Vitest

Vitest is fast, has a great UI, mocks, snapshots, and a Jest-compatible API. The default for new projects.

4 min read Level 1/5 #nodejs#vitest#testing

What you'll learn

Install and run Vitest
Write tests with describe/it/expect
Use mocks and spies

Vitest is the test runner most new Node/Vite projects pick. Fast, TypeScript-friendly, Jest-compatible API.

Install

npm install --save-dev vitest

A First Test

// math.test.js
import { test, expect } from "vitest";
import { add } from "./math.js";

test("add: sums two numbers", () => {
  expect(add(2, 3)).toBe(5);
});

test("add: order doesn't matter", () => {
  expect(add(2, 3)).toBe(add(3, 2));
});

Run

npx vitest        # watch mode by default
npx vitest run    # single pass (for CI)

`describe`, `it`, `expect`

import { describe, it, expect, beforeEach } from "vitest";

describe("Cart", () => {
  let cart;
  beforeEach(() => { cart = new Cart(); });

  it("starts empty", () => {
    expect(cart.items).toHaveLength(0);
  });

  it("adds items", () => {
    cart.add({ id: 1, price: 10 });
    expect(cart.total).toBe(10);
  });
});

expect has dozens of matchers: .toBe, .toEqual, .toContain, .toThrow, .toBeCloseTo, .toMatchObject, .toMatchSnapshot.

Mocks

import { vi, expect, test } from "vitest";
import { sendEmail } from "./email.js";

vi.mock("./email.js", () => ({
  sendEmail: vi.fn().mockResolvedValue({ id: "msg_123" }),
}));

test("signup sends a welcome email", async () => {
  await signup({ email: "ada@example.com" });
  expect(sendEmail).toHaveBeenCalledWith("ada@example.com", /welcome/i);
});

Coverage

npx vitest run --coverage

Outputs an HTML coverage report — great for finding untested branches.

UI

npx vitest --ui

A browser UI showing test status, output, and a file tree. Surprisingly useful for big test suites.

Why It Won

Same API as Jest (easy migration)
Native ESM + TypeScript out of the box
Vite-powered (super fast HMR for tests)
Built-in coverage, mocks, snapshots, UI
Works for backend AND frontend (JSX, jsdom)

For Node-only libraries: node:test is fine. For everything else: Vitest.

Debugging →