cheatsheets/jest.md

title	category	layout	updated	weight
Jest	JavaScript libraries	2017/sheet	2017-08-26	-3

Testing

{: .-three-column}

Quick start

{: .-prime}

npm install --save-dev jest babel-jest

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

{: .-setup}

# Run your tests
npm test -- --watch

{: .-setup}

See: Getting started

Writing tests

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

BDD syntax

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

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

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

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

Expect

{: .-three-column}

Basic expectations

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

See: expect()

Snapshots

expect(value)
  .toMatchSnapshot()

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

Snapshots

const tree = renderer.create(
  <Link page="http://www.facebook.com">Facebook</Link>
).toJSON()

// First run creates a snapshot; subsequent runs match it
expect(tree).toMatchSnapshot()

# To update snapshots
jest --updateSnapshot

Mocks

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

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

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

Timers

jest.useFakeTimers()

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

See: Timers

References

{: .-one-column}

• Based on Jest v19.
• http://facebook.github.io/jest/ {: .-also-see}