promise-based, jQuery-like $.post XHR wrapper

post.js

function reqResponse(xhr) {
  var data = xhr.responseText;
  if ((xhr.getResponseHeader('Content-Type') || '').split(';', 1)[0].indexOf('json') !== -1) {
    try {
      return JSON.parse(data);
    } catch(e) {
      // Oh well.
      return {};
    }
  }
  return data || null;
}

$.post = function(url, params) {
  return new Promise(function(resolve, reject) {
    params = serialize(params || {});

    var xhr = new XMLHttpRequest();
    xhr.open('post', url, true);

    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xhr.setRequestHeader('Content-Length', params.length);
    xhr.setRequestHeader('Connection', 'close');

    xhr.send(params);

    xhr.addEventListener('load', function() {
      var res = reqResponse(xhr);

      var statusCode = xhr.status;
      if (statusCode < 200 || statusCode > 300) {
          return reject(res, xhr);
      }

      return resolve(res, xhr);
    }, false);
  });
};