jest cheet sheet

jest_cheet_sheet.js

test('two plus two is four', () => {
  expect(2 + 2).toBe(4)
})

test('object assignment', () => {
  const data = { one: 1 }
  data['two'] = 2
  expect(data).toEqual({ one: 1, two: 2 })
})

test('adding positive numbers is not zero', () => {
  for (let a = 1; a < 10; a++) {
    for (let b = 1; b < 10; b++) {
      expect(a + b).not.toBe(0)
    }
  }
})

test('null', () => {
  const n = null
  expect(n).toBeNull()
  expect(n).toBeDefined()
  expect(n).not.toBeUndefined()
  expect(n).not.toBeTruthy()
  expect(n).toBeFalsy()
})

test('zero', () => {
  const z = 0
  expect(z).not.toBeNull()
  expect(z).toBeDefined()
  expect(z).not.toBeUndefined()
  expect(z).not.toBeTruthy()
  expect(z).toBeFalsy()
  expect(z).toBe(0)
})

test('two plus two', () => {
  const value = 2 + 2
  expect(value).toBeGreaterThan(3)
  expect(value).toBeGreaterThanOrEqual(3)
  expect(value).toBeLessThan(5)
  expect(value).toBeLessThanOrEqual(4.5)
  expect(value).toBe(4)
  expect(value).toEqual(4)
})

test('adding floating point number', () => {
  const value = 0.1 + 0.2
  expect(value).toBeCloseTo(0.3)
})

test('there is no I in team', () => {
  expect('team').not.toMatch(/I/)
})

test('but there is a "stop" in Christoph', () => {
  expect('Christoph').toMatch(/stop/)
})

test('the shopping list has beer on it', () => {
  const shoppingList = [
    'diapers',
    'kleenex',
    'trash bags',
    'paper towels',
    'beer',
  ];
  expect(shoppingList).toContain('beer')
  expect(new Set(shoppingList)).toContain('beer')
})

class ConfigError extends Error {}
function compileAndroidCode() {
  throw new ConfigError('you are using the wrong JDK')
}

test('compling android goes as expected', () => {
  expect(compileAndroidCode).toThrow()
  expect(compileAndroidCode).toThrow(ConfigError)

  expect(compileAndroidCode).toThrow('you are using the wrong JDK')
  expect(compileAndroidCode).toThrow(/JDK/)
})

// ===================== Callbacks ============================
test('the data is peanut butter', done => {
  function fetchData(callback) {
    callback('peanut butter')
  }

  function callback(data) {
    setTimeout(() => {
      expect(data).toBe('peanut butter')
      done()
    }, 1000)
  }

  fetchData(callback)
})

// ==================== Promise ================================
function promiseFetchData(success) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (!success) return reject('error')
      return resolve('peanut butter')
    }, 100)
  })
}

it('the data is peanut butter', () => {
  const resp = 'qqq'
  const mockFunction = jest.fn(promiseFetchData)
  mockFunction.mockResolvedValue(resp)

  return mockFunction().then(data => {
    expect(data).toBe(resp)
  })
})

test('the fetch fails with an error', () => {
  return promiseFetchData(false).catch(error => {
    expect.assertions(1)
    expect(error).toMatch('error')
  })
})

test('promise: the data is peanut butter', () => {
  return expect(promiseFetchData(true)).resolves.toBe('peanut butter')
})

test('promise: the data is peanut butter', () => {
  return expect(promiseFetchData(false)).rejects.toBe('error')
})

// ==================== Async/Await ================================
async function asyncFetchData(success) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (!success) return reject('error')
      return resolve('peanut butter')
    }, 100)
  })
}

test('the data is peanut butter', async () => {
  expect.assertions(1)
  const data = await asyncFetchData(true)
  expect(data).toBe('peanut butter')
})

test('the fetch data fails with an error', async () => {
  expect.assertions(1)
  try {
    await asyncFetchData(false)
  } catch(e) {
    expect(e).toMatch('error')
  }
})

test('the data is peanut butter', async () => {
  await expect(asyncFetchData(true)).resolves.toBe('peanut butter')
})

test('the fetch fails with an error', async () => {
  await expect(asyncFetchData(false)).rejects.toMatch('error')
})

// ======================= Mock Functions ===============================
function forEach(items, callback) {
  for (let index = 0; index < items.length; index++) {
    callback(items[index], 99)
  }
}

const mockCallback = jest.fn(x => 42 + x)
test('forEach', () => {
  forEach([0, 1], mockCallback)
  expect(mockCallback.mock.calls.length).toBe(2)
  expect(mockCallback.mock.calls[0][0]).toBe(0)
  expect(mockCallback.mock.calls[1][0]).toBe(1)
  expect(mockCallback.mock.results[0].value).toBe(42)
  expect(mockCallback.mock.results[1].value).toBe(43)
})