vitest-tests

Overview

Per vitest.dev/guide:

Vitest is the Vite-native test framework. The model:

"Vitest reads your vite.config.* by default, so your existing Vite plugins and configuration work out-of-the-box."

This is the differentiator vs Jest - Jest needs separate babel-jest/ts-jest transform setup; Vitest reuses Vite's already-configured pipeline. Same code transforms in dev + test.

API is intentionally Jest-compatible (expect, describe, it / test, vi.fn, vi.mock) - migration from Jest is mostly mechanical.

When to use

• The repo uses Vite (Vue, Svelte, Solid, Astro, modern React).
• Migration from Jest where the team wants faster runs + ESM-native.
• New JS/TS project starting from scratch (Vitest is the modern default for greenfield).
• In-source testing pattern (tests colocated with implementation via if (import.meta.vitest)) is desired.

Step 1 - Install

Per vt-guide:

npm install -D vitest

If the project already has Vite + a vite.config.* file, no additional config needed.

Step 2 - First test

Per vt-guide:

// sum.js
export function sum(a, b) { return a + b; }

// sum.test.js
import { expect, test } from 'vitest'
import { sum } from './sum.js'

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3)
})

Wire package.json:

{
  "scripts": {
    "test": "vitest run",
    "test:watch": "vitest",
    "coverage": "vitest run --coverage"
  }
}

vitest (no subcommand) defaults to watch mode; vitest run is the single-pass run.

Step 3 - Configuration

Vitest reads vite.config.ts by default. For Vitest-specific options:

// vite.config.ts
import { defineConfig } from 'vitest/config'   // note: vitest/config wrapper

export default defineConfig({
  test: {
    environment: 'jsdom',           // 'jsdom' (browser-like) | 'node' | 'happy-dom' | 'edge-runtime'
    globals: false,                  // import {test, expect} explicitly (recommended) vs global injection
    include: ['**/*.test.{js,ts}', '**/*.spec.{js,ts}'],
    exclude: ['node_modules', 'dist'],
    setupFiles: ['./vitest.setup.ts'],
    coverage: {
      provider: 'v8',                // 'v8' (default; native) | 'istanbul'
      reporter: ['text', 'json', 'html', 'lcov'],
      thresholds: {
        lines: 80,
        functions: 80,
        branches: 80,
        statements: 80,
      },
      include: ['src/**'],
      exclude: ['**/*.test.ts', '**/types.ts'],
    },
  },
})

provider: 'v8' is Vitest's default; istanbul is more accurate for branch coverage but slower.

Step 4 - Mocking (Jest-compatible API)

import { vi, expect, test } from 'vitest';

// vi.fn() — standalone mock
const myMock = vi.fn();
myMock.mockReturnValue(42);

// vi.mock() — module mock (hoisted to top of file)
vi.mock('./api-client', () => ({
  fetchUser: vi.fn().mockResolvedValue({ id: 1 }),
}));

// vi.spyOn() — wrap existing method
const spy = vi.spyOn(myObject, 'someMethod');

Timer mocks:

vi.useFakeTimers();
setTimeout(callback, 1000);
vi.advanceTimersByTime(1000);
expect(callback).toHaveBeenCalled();
vi.useRealTimers();

Migration from Jest: replace jest. with vi. (mostly mechanical).

Step 5 - In-source testing

Vitest's distinguishing feature - tests live in the implementation file:

// sum.ts
export function sum(a: number, b: number): number {
  return a + b;
}

if (import.meta.vitest) {
  const { test, expect } = import.meta.vitest;
  test('adds', () => {
    expect(sum(1, 2)).toBe(3);
  });
}

Enable in config:

test: {
  includeSource: ['src/**/*.{js,ts}'],
}

In production builds, the if (import.meta.vitest) block is tree-shaken away. Useful for tiny utility files where separate test files feel like overkill - but mainstream test suites should use separate files for greppability.

Step 6 - Browser mode

For tests that need real browser APIs (vs jsdom approximations):

npm install -D @vitest/browser playwright

// vite.config.ts
test: {
  browser: {
    enabled: true,
    name: 'chromium',     // 'chromium' | 'firefox' | 'webkit'
    provider: 'playwright',
    headless: true,
  },
}

Tests run in a real browser instance; tradeoff is speed vs fidelity. For DOM-only assertions, jsdom is faster; for CSS layout / Web API correctness, browser mode catches more.

Step 7 - Type-checking integration

vitest run --typecheck

Runs tsc --noEmit against test files alongside the test run. Without --typecheck, TypeScript type errors in tests don't fail the run.

Step 8 - Coverage

vitest run --coverage

Output formats configured in coverage.reporter (Step 3).

Step 9 - CI integration

- run: npm ci
- run: npx vitest run --coverage --reporter=verbose --reporter=junit --outputFile=junit.xml
- uses: codecov/codecov-action@v4
  with: { files: ./coverage/lcov.info }

vitest run (not vitest) is required in CI - without run, Vitest enters watch mode and hangs the runner.

Anti-patterns

Limitations

• ESM-native; CommonJS-only projects need migration or Jest.
• Browser-mode is newer; some matchers don't work identically in browser vs jsdom environments.
• Snapshot format differs slightly from Jest; migrating snapshots needs care.
• In-source testing is a power feature; over-use harms code readability.

References

• vt-guide - getting started
• vitest.dev/config - full config reference
• vitest.dev/api/vi - vi.* API reference
• vitest.dev/guide/in-source - in-source testing
• vitest.dev/guide/browser - browser mode
• jest-tests, mocha-tests, ava-tests, jasmine-tests - sister tools
• test-code-conventions - cross-plugin: test code hygiene