FakeXMLHttpRequest falsly sets responseXML

[Ninerian]

• Sinon version : 4.2.1
• Environment: Node 9.4.0, Chrome, Firefox, Safari
• Other libraries you are using: karma, chai, mocha

What did you expect to happen?
When a invalid xml file is loaded with XMLHttpRequest, the responseXML is null.

What actually happens
The FakeXMLHttpRequest fills the responseXML with the parsed xml document with included parseErrors.

How to reproduce

const invalidXMLData = `!!!<?xml version="1.0" encoding="UTF-8"?><start>`;

function testXMLRequest(options) {
  var xhr = new XMLHttpRequest();
  xhr.open('GET', options.url, true);

  // If specified, responseType must be empty string or "document"
  xhr.responseType = 'document';

  // overrideMimeType() can be used to force the response to be parsed as XML
  xhr.overrideMimeType('text/xml');

  xhr.onload = function() {
    if (xhr.readyState === xhr.DONE) {
      if (xhr.status === 200) {
        if (xhr.responseXML) {
          options.onSuccess();
        } else {
          options.onErrorCallback();
        }
      }
    }
  };

  xhr.send(null);
}

let xhr, requests, requestor;

before(() => {
  xhr = sinon.useFakeXMLHttpRequest();
  requests = [];
  xhr.onCreate = function(xhr) {
    requests.push(xhr);
  };
});

after(() => {
  xhr.restore();
});

it('Should throw an error, when the xml is invalid', () => {
  let successCallback = sinon.spy();
  let errorCallback = sinon.spy();

  const reqSettings = {
    url: '/xml',
    onSuccess: successCallback,
    onErrorCallback: errorCallback,
  };

  testXMLRequest(reqSettings);

  requests[0].respond(200, { 'Content-Type': 'text/text' }, invalidXMLData);

  expect(successCallback.notCalled, 'Success callback is not called').to.be.true;
  expect(errorCallback.calledOnce, 'Error callback is called').to.be.true;
});

Solution
After parsing the response, you should check for parseerrors and return null.

[mroderick]

Issue migrated to sinonjs/nise#25