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JavaScript Testing Cheatsheet

At a glance​

What it isTest runners for JS/TS: Jest (batteries included), Vitest (Jest-compatible API on top of Vite), Mocha (minimal runner, bring your own assertions)
Use it forUnit and integration tests, mocking modules and timers, snapshot tests, coverage in CI
Latest stableJest v30.5.2 (released 2026-09-18) · Vitest v5.0.5 (released 2026-10-15) · Mocha v12.0.3 (released 2026-10-01) - checked 2026-10-31
Official docsjestjs.io · vitest.dev · mochajs.org

Core concepts​

  • Suite and test: describe groups tests, test / it defines one case.
  • Matchers: expect(value).toBe(...). Jest and Vitest bundle them; Mocha usually pairs with node:assert or Chai.
  • Hooks: beforeAll, beforeEach, afterEach, afterAll (Mocha: before / after).
  • Test doubles: jest.fn() / vi.fn() for mocks, spyOn to watch a real method, module mocks to replace imports.
  • Fake timers: control setTimeout, setInterval and Date without waiting.
  • Environment: node (default) or jsdom / happy-dom for DOM tests. Vitest also has a real-browser Browser Mode.
  • Snapshots: store serialized output and fail when it changes. Review diffs, do not blindly update.
  • Coverage: --coverage, with V8 or Istanbul as the provider.

Syntax & commands​

npm i -D jest                 # Jest
npx jest                      # run all
npx jest user                 # files matching "user"
npx jest -t "adds numbers"    # tests whose name matches
npx jest --watch              # re-run changed
npx jest --coverage

npm i -D vitest               # Vitest
npx vitest                    # watch mode in dev, run once in CI
npx vitest run --coverage

npm i -D mocha                # Mocha
npx mocha "test/**/*.test.js"
// Jest (globals) · Vitest: import { describe, test, expect, vi } from "vitest"
describe("sum", () => {
  beforeEach(() => { /* reset state */ });

  test("adds numbers", () => {
    expect(sum(1, 2)).toBe(3);
  });

  test.each([[1, 1, 2], [2, 3, 5]])("sum(%i, %i) = %i", (a, b, out) => {
    expect(sum(a, b)).toBe(out);
  });
});
MatcherChecks
toBe(x)Object.is equality (primitives, same reference)
toEqual(obj) / toStrictEqual(obj)deep equality (strict also checks undefined props and classes)
toBeTruthy() / toBeNull() / toBeUndefined()truthiness and empty values
toContain(item) / toHaveLength(n)arrays and strings
toMatch(/re/) / toMatchObject(partial)regex / subset of an object
toThrow("msg")function throws (wrap the call: expect(() => fn()))
toHaveBeenCalledWith(...args) / toHaveBeenCalledTimes(n)mock calls
resolves / rejectspromise outcome: await expect(p).rejects.toThrow()

Common patterns​

// Async tests
test("loads user", async () => {
  const user = await getUser(1);
  expect(user).toMatchObject({ id: 1 });
});
test("fails on missing id", async () => {
  await expect(getUser(-1)).rejects.toThrow("not found");
});
// Mocks and spies (Jest; replace jest with vi in Vitest)
const onSave = jest.fn().mockResolvedValue({ ok: true });
const spy = jest.spyOn(api, "fetchUser").mockReturnValue({ id: 1 });

jest.mock("./mailer", () => ({ send: jest.fn() })); // whole module (hoisted)

afterEach(() => jest.restoreAllMocks());
// Fake timers
jest.useFakeTimers();
debouncedSave();
jest.advanceTimersByTime(300);
expect(save).toHaveBeenCalledTimes(1);
jest.useRealTimers();
// Mocha + node:assert
import assert from "node:assert/strict";

describe("Array", () => {
  it("returns -1 when missing", () => {
    assert.equal([1, 2, 3].indexOf(4), -1);
  });
});
JestVitestMocha (+ Chai / Sinon)
jest.fn()vi.fn()sinon.fake() / sinon.stub()
jest.mock(path)vi.mock(path)proxyquire / esmock / node:test mocks
jest.useFakeTimers()vi.useFakeTimers()sinon.useFakeTimers()
jest.config.jsvitest.config.ts (or test in vite.config.ts).mocharc.json

Gotchas​

  • Jest runs CommonJS by default. Native ESM needs node --experimental-vm-modules (or Babel / ts-jest transform). Vitest handles ESM and TS natively.
  • jest.mock() calls are hoisted above imports. Variables used inside the factory must start with mock.
  • Forgetting await (or return) on a promise lets the test pass before the assertion runs. Vitest 5 now fails unawaited resolves / rejects.
  • toBe on objects compares references. Use toEqual for value equality.
  • Mocks leak between tests unless you reset them (clearMocks / restoreMocks in config, or afterEach).
  • Mocha arrow functions have no this, so this.timeout() only works in function () {} callbacks.
  • Mocha does not combine done and a returned promise. Use one style per test.

What's new in recent releases​

  • Vitest 5 (2026-09): needs Node.js 22.12+ and Vite 6.4+, faster pools, unawaited async assertions and expect.poll timeouts now fail, reports and attachments move under a shared .vitest/ folder.
  • Vitest 4 (2025-10): Browser Mode became stable, with visual regression testing (toMatchScreenshot).
  • Jest 30 (2025-06): drops Node.js 14/16/19/21 and needs TypeScript 5.4+, jsdom 26, --testPathPattern renamed to --testPathPatterns, alias matchers such as toBeCalledWith removed (use toHaveBeenCalledWith), using support for spies. Jest 30.x minors continue on the same major.
  • Mocha 12 (2026-08): minimum Node.js raised from 18.18 to 20.19 (or 22.12+), the package is now ESM, the CLI parser moved from yargs to Node's util.parseArgs, and --forbid-only defaults on when CI is set.

In these notes​