Mock a Ajax call with Jest test runner

Feature.js

class Feature {
  load(url, callback) {
    $.ajax({ url })
      .done((data) => {
        callback.apply(this, [null, data])
      })
      .fail((jqXHR) => {
        callback.apply(this, [jqXHR.statusText])
      })
  }
}

Feature.spec.js

import Feature from './Feature'

describe('Feature', () => {
  const fakeData = { x: 0, y: 0 }
  let subject
  let callback
  
  beforeEach(() => {
    callback = jest.fn()
    $.ajax = jest.fn(() => {
      let deferred = $.Deferred()

      deferred.resolve(fakeData)
      return deferred.promise()
    })
    subject = new Feature()
  })
  
  it('loads JSON data', () => {
    url = 'load_path'
    subject.load(url, callback)

    expect($.ajax).toHaveBeenCalledWith({ url })
  })
  it('calls callback when successful', () => {
    expect(callback).toHaveBeenCalledWith(null, fakeData)
  })
})