A micro ajax helper for promises

ajax.js

let ajax = (function() {
		/**
		 * Easily make an XHR request.
		 *
		 * @param {string} protocol
		 * @param {string} url
		 * @param {object} params -- WARNING. Only accepts shallow objects.
		 * @return {Promise}
		 */
		function xhr(protocol, url = '/', params = {}, headers = {}) {
			var paramStr = Object.keys(params)
								 .map(key => `${encodeURIComponent(key)}=${encodeURIComponent(params[key])}`)
								 .join('&');
			return new Promise(function(resolve, reject) {
				var xhr = new XMLHttpRequest();
				xhr.open(protocol, url);

				if (protocol == 'POST') {
					xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
				}

				Object.keys(headers).forEach(key => {
					xhr.setRequestHeader(key, headers[key]);
				});

				xhr.onload = function() {
					if (xhr.status == 200) {
						resolve(xhr.response);
					} else {
						reject(Error(xhr.statusText));
					}
				};
				// Network error, or rejected connection
				xhr.onerror = function() {
					reject(Error("Network Error"));
				};

				if (paramStr) {
					xhr.send(paramStr);
				} else {
					xhr.send();
				}
			});
		}

		/**
		 * Function to GET a page. Passes the response of the XHR in the resolve promise.
		 *
		 * @param {string} url
		 * @param {object} params
		 * @param {object} headers
		 * @return {Promise}
		 */
		function get(url = '/', params = {}, headers = {}) {
			return xhr('GET', url, params, headers);
		}


		/**
		 * Function to make a post xhruest
		 *
		 * @param {string} url
		 * @param {object} params
		 * @param {objct} headers
		 * @return {Promise}
		 */
		function post(url = '/', params = {}, headers = {}) {
			return xhr('POST', url, params, headers);
		}

		return {xhr, get, post};
	}());