Jest / Vitest

Jest and Vitest are the dominant unit testing frameworks for JavaScript and TypeScript. Jest is the battle-tested choice bundled with Create React App and widely adopted across Node.js ecosystems. Vitest is the modern successor - it reuses Vite's transform pipeline, offers a compatible API, and is significantly faster for projects already on Vite. Both share the same describe/it/expect vocabulary, making knowledge transferable. This skill covers writing well-structured tests, mocking strategies, async patterns, snapshot testing, React component testing, and coverage analysis.

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When to use this skill

Trigger this skill when the user:

• Asks to write, review, or improve unit tests in JavaScript or TypeScript
• Mentions Jest, Vitest, describe, it, test, expect, or beforeEach
• Needs to mock a module, function, or dependency (vi.fn, jest.fn, vi.mock)
• Asks about snapshot testing or updating snapshots
• Wants to configure a test runner for a new or existing project
• Needs to test React (or other UI) components with @testing-library
• Asks about test coverage - thresholds, gaps, or measuring it
• Is migrating a test suite from Jest to Vitest

Do NOT trigger this skill for:

• End-to-end or browser automation testing (use Playwright / Cypress skills instead)
• Static analysis or linting - these are not tests

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Key principles

1. Test behavior, not implementation - Tests should verify what a unit does from the outside, not how it does it internally. Tests that reach into private state or assert on internal call sequences break during refactoring even when behavior is unchanged.

2. Arrange-Act-Assert - Every test has three clear sections: set up the preconditions, perform the action under test, then assert the outcome. Keep each section small. Long Arrange sections signal the API is too complex.

3. One assertion concept per test - A test should fail for exactly one reason. Multiple expect calls are fine when they all verify the same behavioral concept. Tests that verify two unrelated concepts hide which behavior broke.

4. Mock at boundaries, not internals - Mock I/O and external services (HTTP clients, databases, file system, timers) at their entry point. Do not mock internal helper functions within the same module - that tests the wiring, not the behavior.

5. Fast tests run more often - A suite that completes in under 10 seconds gets run on every save. One that takes 2 minutes gets run before commits only. Keep unit tests in-memory: no real network, no real filesystem, no real clocks.

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Core concepts

Test lifecycle

beforeAll  → runs once before all tests in a describe block
beforeEach → runs before each individual test
afterEach  → runs after each individual test (cleanup)
afterAll   → runs once after all tests in a describe block

Prefer beforeEach / afterEach over beforeAll / afterAll. Shared state across tests causes order-dependent failures that are painful to debug.

Matchers

Matcher	Use for
toBe(value)	Strict equality (===) for primitives
toEqual(value)	Deep equality for objects and arrays
toStrictEqual(value)	Deep equality including undefined properties and class instances
toMatchObject(partial)	Object contains at least these keys/values
toContain(item)	Array contains item, string contains substring
toThrow(error?)	Function throws (wrap in () => fn())
toHaveBeenCalledWith(...args)	Mock was called with specific arguments
toHaveBeenCalledTimes(n)	Mock call count
resolves / rejects	Chain on Promises: await expect(p).resolves.toBe(x)

Mock types

Type	API	Purpose
Function mock	vi.fn() / jest.fn()	Replaces a function, records calls
Spy	vi.spyOn(obj, 'method')	Wraps an existing method, records calls, can restore
Module mock	vi.mock('module') / jest.mock('module')	Replaces an entire module's exports

Snapshot testing

Snapshots serialize a value to a .snap file on first run, then assert the value matches that serialization on subsequent runs. Use snapshots for stable, complex output (serialized data structures, CLI output). Avoid snapshots for UI components rendered to HTML - they become noisy and get blindly updated.

Update stale snapshots intentionally with --updateSnapshot (-u) after reviewing the diff.

Coverage metrics

Metric	What it measures
Statements	Percentage of executable statements run
Branches	Percentage of if/else/ternary paths taken
Functions	Percentage of functions called at least once
Lines	Percentage of source lines executed

Branch coverage is the most meaningful metric. A function with 100% statement coverage but 60% branch coverage has untested if paths that can fail in production. Aim for 80%+ branch coverage on business logic.

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Common tasks

Write well-structured tests with AAA

// src/cart.test.ts
import { describe, it, expect, beforeEach } from 'vitest';
import { Cart } from './cart';

describe('Cart', () => {
  let cart: Cart;

  beforeEach(() => {
    // Arrange - fresh cart for each test, no shared state
    cart = new Cart();
  });

  it('starts empty', () => {
    // Assert only - trivial arrange already done
    expect(cart.itemCount()).toBe(0);
    expect(cart.total()).toBe(0);
  });

  it('adds items and updates total', () => {
    // Act
    cart.add({ id: '1', name: 'Widget', price: 9.99, quantity: 2 });

    // Assert
    expect(cart.itemCount()).toBe(2);
    expect(cart.total()).toBeCloseTo(19.98);
  });

  it('throws when adding an item with zero quantity', () => {
    expect(() =>
      cart.add({ id: '1', name: 'Widget', price: 9.99, quantity: 0 })
    ).toThrow('Quantity must be positive');
  });
});

Mock modules and dependencies

// src/order-service.test.ts
import { describe, it, expect, vi, beforeEach } from 'vitest';

// Module mock hoisted to top of file by Vitest/Jest
vi.mock('./payment-gateway', () => ({
  charge: vi.fn(),
}));
vi.mock('./mailer', () => ({
  sendConfirmation: vi.fn(),
}));

import { placeOrder } from './order-service';
import { charge } from './payment-gateway';
import { sendConfirmation } from './mailer';

describe('placeOrder', () => {
  beforeEach(() => {
    vi.resetAllMocks();
  });

  it('charges the customer and sends a confirmation on success', async () => {
    // Arrange
    vi.mocked(charge).mockResolvedValue({ success: true, transactionId: 'txn_123' });
    const order = { id: 'ord_1', total: 49.99, customer: { email: 'a@b.com' } };

    // Act
    await placeOrder(order);

    // Assert
    expect(charge).toHaveBeenCalledWith({ amount: 49.99, orderId: 'ord_1' });
    expect(sendConfirmation).toHaveBeenCalledWith('a@b.com', 'ord_1');
  });

  it('throws OrderFailedError when payment is declined', async () => {
    vi.mocked(charge).mockResolvedValue({ success: false, error: 'Insufficient funds' });
    const order = { id: 'ord_2', total: 200, customer: { email: 'a@b.com' } };

    await expect(placeOrder(order)).rejects.toThrow('OrderFailedError');
    expect(sendConfirmation).not.toHaveBeenCalled();
  });
});

Test async code - promises, timers, and events

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
import { fetchUser } from './user-api';

// --- Promises ---
it('resolves with user data', async () => {
  const user = await fetchUser('user-1');
  expect(user).toMatchObject({ id: 'user-1', name: expect.any(String) });
});

it('rejects when user is not found', async () => {
  await expect(fetchUser('nonexistent')).rejects.toThrow('User not found');
});

// --- Fake timers (debounce, throttle, setTimeout) ---
describe('debounced search', () => {
  beforeEach(() => { vi.useFakeTimers(); });
  afterEach(() => { vi.useRealTimers(); });

  it('fires callback once after debounce delay', () => {
    const callback = vi.fn();
    const search = createDebouncedSearch(callback, 300);

    search('re');
    search('rea');
    search('react');

    expect(callback).not.toHaveBeenCalled();
    vi.advanceTimersByTime(300);
    expect(callback).toHaveBeenCalledOnce();
    expect(callback).toHaveBeenCalledWith('react');
  });
});

// --- Event emitters ---
it('emits "ready" after initialization', () =>
  new Promise<void>((resolve) => {
    const service = new DataService();
    service.on('ready', () => {
      expect(service.isReady()).toBe(true);
      resolve();
    });
    service.init();
  })
);

Snapshot testing done right

import { describe, it, expect } from 'vitest';
import { serializeCartSummary } from './cart-serializer';

describe('serializeCartSummary', () => {
  it('produces stable JSON for a standard cart', () => {
    const cart = buildCart([
      { sku: 'A1', qty: 2, price: 10 },
      { sku: 'B3', qty: 1, price: 25.5 },
    ]);

    // Snapshot is useful here: the serialization format is complex and
    // must remain stable for API consumers.
    expect(serializeCartSummary(cart)).toMatchSnapshot();
  });
});

// When output changes intentionally, review the diff then run:
// npx vitest --updateSnapshot
// Do NOT blindly run -u without reading the diff first.

Configure Vitest for a project

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

export default defineConfig({
  plugins: [react()],
  test: {
    environment: 'jsdom',       // use 'node' for server-side code
    globals: true,              // avoids importing describe/it/expect in every file
    setupFiles: ['./src/test-setup.ts'],
    coverage: {
      provider: 'v8',
      reporter: ['text', 'lcov', 'html'],
      thresholds: {
        branches: 80,
        functions: 80,
        lines: 80,
        statements: 80,
      },
      exclude: [
        'src/**/*.d.ts',
        'src/**/index.ts',      // barrel files
        'src/**/*.stories.tsx', // Storybook
      ],
    },
  },
});

// src/test-setup.ts
import '@testing-library/jest-dom'; // extends expect with .toBeInTheDocument() etc.
import { afterEach } from 'vitest';
import { cleanup } from '@testing-library/react';

afterEach(() => {
  cleanup(); // unmount React trees after each test
});

Test React components with testing-library

// src/components/LoginForm.test.tsx
import { describe, it, expect, vi } from 'vitest';
import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { LoginForm } from './LoginForm';

describe('LoginForm', () => {
  it('submits email and password when the form is valid', async () => {
    const user = userEvent.setup();
    const onSubmit = vi.fn().mockResolvedValue(undefined);

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

    await user.type(screen.getByLabelText(/email/i), 'user@example.com');
    await user.type(screen.getByLabelText(/password/i), 'secret123');
    await user.click(screen.getByRole('button', { name: /log in/i }));

    await waitFor(() => {
      expect(onSubmit).toHaveBeenCalledWith({
        email: 'user@example.com',
        password: 'secret123',
      });
    });
  });

  it('shows a validation error when email is empty', async () => {
    const user = userEvent.setup();
    render(<LoginForm onSubmit={vi.fn()} />);

    await user.click(screen.getByRole('button', { name: /log in/i }));

    expect(screen.getByText(/email is required/i)).toBeInTheDocument();
  });
});

Query priority for @testing-library: getByRole > getByLabelText > getByPlaceholderText > getByText > getByTestId. Prefer role-based queries because they reflect how assistive technology sees the page.

Measure and improve coverage

# Run tests with coverage
npx vitest run --coverage

# Or with Jest
npx jest --coverage

# View HTML report (Vitest)
open coverage/index.html

To find untested branches, look for E (else not taken) and I (if not taken) markers in the Istanbul HTML report. Focus on:

1. Error paths - what happens when a fetch fails, input is invalid, or a service throws
2. Guard clauses - early returns and null checks
3. Complex conditionals - expressions with multiple && / || operators

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Anti-patterns

Anti-pattern	Why it's harmful	What to do instead
Testing implementation details	Asserts on private state, internal call order, or mocked internals - breaks during refactoring without catching real bugs	Test observable outputs and public API behavior
One giant test per function	A single test with 15 assertions hides which scenario failed	One test per behavior: happy path, each error case, each edge case
Mocking what you own	Mocking internal helpers inside the module under test leaves the real integration untested	Only mock external boundaries (HTTP, DB, file system, time)
beforeAll shared mutable state	Tests pass individually but fail when run in sequence due to mutated shared objects	Use beforeEach to create fresh instances for every test
Snapshot-everything	Applying .toMatchSnapshot() to all component output means reviewers never read snapshot diffs and always blindly update	Use snapshots only for stable, complex serializations - not HTML
Skipping vi.resetAllMocks()	Mock return values and call counts bleed between tests causing false positives	Call vi.resetAllMocks() in afterEach or enable clearMocks: true in config

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Gotchas

1. vi.mock() calls are hoisted but imports are not - order matters - Vitest (and Jest) hoist vi.mock() calls to the top of the file at compile time, but the imported mock values are still assigned at runtime. If you reference a mock return value before calling vi.mocked(fn).mockReturnValue(...), you'll get undefined. Always configure mock return values inside beforeEach or inside the test body, not at the module scope.

2. jsdom environment makes Node-specific APIs silently undefined - If your test config sets environment: 'jsdom' but the code under test uses Node APIs like fs, path, or process.env, those may behave differently or return undefined without error. Use environment: 'node' for server-side code and restrict jsdom to browser/component tests only.

3. Fake timers must be cleaned up or they leak into subsequent tests - Calling vi.useFakeTimers() in a test without vi.useRealTimers() in afterEach means every subsequent test in the suite runs with fake timers. setTimeout in unrelated tests will never fire, producing mysterious timeouts. Always restore real timers in the teardown of any test that uses fake ones.

4. toMatchSnapshot() on React components captures the entire rendered HTML, making every UI change a snapshot failure - Component snapshots become a maintenance burden because every intentional style or markup change requires running --updateSnapshot, and reviewers stop reading the diffs. Use snapshots for stable serialized data structures, not rendered component output.

5. vi.resetAllMocks() vs vi.clearAllMocks() vs vi.restoreAllMocks() are not the same - clearAllMocks clears call history but keeps implementations. resetAllMocks removes both history and mock implementations. restoreAllMocks also restores spied-on originals. Using the wrong one leaves either stale call counts or stale mock return values, causing false-positive test passes in later tests.

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References

For deep dives, read the relevant file from references/:

• references/migration-guide.md - Step-by-step Jest to Vitest migration, API diffs, config changes