Use this skill when writing unit tests with Jest or Vitest, implementing mocking strategies, configuring test runners, or improving test coverage. Triggers on Jest, Vitest, describe/it/expect, mocking, vi.fn, jest.fn, snapshot testing, test coverage, and any task requiring JavaScript/TypeScript unit testing.
Use this skill when writing unit tests with Jest or Vitest, implementing mocking strategies, configuring test runners, or improving test coverage. Triggers on Jest, Vitest, describe/it/expect, mocking, vi.fn, jest.fn, snapshot testing, test coverage, and any task requiring JavaScript/TypeScript unit testing.
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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.
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
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.
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.
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.
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.
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.
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.
Common tasks
Write well-structured tests with AAA
// src/cart.test.tsimport { describe, it, expect, beforeEach } from'vitest';
import { Cart } from'./cart';
describe('Cart', () => {
letcart: Cart;
beforeEach(() => {
// Arrange - fresh cart for each test, no shared state
cart = newCart();
});
it('starts empty', () => {
// Assert only - trivial arrange already doneexpect(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 });
// Assertexpect(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');
});
});
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.
// src/test-setup.tsimport'@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.tsximport { 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(<LoginFormonSubmit={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 }));
awaitwaitFor(() => {
expect(onSubmit).toHaveBeenCalledWith({
email: 'user@example.com',
password: 'secret123',
});
});
});
it('shows a validation error when email is empty', async () => {
const user = userEvent.setup();
render(<LoginFormonSubmit={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:
Error paths - what happens when a fetch fails, input is invalid, or a service throws
Guard clauses - early returns and null checks
Complex conditionals - expressions with multiple && / || operators
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
Gotchas
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.
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.
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.
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.
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.
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
Companion check
On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install:
