Vitest

Installation and Setup

A project installs Vitest as a dev dependency:

$ npm install --save-dev vitest

Vitest 5 needs Node.js 22.12 or later and Vite 6.4 or later. npm installs a compatible Vite version automatically. The --version flag prints the installed version:

$ npx vitest --version
vitest/5.1.0 linux-x64 node-v26.13.0

Key Features

• Finds test files automatically by their .test.js or .spec.js suffix, including TypeScript variants.
• Exports test(), describe(), and expect() from the vitest package instead of defining globals by default.
• Creates mock functions with vi.fn() and replaces whole modules with vi.mock().
• Reruns only the tests affected by a changed file in watch mode, using Vite’s module graph.
• Reports code coverage with the --coverage flag through the optional @vitest/coverage-v8 package.

Usage

Say a shop’s cart.js module exports a cartTotal() function. A file named cart.test.js imports the test helpers and checks the function with two tests:

import { describe, expect, test } from "vitest";
import { cartTotal } from "./cart.js";

describe("cartTotal()", () => {
  test("adds up price times quantity", () => {
    const items = [
      { price: 5, quantity: 2 },
      { price: 12.5, quantity: 1 },
    ];
    expect(cartTotal(items)).toBe(22.5);
  });

  test("returns 0 for an empty cart", () => {
    expect(cartTotal([])).toBe(0);
  });
});

The run command runs every test file once and prints a summary:

$ npx vitest run

 RUN  v5.1.0 /home/user/shop

 Test Files  1 passed (1)
      Tests  2 passed (2)
   Start at  09:41:07
   Duration  255ms (transform 51%, import 31%, worker 11%, tests 7%)

Without run, the vitest command starts watch mode in an interactive terminal and reruns the related tests on every save. In CI or any other non-interactive shell, it runs the tests once instead:

$ npx vitest

To run only the tests whose names match a pattern, pass -t:

$ npx vitest run -t "empty cart"

Most projects add a test script to package.json, so npm test starts Vitest:

{
  "scripts": {
    "test": "vitest"
  }
}

For the full list of matchers and command-line options, see the Vitest documentation.