Jest extensions

Usage

some-test.js:

const jestExtensions = require('jest-extensions')
jestExtensions.extend(expect)

test('success', () => {
  const mock = jest.fn()
  mock('arg1', 'arg2')
  mock('arg3', 'arg4')
  expect(mock).toBeCalledTimes(2)
  expcet(mock).toBeCalledNthWith(0, 'arg1', 'arg2')
  expcet(mock).toBeCalledNthWith(1, 'arg3', 'arg4')
})

test('fail', () => {
  const mock = jest.fn()
  expect(mock).toBeCalledOnce()
})

Assertions

• toBeCalledTimes(number) is an alias for toHaveBeenCalledTimes(number)
• toBeCalledOnce() is an alias for toHaveBeenCalledTimes(1)
• toBeCalledNthWith(index, arg1, arg2, ...) is equivalent to:

expect(mockedFunction.mock.calls[index][0]).toEqual(arg1)
expect(mockedFunction.mock.calls[index][1]).toEqual(arg2)
// ...

Asymmetric matchers

• expect.and(matcher1, matcher2, ...) - logical AND operator over many matchers.
• expect.or(matcher1, matcher2, ...) - logical OR operator over many matchers.
• expect.toContain(<value>) - checks that the value is contained in the actual collection.
• expect.toHaveLength(<number>) - checks that the actual has a .length property and it is set to a certain numeric value.

Utilities

• mockModuleClass gets a new class with all methods mocked, except those specified in arguments. For example:

./src/MyClass.js:

export default class MyClass {
  
  now() {
    return new Date
  }

  async getFromApi(date) {
    // asynchronously request external API
  }

  async getNowData() {
    return await this.getFromApi(this.now())
  }

}

./src/__tests__/MyClass.test.js:

import { mockModuleClass } from 'jest-extensions'

function mockMyClass(...unmockedMethods) {
  return mockModuleClass(__dirname + '/../MyClass.js', ...unmockedMethods)
}

describe('MyClass', () => {

  describe('getNowData', () => {

    it('succeeds', async () => {
      const sut = new mockMyClass('getNowData')()

      // All other methods are already mocked.
      // Even if mock implementations aren't defined,
      // no accidental side effect evaluations or state changes can occur.
      sut.now.mockImplementationOnce(() => new Date('2017-01-16'))
      sut.getFromApi.mockImplementation(() => ({ data: 'value' }))

      const result = await sut.getNowData()

      expect(sut.getFromApi).toBeCalledWith(new Date('2017-01-16'))
      expect(result).toEqual({ data: 'value' })
    })

  })

})