Vitest Has a Free API — Here's How to Test JavaScript at Blazing Speed

#vitest #testing #javascript #typescript

Vitest is a Vite-native testing framework that is compatible with Jest's API but runs 10-20x faster. It provides instant HMR, TypeScript support, and ESM-first testing.

Installation

npm install -D vitest

Basic Tests

// sum.test.ts
import { describe, it, expect } from "vitest";
import { sum, multiply } from "./math";

describe("math", () => {
  it("adds numbers correctly", () => {
    expect(sum(1, 2)).toBe(3);
    expect(sum(-1, 1)).toBe(0);
  });

  it("multiplies numbers", () => {
    expect(multiply(3, 4)).toBe(12);
  });
});

Async Testing

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

describe("API", () => {
  it("fetches users", async () => {
    const users = await fetchUsers();
    expect(users).toHaveLength(10);
    expect(users[0]).toHaveProperty("name");
  });

  it("handles errors", async () => {
    await expect(fetchUser("invalid")).rejects.toThrow("Not found");
  });
});

Mocking

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

// Mock a module
vi.mock("./database", () => ({
  getUser: vi.fn().mockResolvedValue({ id: 1, name: "Test" })
}));

import { getUser } from "./database";

describe("with mocks", () => {
  it("uses mocked database", async () => {
    const user = await getUser(1);
    expect(user.name).toBe("Test");
    expect(getUser).toHaveBeenCalledWith(1);
  });
});

// Spy on functions
const spy = vi.spyOn(console, "log");
console.log("test");
expect(spy).toHaveBeenCalledWith("test");

// Fake timers
vi.useFakeTimers();
setTimeout(() => callback(), 1000);
vi.advanceTimersByTime(1000);
expect(callback).toHaveBeenCalled();

Snapshot Testing

it("matches snapshot", () => {
  const result = generateHTML({ title: "Test" });
  expect(result).toMatchSnapshot();
});

it("matches inline snapshot", () => {
  expect(formatDate(new Date(2026, 0, 1))).toMatchInlineSnapshot(`"January 1, 2026"`);
});

Configuration

// vitest.config.ts
import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    globals: true,
    environment: "jsdom", // or "node"
    coverage: {
      provider: "v8",
      reporter: ["text", "html"],
      thresholds: { lines: 80, functions: 80 }
    },
    include: ["src/**/*.test.ts"]
  }
});

vitest           # Watch mode
vitest run       # Single run
vitest --coverage  # With coverage

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