Request with XMLHttpRequest and promises

makerequest.js

function makeRequest(opts) {
  return new Promise(function(resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.open(opts.method, opts.url);
    xhr.withCredentials = true;
    xhr.onload = function() {
      if (this.status >= 200 && this.status < 300) {
        resolve(xhr.response);
      } else {
        reject({
          status: this.status,
          statusText: xhr.statusText,
        });
      }
    };
    xhr.onerror = function() {
      reject({
        status: this.status,
        statusText: xhr.statusText,
      });
    };
    if (opts.headers) {
      Object.keys(opts.headers).forEach(function(key) {
        xhr.setRequestHeader(key, opts.headers[key]);
      });
    }
    var params = opts.params;
    var formdata = opts.formdata;
    // We'll need to stringify if we've been given an object
    // If we have a string, this is skipped.
    if (params && typeof params === 'object') {
      params = Object.keys(params)
        .map(function(key) {
          return (
            encodeURIComponent(key) + '=' + encodeURIComponent(params[key])
          );
        })
        .join('&');
    }
    if (params) var data = params;
    else if (formdata) var data = JSON.stringify(formdata);
    else var data = null;
    xhr.send(data);
  });
}