Vitest

Test behavior, not implementation. Mock boundaries, not internals.

Vitest is a Vite-native test framework with Jest-compatible APIs. It shares your app's Vite config (aliases, plugins, transforms) so tests run against the same code you ship.

References

Test Structure

• Import explicitly. import { describe, it, expect, vi } from 'vitest' — do not rely on globals unless the project has globals: true configured.
• One concept per test. Name tests by behavior: 'returns empty array when input is null', not 'test case 1'.
• Use describe for grouping. Group by unit (function, class, component), not by test type.
• Prefer it over test. Both work, but it reads better inside describe: describe('parseUrl', () => { it('extracts hostname', ...) }).
• Use it.each / describe.each for parametrized tests. Supports array form and template literal form with $key interpolation.
• Test modifiers: it.skip, it.only, it.todo, it.fails, it.skipIf(cond), it.runIf(cond).
• Retry: it('name', { retry: 3 }, fn). Repeat: it('name', { repeats: 100 }, fn).
• Concurrent tests: describe.concurrent(...) — use expect from test context (destructured parameter) for correct snapshot/assertion tracking.

Mocking

Function Mocks

• vi.fn() creates a standalone trackable mock. Optionally accepts implementation.
• vi.spyOn(obj, 'method') wraps existing method while preserving original. Also supports vi.spyOn(obj, 'prop', 'get') for getters/setters.
• Prefer vi.spyOn over vi.mock when you only need to observe or override a single export.

Module Mocking

• vi.mock() is hoisted. It moves to top of file regardless of where you write it. Always runs before imports.
• Factory must return an object with explicit exports. ESM requires explicit default key: vi.mock('./mod', () => ({ default: val, namedExport: vi.fn() })).
• Partial mocking with importOriginal: vi.mock(import('./api'), async (importOriginal) => ({ ...await importOriginal(), fetchUser: vi.fn() })).
• vi.doMock() is not hoisted — runs at position. Only affects subsequent dynamic import() calls. Use when you need per-test mock behavior.
• vi.mock cannot intercept internal calls. If foo() calls bar() in the same file, mocking bar externally does not affect foo. Refactor to separate modules or use dependency injection.

Cleanup & Timers

• Always restore mocks. Use restoreMocks: true in config (recommended) or afterEach(() => vi.restoreAllMocks()).
• Pair vi.useFakeTimers() with vi.useRealTimers() — in beforeEach/afterEach or use fakeTimers config option.
• vi.mocked(fn) narrows TypeScript types to mock types without runtime changes.

Full mocking rules (auto-mocking, spy mode, vi.hoisted, __mocks__ directory, env/globals stubbing, async helpers): see references/mocking.md.

Assertions

Value Matchers

Core Rules

• Sync errors: wrap in function: expect(() => throwingFn()).toThrow('message').
• Async errors: await expect(asyncFn()).rejects.toThrow('message').
• Always await async assertions. await expect(promise).resolves.toEqual(...) — an un-awaited assertion silently passes.
• expect.poll(() => value, { timeout, interval }) — retries assertion until pass or timeout. Prefer over manual waitFor loops.
• expect.soft(val) continues after failure, reports all errors at end.
• Asymmetric matchers inside toEqual, toHaveBeenCalledWith: expect.any(Number), expect.arrayContaining([...]), expect.objectContaining({}), expect.stringMatching(/regex/). Negate with expect.not.*.
• expect.assertions(n) — exactly n assertions must run. expect.hasAssertions() — at least one. Guard against missing assertions in async code.

Truthiness, number, string/array/object matchers, type checks, spy assertions, custom error messages, expect.unreachable: see references/assertions.md.

Snapshots

• File snapshots: expect(val).toMatchSnapshot() — writes to .snap file.
• Inline snapshots: expect(val).toMatchInlineSnapshot(\"expected"`)` — Vitest auto-updates the string argument.
• Property matchers for volatile data: toMatchSnapshot({ id: expect.any(String) }). Never snapshot timestamps, random IDs, or other volatile data without matchers.
• Prefer inline snapshots for small values — easier to review.
• Avoid large snapshots — they become rubber-stamp reviews.
• Commit snapshot files. Review them in PRs like any other code.
• Update with vitest -u or press u in watch mode.

Lifecycle

Core Rules

• beforeAll/afterAll run once per describe block (or per file at top level).
• beforeEach/afterEach run before/after every test in current scope.
• Teardown via return value: if beforeAll or beforeEach returns a function, it runs as teardown. Vitest-specific, not in Jest. Be careful not to accidentally return values — wrap in braces: beforeEach(() => { setupFn() }).
• onTestFinished(fn) — register cleanup inside a test. Always runs regardless of pass/fail.
• onTestFailed(fn) — runs only on failure. Useful for diagnostics.

Setup Files

• setupFiles: ['./test/setup.ts'] — runs before each test file in the same process. Use for global hooks, custom matchers, shared setup.
• globalSetup: ['./test/global-setup.ts'] — runs once before any test workers. Use for expensive one-time setup (database seeding, server startup). Return a function for teardown.

Hook execution order, test context, hook order config, provide/inject, and global vs setup file comparison: see references/lifecycle.md.

Configuration

• Prefer vitest.config.ts with defineConfig from 'vitest/config'. Inherits Vite plugins and aliases automatically.
• If using vite.config.ts, add /// <reference types="vitest/config" /> directive.
• Use projects (v3.2+, replaces deprecated workspace) for multi-environment setups. Every project must have a unique name.

Config file merging, key options table, pools and parallelism, environment variables, in-source testing, and sharding: see references/configuration.md.

Coverage

Set coverage.include to catch uncovered files. Use v8 provider (default, recommended). Set thresholds in config. Run coverage only in CI, not in watch mode.

Provider comparison, reporters, ignore comments, and performance tips: see references/coverage.md.

Extending Matchers

Define via expect.extend({ matcherName(received, ...args) {} }). Return { pass, message }. Add TypeScript declarations via interface Matchers<T> in vitest.d.ts.

Matcher context, TypeScript setup, and diff output: see references/assertions.md.

Jest Migration

Replace jest.* with vi.*. Key differences: mock factory must return object with explicit exports, mockReset restores original impl, auto-mocking requires explicit vi.mock() call, hook return values are teardown functions.

Full API translation table, behavioral differences, and Mocha/Sinon migration: see references/jest-migration.md.

Application

When writing tests:

• Apply all conventions silently — don't narrate each rule being followed.
• Match the project's existing test style (naming, structure, assertion library).
• If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.

When reviewing tests:

• Cite the specific issue and show the fix inline.
• Don't lecture — state what's wrong and how to fix it.

Integration

The javascript skill governs language choices; this skill governs Vitest testing decisions. Activate the relevant runtime skill (nodejs or bun) for runtime-specific behavior.

Test behavior, not implementation. When in doubt, mock less.