTimersByTime(5000);
    });
 
    expect(screen.queryByText("Saved!")).not.toBeInTheDocument();
  });
});

API Mocking with MSW#

typescript

import { http, HttpResponse } from "msw";
import { setupServer } from "msw/node";
import { render, screen } from "@testing-library/react";
 
const server = setupServer(
  http.get("/api/users", () => {
    return HttpResponse.json([
      { id: 1, name: "Alice", email: "alice@example.com" },
      { id: 2, name: "Bob", email: "bob@example.com" },
    ]);
  }),
  http.get("/api/users/:id", ({ params }) => {
    return HttpResponse.json({
      id: Number(params.id),
      name: "Alice",
      email: "alice@example.com",
    });
  })
);
 
beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());
 
describe("UserList", () => {
  it("displays users from API", async () => {
    render(<UserList />);
 
    expect(await screen.findByText("Alice")).toBeInTheDocument();
    expect(await screen.findByText("Bob")).toBeInTheDocument();
  });
 
  it("shows error state when API fails", async () => {
    // Override the default handler for this one test
    server.use(
      http.get("/api/users", () => {
        return new HttpResponse(null, { status: 500 });
      })
    );
 
    render(<UserList />);
 
    expect(await screen.findByText(/failed to load/i)).toBeInTheDocument();
  });
});

MSW is better than mocking fetch or axios directly because:

Your component's actual data fetching code runs — you test the real integration.
You can test error handling by overriding handlers per test.
The same handlers work in both tests and browser dev mode (Storybook, for instance).

Integration Tests#

Unit Tests vs Integration Tests#

Integration tests catch these bugs. They're slower than unit tests and harder to debug when they fail, but they give more confidence per test.

Testing Multiple Components Together#

typescript

import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { ShoppingCart } from "@/components/ShoppingCart";
import { CartProvider } from "@/contexts/CartContext";
 
// Only mock the API layer — everything else is real
vi.mock("@/lib/api", () => ({
  checkout: vi.fn().mockResolvedValue({ orderId: "ORD-123" }),
}));
 
describe("Shopping Cart Flow", () => {
  const renderCart = (initialItems = []) => {
    return render(
      <CartProvider initialItems={initialItems}>
        <ShoppingCart />
      </CartProvider>
    );
  };
 
  it("displays item count and total", () => {
    renderCart([
      { id: "1", name: "Keyboard", price: 79.99, quantity: 1 },
      { id: "2", name: "Mouse", price: 49.99, quantity: 2 },
    ]);
 
    expect(screen.getByText("3 items")).toBeInTheDocument();
    expect(screen.getByText("$179.97")).toBeInTheDocument();
  });
 
  it("updates quantity and recalculates total", async () => {
    const user = userEvent.setup();
 
    renderCart([
      { id: "1", name: "Keyboard", price: 79.99, quantity: 1 },
    ]);
 
    const incrementButton = screen.getByRole("button", { name: /increase quantity/i });
    await user.click(incrementButton);
 
    expect(screen.getByText("$159.98")).toBeInTheDocument();
  });
 
  it("completes checkout flow", async () => {
    const user = userEvent.setup();
    const { checkout } = await import("@/lib/api");
 
    renderCart([
      { id: "1", name: "Keyboard", price: 79.99, quantity: 1 },
    ]);
 
    await user.click(screen.getByRole("button", { name: /checkout/i }));
 
    expect(checkout).toHaveBeenCalledWith({
      items: [{ id: "1", quantity: 1 }],
    });
 
    expect(await screen.findByText(/order confirmed/i)).toBeInTheDocument();
    expect(screen.getByText("ORD-123")).toBeInTheDocument();
  });
});

When to Use Integration vs Unit Tests#

Use unit tests when:

The logic is complex and has many edge cases (a date parser, a state machine, a calculation).
You need fast feedback on a specific function's behavior.
The unit is relatively isolated and doesn't depend heavily on other units.

Use integration tests when:

Multiple components need to work together correctly (a form with validation and submission).
Data flows through several layers (context → component → child component).
You're testing a user workflow, not a function's return value.

Coverage#

Running Coverage#

bash

vitest run --coverage

You'll need a coverage provider. Vitest supports two:

bash

# V8 — faster, uses V8's built-in coverage
npm install -D @vitest/coverage-v8
 
# Istanbul — more mature, more configuration options
npm install -D @vitest/coverage-istanbul

Configure it in your Vitest config:

typescript

export default defineConfig({
  test: {
    coverage: {
      provider: "v8",
      reporter: ["text", "html", "lcov"],
      include: ["src/**/*.{ts,tsx}"],
      exclude: [
        "src/**/*.test.{ts,tsx}",
        "src/**/*.spec.{ts,tsx}",
        "src/test/**",
        "src/**/*.d.ts",
        "src/**/types.ts",
      ],
      thresholds: {
        statements: 80,
        branches: 75,
        functions: 80,
        lines: 80,
      },
    },
  },
});

Istanbul vs V8#

I use V8 for local development (faster feedback) and Istanbul in CI (more accurate enforcement). You can configure different providers per environment if needed.

What Coverage Actually Means#

Coverage tells you which lines of code were executed during tests. That's it. It does not tell you whether those lines were tested correctly. Consider this:

typescript

function divide(a: number, b: number): number {
  return a / b;
}
 
it("divides numbers", () => {
  divide(10, 2);
  // No assertion!
});

This test gives 100% coverage of the divide function. It also tests absolutely nothing. The test would pass if divide returned null, threw an error, or launched missiles.

Lines vs Branches#

Line coverage is the most common metric but branch coverage is more valuable:

typescript

function getDiscount(user: User): number {
  if (user.isPremium) {
    return user.yearsActive > 5 ? 0.2 : 0.1;
  }
  return 0;
}

Branch coverage catches this. It tracks every possible path through conditional logic. If you're going to enforce a coverage threshold, use branch coverage.

Ignoring Generated Code#

Some code shouldn't be counted in coverage. Generated files, type definitions, configuration — these inflate your metrics without adding value:

typescript

// vitest.config.ts
coverage: {
  exclude: [
    "src/**/*.d.ts",
    "src/**/types.ts",
    "src/**/*.stories.tsx",
    "src/generated/**",
    ".velite/**",
  ],
}

You can also ignore specific lines or blocks in your source code:

typescript

/* v8 ignore start */
if (process.env.NODE_ENV === "development") {
  console.log("Debug info:", data);
}
/* v8 ignore stop */
 
// Or for Istanbul
/* istanbul ignore next */
function devOnlyHelper() { /* ... */ }

Use this sparingly. If you find yourself ignoring large chunks of code, either those chunks need tests or they shouldn't be in the coverage report to begin with.

Testing Next.js#

Mocking next/navigation#

Next.js components that use useRouter, usePathname, or useSearchParams need mocks:

typescript

import { vi } from "vitest";
 
vi.mock("next/navigation", () => ({
  useRouter: () => ({
    push: vi.fn(),
    replace: vi.fn(),
    back: vi.fn(),
    prefetch: vi.fn(),
    refresh: vi.fn(),
  }),
  usePathname: () => "/en/blog",
  useSearchParams: () => new URLSearchParams("?page=1"),
  useParams: () => ({ locale: "en" }),
}));

For tests that need to verify navigation was called:

typescript

import { useRouter } from "next/navigation";
 
vi.mock("next/navigation", () => ({
  useRouter: vi.fn(),
}));
 
describe("LogoutButton", () => {
  it("redirects to home after logout", async () => {
    const mockPush = vi.fn();
    vi.mocked(useRouter).mockReturnValue({
      push: mockPush,
      replace: vi.fn(),
      back: vi.fn(),
      prefetch: vi.fn(),
      refresh: vi.fn(),
      forward: vi.fn(),
    });
 
    const user = userEvent.setup();
    render(<LogoutButton />);
 
    await user.click(screen.getByRole("button", { name: /log out/i }));
 
    expect(mockPush).toHaveBeenCalledWith("/");
  });
});

Mocking next-intl#

For internationalized components using next-intl:

typescript

vi.mock("next-intl", () => ({
  useTranslations: () => (key: string) => key,
  useLocale: () => "en",
}));

If you need actual translations in a specific test:

typescript

vi.mock("next-intl", () => ({
  useTranslations: () => {
    const translations: Record<string, string> = {
      "hero.title": "Welcome to My Site",
      "hero.subtitle": "Building things for the web",
    };
    return (key: string) => translations[key] ?? key;
  },
}));

Testing Route Handlers#

Next.js Route Handlers are regular functions that take a Request and return a Response. They're straightforward to test:

typescript

import { GET, POST } from "@/app/api/users/route";
import { NextRequest } from "next/server";
 
describe("GET /api/users", () => {
  it("returns users list", async () => {
    const request = new NextRequest("http://localhost:3000/api/users");
    const response = await GET(request);
    const data = await response.json();
 
    expect(response.status).toBe(200);
    expect(data).toEqual(
      expect.arrayContaining([
        expect.objectContaining({ id: expect.any(Number), name: expect.any(String) }),
      ])
    );
  });
 
  it("supports pagination via search params", async () => {
    const request = new NextRequest("http://localhost:3000/api/users?page=2&limit=10");
    const response = await GET(request);
    const data = await response.json();
 
    expect(data.page).toBe(2);
    expect(data.items).toHaveLength(10);
  });
});
 
describe("POST /api/users", () => {
  it("creates a new user", async () => {
    const request = new NextRequest("http://localhost:3000/api/users", {
      method: "POST",
      body: JSON.stringify({ name: "Alice", email: "alice@test.com" }),
    });
 
    const response = await POST(request);
    const data = await response.json();
 
    expect(response.status).toBe(201);
    expect(data.name).toBe("Alice");
  });
 
  it("returns 400 for invalid body", async () => {
    const request = new NextRequest("http://localhost:3000/api/users", {
      method: "POST",
      body: JSON.stringify({ name: "" }),
    });
 
    const response = await POST(request);
    expect(response.status).toBe(400);
  });
});

Testing Middleware#

Next.js middleware runs at the edge and processes every request. Test it as a function:

typescript

import { middleware } from "@/middleware";
import { NextRequest } from "next/server";
 
function createRequest(path: string, headers: Record<string, string> = {}): NextRequest {
  const url = new URL(path, "http://localhost:3000");
  return new NextRequest(url, { headers });
}
 
describe("middleware", () => {
  it("redirects unauthenticated users from protected routes", async () => {
    const request = createRequest("/dashboard");
    const response = await middleware(request);
 
    expect(response.status).toBe(307);
    expect(response.headers.get("location")).toContain("/login");
  });
 
  it("allows authenticated users through", async () => {
    const request = createRequest("/dashboard", {
      cookie: "session=valid-token",
    });
    const response = await middleware(request);
 
    expect(response.status).toBe(200);
  });
 
  it("adds security headers", async () => {
    const request = createRequest("/");
    const response = await middleware(request);
 
    expect(response.headers.get("x-frame-options")).toBe("DENY");
    expect(response.headers.get("x-content-type-options")).toBe("nosniff");
  });
 
  it("handles locale detection", async () => {
    const request = createRequest("/", {
      "accept-language": "tr-TR,tr;q=0.9,en;q=0.8",
    });
    const response = await middleware(request);
 
    expect(response.headers.get("location")).toContain("/tr");
  });
});

Testing Server Actions#

Server Actions are async functions that run on the server. Since they're just functions, you can test them directly — but you may need to mock server-only dependencies:

typescript

vi.mock("@/lib/db", () => ({
  db: {
    user: {
      update: vi.fn().mockResolvedValue({ id: "1", name: "Updated" }),
      findUnique: vi.fn().mockResolvedValue({ id: "1", name: "Original" }),
    },
  },
}));
 
vi.mock("next/cache", () => ({
  revalidatePath: vi.fn(),
  revalidateTag: vi.fn(),
}));
 
import { updateProfile } from "@/app/actions/profile";
import { revalidatePath } from "next/cache";
 
describe("updateProfile", () => {
  it("updates user and revalidates profile page", async () => {
    const formData = new FormData();
    formData.set("name", "New Name");
    formData.set("bio", "New bio text");
 
    const result = await updateProfile(formData);
 
    expect(result.success).toBe(true);
    expect(revalidatePath).toHaveBeenCalledWith("/profile");
  });
 
  it("returns error for invalid data", async () => {
    const formData = new FormData();
    // Missing required fields
 
    const result = await updateProfile(formData);
 
    expect(result.success).toBe(false);
    expect(result.error).toBeDefined();
  });
});

Practical Patterns#

Custom Render Function#

Most projects need the same providers wrapped around every component. Create a custom render:

typescript

// src/test/utils.tsx
import { render, type RenderOptions } from "@testing-library/react";
import { ReactElement } from "react";
import { ThemeProvider } from "@/contexts/ThemeContext";
import { CartProvider } from "@/contexts/CartContext";
 
interface CustomRenderOptions extends RenderOptions {
  theme?: "light" | "dark";
  initialCartItems?: CartItem[];
}
 
function AllProviders({ children, theme = "light", initialCartItems = [] }: {
  children: React.ReactNode;
  theme?: "light" | "dark";
  initialCartItems?: CartItem[];
}) {
  return (
    <ThemeProvider defaultTheme={theme}>
      <CartProvider initialItems={initialCartItems}>
        {children}
      </CartProvider>
    </ThemeProvider>
  );
}
 
export function renderWithProviders(
  ui: ReactElement,
  options: CustomRenderOptions = {}
) {
  const { theme, initialCartItems, ...renderOptions } = options;
 
  return render(ui, {
    wrapper: ({ children }) => (
      <AllProviders theme={theme} initialCartItems={initialCartItems}>
        {children}
      </AllProviders>
    ),
    ...renderOptions,
  });
}
 
// Re-export everything from testing library
export * from "@testing-library/react";
export { renderWithProviders as render };

Now every test file imports from your custom utils instead of @testing-library/react:

typescript

import { render, screen } from "@/test/utils";
 
it("renders in dark mode", () => {
  render(<Header />, { theme: "dark" });
  // Header and all its children have access to ThemeProvider and CartProvider
});

Testing Custom Hooks#

Vitest works with @testing-library/react's renderHook:

typescript

import { renderHook, act } from "@testing-library/react";
import { useCounter } from "@/hooks/useCounter";
 
describe("useCounter", () => {
  it("starts at initial value", () => {
    const { result } = renderHook(() => useCounter(10));
    expect(result.current.count).toBe(10);
  });
 
  it("increments", () => {
    const { result } = renderHook(() => useCounter(0));
 
    act(() => {
      result.current.increment();
    });
 
    expect(result.current.count).toBe(1);
  });
 
  it("decrements with floor", () => {
    const { result } = renderHook(() => useCounter(0, { min: 0 }));
 
    act(() => {
      result.current.decrement();
    });
 
    expect(result.current.count).toBe(0); // doesn't go below min
  });
});

Testing Error Boundaries#

typescript

import { render, screen } from "@testing-library/react";
import { ErrorBoundary } from "@/components/ErrorBoundary";
 
const ThrowingComponent = () => {
  throw new Error("Test explosion");
};
 
describe("ErrorBoundary", () => {
  // Suppress console.error for expected errors
  beforeEach(() => {
    vi.spyOn(console, "error").mockImplementation(() => {});
  });
 
  afterEach(() => {
    vi.restoreAllMocks();
  });
 
  it("displays fallback UI when child throws", () => {
    render(
      <ErrorBoundary fallback={<div>Something went wrong</div>}>
        <ThrowingComponent />
      </ErrorBoundary>
    );
 
    expect(screen.getByText("Something went wrong")).toBeInTheDocument();
  });
 
  it("renders children when no error", () => {
    render(
      <ErrorBoundary fallback={<div>Error</div>}>
        <div>All good</div>
      </ErrorBoundary>
    );
 
    expect(screen.getByText("All good")).toBeInTheDocument();
    expect(screen.queryByText("Error")).not.toBeInTheDocument();
  });
});

Snapshot Testing (Carefully)#

But targeted snapshots can be useful:

typescript

import { render } from "@testing-library/react";
import { formatCurrency } from "@/lib/format";
 
// Good — small, targeted snapshot of a pure function's output
it("formats various currency values consistently", () => {
  expect(formatCurrency(0)).toMatchInlineSnapshot('"$0.00"');
  expect(formatCurrency(1234.5)).toMatchInlineSnapshot('"$1,234.50"');
  expect(formatCurrency(-99.99)).toMatchInlineSnapshot('"-$99.99"');
  expect(formatCurrency(1000000)).toMatchInlineSnapshot('"$1,000,000.00"');
});
 
// Bad — giant snapshot nobody will review
it("renders the dashboard", () => {
  const { container } = render(<Dashboard />);
  expect(container).toMatchSnapshot(); // Don't do this
});

The Testing Philosophy#

Test Behavior, Not Implementation#

This principle is so important that it's worth a dedicated section. Consider two tests for the same feature:

typescript

// Implementation test — brittle, breaks on refactors
it("calls setState with new count", () => {
  const setStateSpy = vi.spyOn(React, "useState");
  render(<Counter />);
  fireEvent.click(screen.getByText("+"));
  expect(setStateSpy).toHaveBeenCalledWith(expect.any(Function));
});
 
// Behavior test — resilient, tests what the user sees
it("increments the displayed count when plus button is clicked", async () => {
  const user = userEvent.setup();
  render(<Counter />);
  expect(screen.getByText("Count: 0")).toBeInTheDocument();
 
  await user.click(screen.getByRole("button", { name: "+" }));
 
  expect(screen.getByText("Count: 1")).toBeInTheDocument();
});

The litmus test is simple: can you refactor the implementation without changing the test? If yes, you're testing behavior. If no, you're testing implementation.

The Testing Trophy#

Kent C. Dodds proposed the "Testing Trophy" as an alternative to the traditional testing pyramid:

    ╭─────────╮
    │  E2E    │   Few — expensive, slow, high confidence
    ├─────────┤
    │         │
    │ Integr. │   Most — good confidence-to-cost ratio
    │         │
    ├─────────┤
    │  Unit   │   Some — fast, focused, low-cost
    ├─────────┤
    │ Static  │   Always — TypeScript, ESLint
    ╰─────────╯

The traditional pyramid puts unit tests at the bottom (lots of them) and integration tests in the middle (fewer). The trophy inverts this: integration tests are the sweet spot. Here's why:

Static analysis (TypeScript, ESLint) catches typos, wrong types, and simple logical errors for free. You don't even have to run anything.
Unit tests are great for complex pure logic but don't tell you if the pieces work together.
Integration tests verify that components, hooks, and contexts work together. They give the most confidence per test written.
End-to-end tests verify the entire system but are slow, flaky, and expensive to maintain. You need a few for critical paths but not hundreds.

What Gives Confidence vs What Wastes Time#

Tests exist to give you confidence to ship. Not confidence that every line of code runs — confidence that the application works for users. These are different things.

High confidence, high value:

Integration test of a checkout flow — covers form validation, API calls, state updates, and success/error UI.
Unit test of a price calculation function with edge cases — floating point, rounding, discounts, zero/negative values.
Test that protected routes redirect unauthenticated users.

Low confidence, time wasters:

Snapshot test of a static marketing page — breaks every time copy changes, catches nothing meaningful.
Unit test that a component passes a prop to a child — testing React itself, not your code.
Test that useState is called — testing the framework, not behavior.
100% coverage of a configuration file — it's static data, TypeScript already validates its shape.

Testing as Design Feedback#

typescript

// Hard to test — function does too much
async function processOrder(orderId: string) {
  const order = await db.orders.findById(orderId);
  const user = await db.users.findById(order.userId);
  const inventory = await checkInventory(order.items);
  if (!inventory.available) {
    await sendEmail(user.email, "out-of-stock", { items: inventory.unavailable });
    return { success: false, reason: "out-of-stock" };
  }
  const payment = await chargeCard(user.paymentMethod, order.total);
  if (!payment.success) {
    await sendEmail(user.email, "payment-failed", { error: payment.error });
    return { success: false, reason: "payment-failed" };
  }
  await db.orders.update(orderId, { status: "confirmed" });
  await sendEmail(user.email, "order-confirmed", { orderId });
  return { success: true };
}
 
// Easier to test — separated concerns
function determineOrderAction(
  inventory: InventoryResult,
  payment: PaymentResult
): OrderAction {
  if (!inventory.available) return { type: "out-of-stock", items: inventory.unavailable };
  if (!payment.success) return { type: "payment-failed", error: payment.error };
  return { type: "confirmed" };
}

Write Tests You Trust#

Build a test suite you'd bet your weekend on. Nothing less is worth maintaining.

Why Vitest Over Jest#

Native ESM Support#

Same Config as Your Dev Server#

Speed#

Benchmarking Built In#

Setup#

Installation#

Configuration#

The globals: true Decision#

Environment: jsdom vs happy-dom vs node#

Setup Files#

Writing Good Tests#

The AAA Pattern#

Test Naming#

One Assertion Per Test vs Practical Grouping#

Test Descriptions as Documentation#

Mocking#

vi.fn() — Creating Mock Functions#

vi.spyOn() — Watching Real Methods#

vi.mock() — Module-Level Mocking#

Automatic Mocking#

Clearing vs Resetting vs Restoring#

The Danger of Over-Mocking#

Testing React Components#

The Basics with Testing Library#

Queries: getBy vs queryBy vs findBy#

Accessibility-First Queries#

User Events#

Testing Component Interactions#

Testing Async Code#

waitFor#

Fake Timers#

Testing with Fake Timers and Async Rendering#

API Mocking with MSW#

Integration Tests#

Unit Tests vs Integration Tests#

Testing Multiple Components Together#

When to Use Integration vs Unit Tests#

Coverage#

Running Coverage#

Istanbul vs V8#

What Coverage Actually Means#

Lines vs Branches#

Ignoring Generated Code#

Testing Next.js#

Mocking next/navigation#

Mocking next-intl#

Testing Route Handlers#

Testing Middleware#

Testing Server Actions#

Practical Patterns#

Custom Render Function#

Testing Custom Hooks#

Testing Error Boundaries#

Snapshot Testing (Carefully)#

The Testing Philosophy#

Test Behavior, Not Implementation#

The Testing Trophy#

What Gives Confidence vs What Wastes Time#

Testing as Design Feedback#

Write Tests You Trust#

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Why Vitest Over Jest#

Native ESM Support#

Same Config as Your Dev Server#

Speed#

Benchmarking Built In#

Setup#

Installation#

Configuration#

The globals: true Decision#

Environment: jsdom vs happy-dom vs node#

Setup Files#

Writing Good Tests#

The AAA Pattern#

Test Naming#

One Assertion Per Test vs Practical Grouping#

Test Descriptions as Documentation#

The `globals: true` Decision#

The `globals: true` Decision#