@@ -103,26 +103,16 @@
       /***/ function (module, exports, __webpack_require__) {
         'use strict';

+        module.exports = function (url, etag) {
+          const req = new XMLHttpRequest();
-        module.exports = function (url) {
-          var req = new XMLHttpRequest();
           return new Promise(function (resolve, reject) {
+            req.onreadystatechange = () => {
+              if (req.readyState === XMLHttpRequest.DONE) {
+                if (req.status === 0 || (req.status >= 200 && req.status < 400)) {
+                  if (req.status === 304) {
+                    resolve({ status: req.status, data: null });
+                    return;
+                  }
+                  // The request has been completed successfully
+                  try {
+                    const response = JSON.parse(req.responseText);
+                    resolve(response);
+                  } catch (error) {
+                    console.log('cannot parse', req.responseText);
+                    resolve(req.responseText);
+                  }
-            req.onreadystatechange = function () {
-              if (this.readyState == 4) {
-                if (this.status == 200) {
-                  var response = JSON.parse(this.response);
-                  resolve(response);
                 } else {
+                  const err = JSON.parse(req.responseText);
-                  var err = JSON.parse(this.response);
                   reject(err);
                 }
               }
@@ -131,11 +121,6 @@
               reject(new Error(this.statusText));
             };
             req.open('GET', url, true);
+            if (etag) {
+              req.setRequestHeader('If-None-Match', etag);
+            }
-
             req.send(null);
           });
         };
@@ -153,18 +138,16 @@
           if (!key && typeof cb === 'function') return cb(new Error('API Key is required'));
           else if (!key) throw new Error('API Key is required');
           fillData(key, options);
           var q = querystring.stringify(config.options);
           if (typeof cb === 'function')
+            request(config.URL + q, config.options.etag)
-            request(config.URL + q)
               .then(function (res) {
                 return cb(null, res);
               })
               .catch(function (err) {
                 return cb(err);
               });
+          else return request(config.URL + q, config.options.etag);
-          else return request(config.URL + q);