src.js

'use strict';

(function() {
  function common(method, url, data, options) {
    return new Promise(function (resolve, reject) {
      let xhr = new XMLHttpRequest();
      options = options || {};
      xhr.open(method, url);

      if (/POST|PUT/i.test(method)) {
        xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
      }

      if (data && typeof data === 'object') {
        let pairs = [];
        if (data instanceof FormData) {
          for(let pair of data.entries()) {
            pairs.push(encodeURIComponent(pair[0]) + '=' + encodeURIComponent(pair[1]));
          }
        }
        else {
          for(let key in data) {
            pairs.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
          }
        }
        console.log(pairs);
        data = pairs.join('&');
      }

      if (options.before) {
        options.before(xhr);
      }

      xhr.onreadystatechange = function() {
        if (xhr.readyState === xhr.DONE) {
          if (xhr.status === 200) {
            resolve(xhr);
          }
          else {
            reject(xhr);
          }
        }
      };

      xhr.send(data);
    });
  }

  let request = {};
  
  request.get = function(url, options) {
    return common('GET', url, undefined, options);
  }

  request.post = function(url, data, options) {
    return common('POST', url, data, options);
  }

  request.put = function(url, data, options) {
    return common('PUT', url, data, options);
  }

  request.delete = function(url, options) {
    return common('DELETE', url, undefined, options);
  }

  request.head = function(url, options) {
    return common('HEAD', url, undefined, options);
  }

  window.request = request;
})();