XHR testing in Jest

Question

I want to test AJAX methods (vanilla XHR) and I can\'t find the way to do it with Jest framework. I found mock-ajax.js for Jasmine, the problem

Answer 1

This is how I did it.

test.js

const open = jest.fn();
const onload = jest.fn((x) => {/* <your response data> */});
const onerror = jest.fn();
const send = jest.fn(function(){
    this.onload()
})

const xhrMockClass = function () {
    return {
        open,
        send,
        onerror,
        onload
    };
};

global.XMLHttpRequest = jest.fn().mockImplementation(xhrMockClass);

// ...

test('Should make a request', () => {
 // do stuff to make request

 expect(send).toHaveBeenCalled()
 expect(onload).toHaveBeenCalledWith(/* <your response data> */)
 expect(open).toHaveBeenCalledWith('GET', 'some/url', true)
})

Answer 2

You can test XHR in Jest without additional packages. This is helper function which creates mock object for XMLHttpRequest:

let open, send, onload, onerror;

function createXHRmock() {
    open = jest.genMockFn();

    // be aware we use *function* because we need to get *this* 
    // from *new XmlHttpRequest()* call
    send = jest.genMockFn().mockImpl(function(){   
        onload = this.onload.bind(this);
        onerror = this.onerror.bind(this);
    });

    const xhrMockClass = function () {
        return {
            open,
            send
        };
    };

    window.XMLHttpRequest = jest.genMockFn().mockImpl(xhrMockClass);
}

And you can use it in test as follows:

it('XHR success', () => {
    createXHRmock();

    // here you should call GET request

    expect(open).toBeCalledWith('GET', 'http://example.com', true);
    expect(send).toBeCalled();

    // call onload or onerror
    onload();

    // here you can make your assertions after onload
});

Answer 3

Here's a TypeScript example working with Jest 26:

function performRequest(callback: any) {
  const xhr = new XMLHttpRequest();
  xhr.open('GET', 'https://example.com/');
  xhr.onreadystatechange = () => {
    if (xhr.readyState !== 4 || xhr.status !== 200) return;
    callback(xhr.response);
  };
  xhr.responseType = 'json';
  xhr.setRequestHeader('Accept', 'application/json');
  xhr.send(null);
}

describe('request', () => {
  const xhrMock: Partial<XMLHttpRequest> = {
    open: jest.fn(),
    send: jest.fn(),
    setRequestHeader: jest.fn(),
    readyState: 4,
    status: 200,
    response: 'Hello World!'
  };

  jest.spyOn(window, 'XMLHttpRequest').mockImplementation(() => xhrMock as XMLHttpRequest);
  const callback = jest.fn();
  performRequest(callback);
  expect(xhrMock.open).toBeCalledWith('GET', 'https://example.com/');
  expect(xhrMock.setRequestHeader).toBeCalledWith('Accept', 'application/json');
  (xhrMock.onreadystatechange as EventListener)(new Event(''));
  expect(callback.mock.calls).toEqual([['Hello World!']]);
});

Answer 4

Here's a function that will create a XMLHttpRequest mock and install it into window for you.

const mockXMLHttpRequest = () => {
  const mock = {
    open: jest.fn(),
    addEventListener: jest.fn(),
    setRequestHeader: jest.fn(),
    send: jest.fn(),
    getResponseHeader: jest.fn(),

    upload: {
      addEventListener: jest.fn(),
    },
  };

  window.XMLHttpRequest = jest.fn(() => mock);

  return mock;
};

Which you can then use in a test like so:

const mock = mockXMLHttpRequest();

// Get the upload progress callback
// This assumes you attach your listeners in a stable order
expect(mock.upload.addEventListener).toHaveBeenCalledTimes(1);
const [[, progress]] = mock.upload.addEventListener.mock.calls;

// Get the load callback
expect(mock.addEventListener).toHaveBeenCalledTimes(1);
const [[, load]] = mock.addEventListener.mock.calls;

expect(mock.open).toHaveBeenCalled();
expect(mock.send).toHaveBeenCalled();

// Fire a progress event
progress({ loaded: 12, total: 100 });

// ...

mock.status = 201;
mock.getResponseHeader.mockReturnValue('application/json');
mock.response = JSON.stringify({ id: 111 });

// Fire a load event
load();

// ...

Answer 5

The jest api has changed a bit. This is what I use. It doesn't do anything but it's enough to render my components.

const xhrMockClass = () => ({
  open            : jest.fn(),
  send            : jest.fn(),
  setRequestHeader: jest.fn()
})

window.XMLHttpRequest = jest.fn().mockImplementation(xhrMockClass)

and in the test file:

import '../../__mock__/xhr-mock.js'

Answer 6

As mentioned you don't need additional libraries:

 // mocks.js is where you could put your mocks (example below)
 const mocks = require('./mocks.js')

 test('XHR test', () => {
   let xhrMock = {
      open: jest.fn(),
      setRequestHeader: jest.fn(),
      onreadystatechange: jest.fn(),
      send: jest.fn(),
      readyState: 4,
      responseText: JSON.stringify(mocks),
      status: 200
    }

   window.XMLHttpRequest = jest.fn(() => xhrMock)

   sendData().then((response) => {
      // You could do you checks here. Some examples:
      expect(xhrMock.setRequestHeader).toBeCalledWith('Cache-Control', 'no-cache')
      expect(xhrMock.open).toBeCalledWith('POST', 'you_api_url.com/end_point/')
      expect(xhrMock.withCredentials).toBe(false)

      let formData = new FormData()
      formData.append('firstParam', 'firstParamValue')  

      expect(yoloRequestMock.send).toBeCalledWith(formData)
      expect(JSON.stringify(response)).toBe(JSON.stringify(mocks))
   })
   // when onload is called, resolve and reject has been initialed.
   xhrMock.onreadystatechange()

    // the following function is the one which sends the request (to be tested)
    function sendData() {
       return new Promise(function(resolve, reject) {
         let formData = new FormData()
         formData.append('firstParam', 'firstParamValue')
         let xhr = new XMLHttpRequest()
         xhr.withCredentials = false
         xhr.onreadystatechange = function () {
           if (this.readyState === 4) {
             if(xhr.status === 200) {
               resolve(JSON.parse(xhr.responseText))
             } else {
               reject(xhr.responseText)
             }
           }
         }
         xhr.open('POST', T2S_VISUAL_SEARCH_PARAMS.t2sVisualSeacchApiUrl)
         xhr.setRequestHeader("Cache-Control", "no-cache");
         xhr.send(formData)
       })
    }
  }

The file mocks.js contains the mocks:

module.exports =
  {
    response: {
      body: { ... }
    }
  }