XHR + Promises + responseType

xhr_promises.js

var xhr = o => {

  return new Promise(function(resolve, reject) {
    var req = new XMLHttpRequest();
    o.method = o.method || 'GET';
    // third parameter must be there and be true to use o.formData
    req.open(o.method, o.url, true);
    // default text; can send json
    req.responseType = o.responseType || 'text';
    // get reply back from API in proper language
    req.setRequestHeader('Accept-Language', window.navigator.language);
    // for writing to DB
    if (o.csrfToken) {
      req.setRequestHeader('X-CSRFToken', o.csrfToken);
    }
    // got something
    req.onload = () => {
      // somewhere in the 200 range is good
      if (req.status > 199 && req.status < 300 ) {
        resolve(req.response);
      }
      else {
        // status outside the 200 range = error
        reject(Error("Status Error " + req.statusText));
      }
    };
    // got nothing
    req.onerror = () => {
      reject(Error("Network Error "  + req.statusText));
    };
    // are we posting something?
    if (o.formData) {
      req.send(o.formData);
    } else {
      req.send();
    }
  });

}

var file = {
  // we know this should work, since Pinterest widgets have CORS
  good_xhr_widget_reply_json: {
    url: 'https://widgets.pinterest.com/v3/pidgets/pins/info/?pin_ids=99360735500167749',
    responseType: 'json'
  },
  good_xhr_cors_reply_html: {
    url: 'http://html5rocks-cors.s3-website-us-east-1.amazonaws.com/index.html'
  },
  // XHR Access-Control-Allow-Origin fail
  bad_xhr_github_cors: {
    url: 'https://github.com'
  },
  // 404 not found
  bad_xhr_name_not_resolved: {
    url: 'http://githuuuuuub.com'
  },
  // 404 not found
  bad_xhr_github_cors_and_404: {
    url: 'http://github.com/splootius-maxiumus'
  }
};

// where our results will go
var win = {};
var fail = {};

// when we're all done, show results
var done = () => {
  console.log('WIN', win);
  console.log('FAIL', fail);
};

// file counter
var toGo = 0;

// are we there yet?
var checkDoneLoading = () => {
  toGo = toGo - 1;
  if (!toGo) {
    done();
  }
};
      
// request all files
for (let k in file) {
  toGo = toGo + 1;
  xhr(file[k]).then(
    // worked
    response => {
      win[k] = response;
      checkDoneLoading();
    }, 
    // failed
    error => {
      fail[k] = error;
      checkDoneLoading();
    }
  );
}