Vitest

Vitest

Vitest is a next-generation testing framework powered by Vite. It uses the same configuration (vite.config.ts), transforms, and plugins as your app, making it incredibly fast and creating a unified dev/test environment.

When to Use

• Vite Projects: If you use Vite, Vitest is the no-brainer choice. It reuses your plugins/config.
• Performance: It is significantly faster than Jest because it uses native ESM and doesn't bundle the app.
• Watch Mode: Instant HMR-style updates for tests.

Quick Start

// vite.config.ts
/// <reference types="vitest" />
import { defineConfig } from "vite";

export default defineConfig({
  test: {
    globals: true, // enables 'describe', 'test', 'expect' globally
    environment: "jsdom",
  },
});

// basic.test.ts
import { expect, test } from "vitest";

test("math", () => {
  expect(1 + 1).toBe(2);
});

Core Concepts

Jest Compatible

Vitest's API (describe, it, expect, vi.fn) is designed to be compatible with Jest. Migration is often just changing imports.

In-Source Testing

Vitest allows running tests within your source code blocks (Rust-style), though separating them is still common practice.

if (import.meta.vitest) {
  const { it, expect } = import.meta.vitest;
  it("add", () => {
    expect(add()).toBe(0);
  });
}

Best Practices (2025)

Do:

• Use Native ESM: Stop fighting with Babel/ts-jest. Vitest handles ESM natively.
• Use Workspace Mode: For monorepos, Vitest works beautifully with workspace packages.
• Switch from Jest: If you are starting a new project, choose Vitest.

Don't:

• Don't rely on global state: Vitest runs tests in parallel.
• Don't import Jest types: Ensure types in tsconfig.json points to vitest/globals if using globals.

References

• Vitest Documentation