A quick overview to Jest, a test framework for Node.js. This guide targets Jest v20.

Testing

Quick start

npm install --save-dev jest babel-jest

/* Add to package.json */
"scripts": {
  "test": "jest"
}

# Run your tests
npm test -- --watch

See: Getting started

Writing tests

describe('My work', () => {
  test('works', () => {
    expect(2).toEqual(2)
  })
})

See: describe(), test(), expect()

BDD syntax

describe('My work', () => {
  it('works', () => {
    ···
  })
})

it is an alias for test. See: test()

Setup

beforeEach(() => { ... })
afterEach(() => { ... })

beforeAll(() => { ... })
afterAll(() => { ... })

See: afterAll() and more

Focusing tests

describe.only(···)
it.only(···) // alias: fit()

See: test.only

Skipping tests

describe.skip(···)
it.skip(···) // alias: xit()

See: test.skip

Optional flags

Flag Description
--coverage See a summary of test coverage
--detectOpenHandles See a summary of ports that didn’t close
--runInBand Run all tests one after the other

Expect

Basic expectations

expect(value)
  .not
  .toBe(value)
  .toEqual(value)
  .toBeTruthy()

Note that toEqual is a deep equality check. See: expect()

Snapshots

expect(value)
  .toMatchSnapshot()
  .toMatchInlineSnapshot()

Note that toMatchInlineSnapshot() requires Prettier to be set up for the project. See: Inline snapshots

Errors

expect(value)
  .toThrow(error)
  .toThrowErrorMatchingSnapshot()

Booleans

expect(value)
  .toBeFalsy()
  .toBeNull()
  .toBeTruthy()
  .toBeUndefined()
  .toBeDefined()

Numbers

expect(value)
  .toBeCloseTo(number, numDigits)
  .toBeGreaterThan(number)
  .toBeGreaterThanOrEqual(number)
  .toBeLessThan(number)
  .toBeLessThanOrEqual(number)

Objects

expect(value)
  .toBeInstanceOf(Class)
  .toMatchObject(object)
  .toHaveProperty(keyPath, value)

Objects

expect(value)
  .toContain(item)
  .toContainEqual(item)
  .toHaveLength(number)

Strings

expect(value)
  .toMatch(regexpOrString)

Others

expect.extend(matchers)
expect.any(constructor)
expect.addSnapshotSerializer(serializer)

expect.assertions(1)

More features

Asynchronous tests

test('works with promises', () => {
  return new Promise((resolve, reject) => {
    ···
  })
})

test('works with async/await', async () => {
  const hello = await foo()
  ···
})

Return promises, or use async/await. See: Async tutorial

Snapshots

it('works', () => {
  const output = something()
  expect(output).toMatchSnapshot()
})

First run creates a snapshot. Subsequent runs match the saved snapshot. See: Snapshot testing

React test renderer

import renderer from 'react-test-renderer'

it('works', () => {
  const tree = renderer.create(
    <Link page="https://www.facebook.com">Facebook</Link>
  ).toJSON()

  expect(tree).toMatchSnapshot()
})

React’s test renderer can be used for Jest snapshots. See: Snapshot test

Timers

jest.useFakeTimers()

it('works', () => {
  jest.runOnlyPendingTimers()
  jest.runTimersToTime(1000)
  jest.runAllTimers()
})

See: Timer Mocks

Mock functions

Mock functions

const fn = jest.fn()

const fn = jest.fn(n => n * n)

See: Mock functions

Assertions

expect(fn)
  .toHaveBeenCalled()
  .toHaveBeenCalledTimes(number)
  .toHaveBeenCalledWith(arg1, arg2, ...)
  .toHaveBeenLastCalledWith(arg1, arg2, ...)

expect(fn)
  .toHaveBeenCalledWith(expect.anything())
  .toHaveBeenCalledWith(expect.any(constructor))
  .toHaveBeenCalledWith(expect.arrayContaining([ values ]))
  .toHaveBeenCalledWith(expect.objectContaining({ props }))
  .toHaveBeenCalledWith(expect.stringContaining(string))
  .toHaveBeenCalledWith(expect.stringMatching(regexp))

Instances

const Fn = jest.fn()

a = new Fn()
b = new Fn()

Fn.mock.instances
// → [a, b]

See: .mock property

Calls

const fn = jest.fn()
fn(123)
fn(456)

fn.mock.calls.length   // → 2
fn.mock.calls[0][0]    // → 123
fn.mock.calls[1][0]    // → 456

See: .mock property

Return values

const fn = jest.fn(() => 'hello')

or:

jest.fn().mockReturnValue('hello')
jest.fn().mockReturnValueOnce('hello')

Mock implementations

const fn = jest.fn()
  .mockImplementationOnce(() => 1)
  .mockImplementationOnce(() => 2)

fn()    // → 1
fn()    // → 2

References

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