Jest Cheatsheet - Testing Framework

Tests only earn their keep when a failure tells you precisely what broke. This reference covers describe/it grouping, matchers beyond toBe, async and promise testing, jest.fn/jest.mock module mocking, snapshot testing, and coverage output. It emphasizes the matchers and mock strategies that pinpoint broken behavior instead of vague red. For teams adding real unit coverage to JS/TS code. After reading you write focused assertions and mock the exact seams you intend to.

Languages·44 commands·Last updated 2026-07-21

Basic Tests 6

test("desc", () => { expect(1 + 2).toBe(3) })

Define a test case

it("desc", fn)

Alias of test, more natural BDD style

describe("module", () => { it(...) })

Group related tests

test.skip("skip", () => {})

Skip this test temporarily

test.only("focus", () => {})

Run only this test (debug)

test.todo("todo")

Placeholder for a pending test

Matchers 8

expect(value).toBe(other)

Strict equality (Object.is)

expect(obj).toEqual({ a: 1 })

Deep equality (recursive)

expect(arr).toHaveLength(3)

Length of array/string

expect(str).toMatch(/regex/)

Match a string by regex

expect(arr).toContain(item)

Array contains an item

expect(value).toBeTruthy()

Truthy check (toBeFalsy opposite)

expect(fn).toThrow("error")

Expect a thrown error

expect(value).toBeNull()

Null check (also toBeUndefined/toBeDefined)

Async Tests 6

test("async", async () => { await fn() })

async/await style

test("promise", () => fn().then(d => expect(d).toBe(1)))

Return a promise for Jest to await

expect(promise).resolves.toBe(value)

Assert the resolved value

expect(promise).rejects.toThrow("err")

Assert the rejection reason

test("done", done => { fn(done) })

done callback style (manual)

jest.useFakeTimers()

Fake timers (with runAllTimers)

Hooks 4

beforeEach(() => setup())

Before each test

afterEach(() => cleanup())

After each test

beforeAll(() => init())

Once before all tests

afterAll(() => teardown())

Once after all tests

Mock Functions 8

const fn = jest.fn()

Create a mock function

jest.fn().mockReturnValue(42)

Set a fixed return value

jest.fn().mockResolvedValue(val)

Return a resolved promise

jest.fn().mockImplementation(n => n * 2)

Custom implementation

jest.spyOn(obj, "method")

Spy on a method (keeps impl)

jest.mock("./module")

Auto-mock a whole module

jest.requireActual("./module")

Require the real module in a mock

fn.mockClear() / mockReset() / mockRestore()

Clear/reset/restore a mock

Assertions & Call Verification 5

expect(fn).toHaveBeenCalled()

Verify a function was called

expect(fn).toHaveBeenCalledWith(arg)

Verify call arguments

expect(fn).toHaveBeenCalledTimes(2)

Verify call count

expect(fn).toHaveBeenLastCalledWith(arg)

Verify the last call args

expect(obj).toMatchObject({ a: 1 })

Partial object match

Snapshots & CLI 7

expect(tree).toMatchSnapshot()

Create/compare snapshot (-u to update)

expect(obj).toMatchInlineSnapshot()

Inline snapshot (written to file)

expect(value).toBeInstanceOf(Class)

Instance type assertion

npx jest --coverage

Generate a coverage report

npx jest --watch

Watch mode (rerun related; --watchAll for all)

npx jest --bail

Stop at first failure (--bail)

npx jest -t "case name"

Filter by test name regex (-t)

Tips

jest --coverage reports coverage; --watch enables watch mode.
jest.mock() auto-mocks all exports, returning jest.fn().
toMatchSnapshot suits UI/serialized-structure tests; update with -u.
jest.spyOn spies on a method without fully mocking it.
useFakeTimers with runAllTimers tests timer logic synchronously.
Remove test.only after debugging, or other tests get skipped.

Official References

Each command links to its official documentation below, so you can verify the latest usage and read deeper.

Node.js Docs

Maintained by LaoHand

Publicly updated on Jul 21, 2026, continuously proofread against official docs.

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