The Jest Book

describe('how to test asynchronous code in Jest', () => {
  
  it('can use done() to signal when to end the test', (done) => {
    fetch(endpoint)
      .then((response) => response.json())
      .then((data) => { expect(data).toBeTruthy(); done() })
  })
  
  it('can return a Promise and Jest will handle it', () => {
    return fetch(endpoint)
      .then((response) => response.json())
      .then((data) => expect(data).toBeTruthy())
  })  
  
  it('can use async and await to pause appropriately', async () => {
    const response = await fetch(endpoint)
    const data = await response.json()
    expect(data).toBeTruthy()
  })
  
})

// How to confirm your test made the correct number of assertions
it('can use done() to signal when to end the test', (done) => {
  expect.assertions(1)
  fetch(endpoint)
    .then((response) => response.json())
    .then((data) => { expect(data).toBeTruthy(); done() })
})

// How to mock fetch() in Jest
const mockedFetch = (data) => jest.fn().mockReturnValue(Promise.resolve({ json: () => Promise.resolve(data) }))

swapi.getPeople(mockedFetch).then(() => {
  
  // How to check mocked fetch was called the correct number of times
  expect(mockedFetch.mock.calls.length).toBe(1)
  
  // How to check mocked fetch was called with the correct arguments
  expect(mockedFetch).toBeCalledWith('http://swapi.co/api/people')  
})

// General Function Used In All Component Tests
const getPageObject = 
    (Tag, props, renderer, builder) => 
        builder(renderer(<Tag {...props} />))

// Specific Function, Defined Per Test Suite
const builder = (wrapper) => {
    const pageObject = {
        getClicks: wrapper.instance().getClicks(),
        debug: wrapper.debug()        
    }
    return pageObject
}

const pageObject = getPageObject(Header, props, shallow, builder)