Switch to Fetch API instead of XHR

Driver created with either HTTP or HTTPS url needs to execute REST
requests. This commit makes it use new standard Fetch API instead of
3rd party XHR library, which is based on `XMLHttpRequest`.

Author: lutovich

package-lock.json

@@ -71,7 +71,6 @@
   },
   "dependencies": {
     "babel-runtime": "^6.18.0",
-    "url-parse": "^1.2.0",
-    "xhr": "^2.4.1"
+    "url-parse": "^1.2.0"
   }
 }

package.json

@@ -17,7 +17,6 @@
  * limitations under the License.
  */
 
-import xhr from 'xhr';
 import StreamObserver from '../stream-observer';
 import Result from '../../result';
 import {EMPTY_CONNECTION_HOLDER} from '../connection-holder';
@@ -50,24 +49,26 @@ function createTransactionCommitUrl(url) {
 }
 
 function createHttpHeaders(authToken) {
-  const basicAuthToken = 'Basic ' + btoa(authToken.principal + ':' + authToken.credentials);
-  return {
-    'Accept': 'application/json; charset=UTF-8',
-    'Content-Type': 'application/json',
-    'Authorization': basicAuthToken
-  };
+  const headers = new Headers();
+  headers.append('Accept', 'application/json; charset=UTF-8');
+  headers.append('Content-Type', 'application/json');
+  headers.append('Authorization', 'Basic ' + btoa(authToken.principal + ':' + authToken.credentials));
+  return headers;
 }
 
 function sendPostRequest(statement, parameters, streamObserver, transactionCommitUrl, headers, converter) {
   try {
-    xhr.post(
-      transactionCommitUrl,
-      {
-        headers: headers,
-        body: createStatementJson(statement, parameters, converter)
-      },
-      (error, response) => processPostResponse(error, response, converter, streamObserver)
-    );
+    const options = {
+      method: 'POST',
+      headers: headers,
+      body: createStatementJson(statement, parameters, converter)
+    };
+
+    fetch(transactionCommitUrl, options)
+      .then(response => response.json())
+      .catch(error => processResponseError(error, converter, streamObserver))
+      .then(responseJson => processResponseJson(responseJson, converter, streamObserver));
+
   } catch (e) {
     streamObserver.onError(e);
   }
@@ -93,33 +94,36 @@ function createStatementJsonOrThrow(statement, parameters, converter) {
   });
 }
 
-function processPostResponse(error, response, converter, streamObserver) {
+function processResponseError(error, converter, streamObserver) {
+  const neo4jError = converter.convertNetworkError(error);
+  streamObserver.onError(neo4jError);
+}
+
+function processResponseJson(responseJson, converter, streamObserver) {
+  if (!responseJson) {
+    // request failed and there is no response
+    return;
+  }
+
   try {
-    processPostResponseOrThrow(error, response, converter, streamObserver);
+    processResponseJsonOrThrow(responseJson, converter, streamObserver);
   } catch (e) {
     streamObserver.onError(e);
   }
 }
 
-function processPostResponseOrThrow(error, response, converter, streamObserver) {
-  if (error) {
-    const neo4jError = converter.convertNetworkError(error);
+function processResponseJsonOrThrow(responseJson, converter, streamObserver) {
+  const neo4jError = converter.extractError(responseJson);
+  if (neo4jError) {
     streamObserver.onError(neo4jError);
   } else {
-    const responseJson = JSON.parse(response.body);
-
-    const neo4jError = converter.extractError(responseJson);
-    if (neo4jError) {
-      streamObserver.onError(neo4jError);
-    } else {
-      const recordMetadata = converter.extractRecordMetadata(responseJson);
-      streamObserver.onCompleted(recordMetadata);
+    const recordMetadata = converter.extractRecordMetadata(responseJson);
+    streamObserver.onCompleted(recordMetadata);
 
-      const rawRecords = converter.extractRawRecords(responseJson);
-      rawRecords.forEach(rawRecord => streamObserver.onNext(rawRecord));
+    const rawRecords = converter.extractRawRecords(responseJson);
+    rawRecords.forEach(rawRecord => streamObserver.onNext(rawRecord));
 
-      const statementMetadata = converter.extractStatementMetadata(responseJson);
-      streamObserver.onCompleted(statementMetadata);
-    }
+    const statementMetadata = converter.extractStatementMetadata(responseJson);
+    streamObserver.onCompleted(statementMetadata);
   }
 }