The Expect object

The expect global object is not only used to retrieve matcher for a given value, but it also has very handy utilities for assertions too.

expect.anything

expect.anything() matches anything but null or undefined. You can use it inside toEqual or toBeCalledWith instead of a literal value. For example, if you want to check that a mock function is called with a non-null argument

test('anything', () => {
  const mock = jest.fn()
  mock.fn(55)
  expect(mock).toBeCalledWith(expect.anything()) // true
})

expect.any(constructor)

expect.any matches with anything that was created with a given constructor

class MyClass {}

test('any', () => {
  expect(Math.random()).toEqual(expect.any(Number))
  expect('aaaaaa').toEqual(expect.any(String))
  expect({ a: 1 }).toEqual(expect.any(Object))
  expect(() => {
    console.log('x')
  }).toEqual(expect.any(Function))

  expect(new MyClass()).toEqual(expect.any(MyClass))
})

expect.arrayContaining(array)

test('matches even if received contains additional elements', () => {
  const expected = ['Alice', 'Bob']
  expect(['Alice', 'Bob', 'Eve']).toEqual(expect.arrayContaining(expected))
})
test('does not match if received does not contain expected elements', () => {
  const expected = ['Alice', 'Bob']
  expect(['Bob', 'Eve']).not.toEqual(expect.arrayContaining(expected))
})

expect.stringContaining(string) and expect.stringMatching(string | regex)

test('string', () => {
  expect('im no superman').toEqual(expect.stringContaining('man'))
  expect('im no superman').toEqual(expect.stringMatching('man'))
})

expect.objectContaining(object)

test('anything', () => {
  const data = {
    name: 'bob',
    age: 44,
    address: {
      city: 'milan',
    },
  }
  expect(data).toEqual(
    expect.objectContaining({
      address: {
        city: expect.any(String),
      },
    }),
  )

  expect(data).toMatchObject({
    name: expect.any(String),
  })
})

Author: Jaga Santagostino