diff --git a/.eslintrc.js b/.eslintrc.js
index 6dc44d1db0755..627c44ff82047 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -49,6 +49,7 @@ module.exports = {
         "arrow-parens": [2, "as-needed"],
 
         // anti-patterns
+        "no-var": 2,
         "no-with": 2,
         "no-multi-str": 2,
         "no-caller": 2,
@@ -99,13 +100,11 @@ module.exports = {
         "no-mixed-spaces-and-tabs": 2,
         "no-trailing-spaces": 2,
         "linebreak-style": [ 2, "unix" ],
+        "indent": [2, 2, { "SwitchCase": 1, "CallExpression": {"arguments": 2}, "MemberExpression": 2 }],
 
         /**
          * Disabled, aspirational rules
          */
-
-        "indent": [2, 2, { "SwitchCase": 1, "CallExpression": {"arguments": 2}, "MemberExpression": 2 }],
-
         // brace-style is disabled, as eslint cannot enforce 1tbs as default, but allman for functions
         "brace-style": [0, "allman", { "allowSingleLine": true }],
 
diff --git a/install.js b/install.js
index e1ee4f319f30b..3b07d20ca170e 100644
--- a/install.js
+++ b/install.js
@@ -14,9 +14,9 @@
  * limitations under the License.
  */
 
-var Downloader = require('./utils/ChromiumDownloader');
-var revision = require('./package').puppeteer.chromium_revision;
-var ProgressBar = require('progress');
+let Downloader = require('./utils/ChromiumDownloader');
+let revision = require('./package').puppeteer.chromium_revision;
+let ProgressBar = require('progress');
 
 // Do nothing if the revision is already downloaded.
 if (Downloader.revisionInfo(Downloader.currentPlatform(), revision))
@@ -27,7 +27,7 @@ Downloader.downloadRevision(Downloader.currentPlatform(), revision, onProgress)
       console.error('Download failed: ' + error.message);
     });
 
-var progressBar = null;
+let progressBar = null;
 function onProgress(bytesTotal, delta) {
   if (!progressBar) {
     progressBar = new ProgressBar(`Downloading Chromium - ${toMegabytes(bytesTotal)} [:bar] :percent :etas `, {
@@ -41,7 +41,7 @@ function onProgress(bytesTotal, delta) {
 }
 
 function toMegabytes(bytes) {
-  var mb = bytes / 1024 / 1024;
+  let mb = bytes / 1024 / 1024;
   return (Math.round(mb * 10) / 10) + ' Mb';
 }
 
diff --git a/lib/Browser.js b/lib/Browser.js
index 6f7d569ea49d1..a24018ef3025c 100644
--- a/lib/Browser.js
+++ b/lib/Browser.js
@@ -14,18 +14,18 @@
  * limitations under the License.
  */
 
-var http = require('http');
-var path = require('path');
-var removeRecursive = require('rimraf').sync;
-var Page = require('./Page');
-var childProcess = require('child_process');
-var Downloader = require('../utils/ChromiumDownloader');
-var Connection = require('./Connection');
+let http = require('http');
+let path = require('path');
+let removeRecursive = require('rimraf').sync;
+let Page = require('./Page');
+let childProcess = require('child_process');
+let Downloader = require('../utils/ChromiumDownloader');
+let Connection = require('./Connection');
 
-var CHROME_PROFILE_PATH = path.resolve(__dirname, '..', '.dev_profile');
-var browserId = 0;
+let CHROME_PROFILE_PATH = path.resolve(__dirname, '..', '.dev_profile');
+let browserId = 0;
 
-var DEFAULT_ARGS = [
+let DEFAULT_ARGS = [
   '--disable-background-timer-throttling',
   '--no-first-run',
 ];
@@ -54,8 +54,8 @@ class Browser {
     if (typeof options.executablePath === 'string') {
       this._chromeExecutable = options.executablePath;
     } else {
-      var chromiumRevision = require('../package.json').puppeteer.chromium_revision;
-      var revisionInfo = Downloader.revisionInfo(Downloader.currentPlatform(), chromiumRevision);
+      let chromiumRevision = require('../package.json').puppeteer.chromium_revision;
+      let revisionInfo = Downloader.revisionInfo(Downloader.currentPlatform(), chromiumRevision);
       console.assert(revisionInfo, 'Chromium revision is not downloaded. Run npm install');
       this._chromeExecutable = revisionInfo.executablePath;
     }
@@ -72,8 +72,8 @@ class Browser {
     await this._ensureChromeIsRunning();
     if (!this._chromeProcess || this._terminated)
       throw new Error('ERROR: this chrome instance is not alive any more!');
-    var client = await Connection.create(this._remoteDebuggingPort);
-    var page = await Page.create(client);
+    let client = await Connection.create(this._remoteDebuggingPort);
+    let page = await Page.create(client);
     return page;
   }
 
@@ -91,7 +91,7 @@ class Browser {
      */
   async version() {
     await this._ensureChromeIsRunning();
-    var version = await Connection.version(this._remoteDebuggingPort);
+    let version = await Connection.version(this._remoteDebuggingPort);
     return version.Browser;
   }
 
@@ -99,7 +99,7 @@ class Browser {
     if (this._chromeProcess)
       return;
     this._chromeProcess = childProcess.spawn(this._chromeExecutable, this._chromeArguments, {});
-    var stderr = '';
+    let stderr = '';
     this._chromeProcess.stderr.on('data', data => stderr += data.toString('utf8'));
     // Cleanup as processes exit.
     process.on('exit', () => this._chromeProcess.kill());
@@ -123,20 +123,20 @@ class Browser {
 module.exports = Browser;
 
 function waitForChromeResponsive(remoteDebuggingPort, shouldWaitCallback) {
-  var fulfill;
-  var promise = new Promise(x => fulfill = x);
-  var options = {
+  let fulfill;
+  let promise = new Promise(x => fulfill = x);
+  let options = {
     method: 'GET',
     host: 'localhost',
     port: remoteDebuggingPort,
     path: '/json/list'
   };
-  var probeTimeout = 100;
+  let probeTimeout = 100;
   sendRequest();
   return promise;
 
   function sendRequest() {
-    var req = http.request(options, res => {
+    let req = http.request(options, res => {
       fulfill();
     });
     req.on('error', e => {
diff --git a/lib/Connection.js b/lib/Connection.js
index 25f924598da35..fa52efc9b86ac 100644
--- a/lib/Connection.js
+++ b/lib/Connection.js
@@ -14,9 +14,9 @@
  * limitations under the License.
  */
 
-var EventEmitter = require('events');
-var WebSocket = require('ws');
-var http = require('http');
+let EventEmitter = require('events');
+let WebSocket = require('ws');
+let http = require('http');
 const COMMAND_TIMEOUT = 10000;
 
 class Connection extends EventEmitter {
@@ -44,8 +44,8 @@ class Connection extends EventEmitter {
    * @return {!Promise<?Object>}
    */
   send(method, params = {}) {
-    var id = ++this._lastId;
-    var message = JSON.stringify({id, method, params});
+    let id = ++this._lastId;
+    let message = JSON.stringify({id, method, params});
     this._ws.send(message);
     return new Promise((resolve, reject) => {
       this._callbacks.set(id, {resolve, reject, method});
@@ -56,9 +56,9 @@ class Connection extends EventEmitter {
    * @param {string} message
    */
   _onMessage(message) {
-    var object = JSON.parse(message);
+    let object = JSON.parse(message);
     if (object.id && this._callbacks.has(object.id)) {
-      var callback = this._callbacks.get(object.id);
+      let callback = this._callbacks.get(object.id);
       this._callbacks.delete(object.id);
       if (object.error)
         callback.reject(new Error(`Protocol error (${callback.method}): ${object.error.message}`));
@@ -87,11 +87,11 @@ class Connection extends EventEmitter {
    * @return {!Promise<!Connection>}
    */
   static async create(port) {
-    var newTab = await runJsonCommand(port, 'new');
-    var url = newTab.webSocketDebuggerUrl;
+    let newTab = await runJsonCommand(port, 'new');
+    let url = newTab.webSocketDebuggerUrl;
 
     return new Promise((resolve, reject) => {
-      var ws = new WebSocket(url, { perMessageDeflate: false });
+      let ws = new WebSocket(url, { perMessageDeflate: false });
       ws.on('open', () => resolve(new Connection(port, newTab.id, ws)));
       ws.on('error', reject);
     });
@@ -112,17 +112,17 @@ class Connection extends EventEmitter {
  * @return {!Promise<!Object>}
  */
 function runJsonCommand(port, command) {
-  var request = http.get({
+  let request = http.get({
     hostname: 'localhost',
     port: port,
     path: '/json/' + command
   }, onResponse);
   request.setTimeout(COMMAND_TIMEOUT, onTimeout);
-  var resolve, reject;
+  let resolve, reject;
   return new Promise((res, rej) => { resolve = res; reject = rej; });
 
   function onResponse(response) {
-    var data = '';
+    let data = '';
     response.on('data', chunk => data += chunk);
     response.on('end', () => {
       if (response.statusCode !== 200) {
diff --git a/lib/FrameManager.js b/lib/FrameManager.js
index 8763229aa4aef..0c8e129dea541 100644
--- a/lib/FrameManager.js
+++ b/lib/FrameManager.js
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-var EventEmitter = require('events');
+let EventEmitter = require('events');
 
 class FrameManager extends EventEmitter {
   /**
@@ -22,7 +22,7 @@ class FrameManager extends EventEmitter {
    * @return {!FrameManager}
    */
   static async create(client) {
-    var mainFramePayload = await client.send('Page.getResourceTree');
+    let mainFramePayload = await client.send('Page.getResourceTree');
     return new FrameManager(client, mainFramePayload.frameTree);
   }
 
@@ -70,8 +70,8 @@ class FrameManager extends EventEmitter {
       this._navigateFrame(this._mainFrame, frameId, null);
       return;
     }
-    var parentFrame = this._frames.get(parentFrameId);
-    var frame = new Frame(parentFrame, frameId, null);
+    let parentFrame = this._frames.get(parentFrameId);
+    let frame = new Frame(parentFrame, frameId, null);
     this._frames.set(frame._id, frame);
     this.emit(FrameManager.Events.FrameAttached, frame);
   }
@@ -80,7 +80,7 @@ class FrameManager extends EventEmitter {
    * @param {!Object} framePayload
    */
   _frameNavigated(framePayload) {
-    var frame = this._frames.get(framePayload.id);
+    let frame = this._frames.get(framePayload.id);
     if (!frame) {
       // Simulate missed "frameAttached" for a main frame navigation to the new backend process.
       console.assert(!framePayload.parentId, 'Main frame shouldn\'t have parent frame id.');
@@ -93,7 +93,7 @@ class FrameManager extends EventEmitter {
    * @param {string} frameId
    */
   _frameDetached(frameId) {
-    var frame = this._frames.get(frameId);
+    let frame = this._frames.get(frameId);
     if (frame)
       this._removeFramesRecursively(frame);
   }
@@ -105,7 +105,7 @@ class FrameManager extends EventEmitter {
    */
   _navigateFrame(frame, newFrameId, newFramePayload) {
     // Detach all child frames first.
-    for (var child of frame.childFrames())
+    for (let child of frame.childFrames())
       this._removeFramesRecursively(child);
     this._frames.delete(frame._id, frame);
     frame._id = newFrameId;
@@ -120,11 +120,11 @@ class FrameManager extends EventEmitter {
    * @return {!Frame}
    */
   _addFramesRecursively(parentFrame, frameTreePayload) {
-    var framePayload = frameTreePayload.frame;
-    var frame = new Frame(parentFrame, framePayload.id, framePayload);
+    let framePayload = frameTreePayload.frame;
+    let frame = new Frame(parentFrame, framePayload.id, framePayload);
     this._frames.set(frame._id, frame);
 
-    for (var i = 0; frameTreePayload.childFrames && i < frameTreePayload.childFrames.length; ++i)
+    for (let i = 0; frameTreePayload.childFrames && i < frameTreePayload.childFrames.length; ++i)
       this._addFramesRecursively(frame, frameTreePayload.childFrames[i]);
     return frame;
   }
@@ -133,7 +133,7 @@ class FrameManager extends EventEmitter {
    * @param {!Frame} frame
    */
   _removeFramesRecursively(frame) {
-    for (var child of frame.childFrames())
+    for (let child of frame.childFrames())
       this._removeFramesRecursively(child);
     frame._detach();
     this._frames.delete(frame._id);
diff --git a/lib/Navigator.js b/lib/Navigator.js
index 38dda16ebf64d..c12e8b8cdaecf 100644
--- a/lib/Navigator.js
+++ b/lib/Navigator.js
@@ -39,7 +39,7 @@ class Navigator {
     this._minimumTimer = setTimeout(this._completeNavigation.bind(this, false), this._minTime);
     let onload = new Promise(fulfill => this._client.once('Page.loadEventFired', fulfill));
     let networkIdle = new Promise(fulfill => this._navigationLoadCallback = fulfill);
-    var interstitialPromise = new Promise(fulfill => this._client.once('Security.certificateError', fulfill)).then(() => false);
+    let interstitialPromise = new Promise(fulfill => this._client.once('Security.certificateError', fulfill)).then(() => false);
 
     this._inflightRequests = 0;
     // Await for the command to throw exception in case of illegal arguments.
diff --git a/lib/Page.js b/lib/Page.js
index 83d60cd004f49..446eae838e27f 100644
--- a/lib/Page.js
+++ b/lib/Page.js
@@ -14,13 +14,13 @@
  * limitations under the License.
  */
 
-var fs = require('fs');
-var EventEmitter = require('events');
-var mime = require('mime');
-var Request = require('./Request');
-var Navigator = require('./Navigator');
-var Dialog = require('./Dialog');
-var FrameManager = require('./FrameManager');
+let fs = require('fs');
+let EventEmitter = require('events');
+let mime = require('mime');
+let Request = require('./Request');
+let Navigator = require('./Navigator');
+let Dialog = require('./Dialog');
+let FrameManager = require('./FrameManager');
 
 class Page extends EventEmitter {
   /**
@@ -34,10 +34,10 @@ class Page extends EventEmitter {
       client.send('Runtime.enable', {}),
       client.send('Security.enable', {}),
     ]);
-    var expression = Page._evaluationString(() => window.devicePixelRatio);
-    var {result:{value: screenDPI}} = await client.send('Runtime.evaluate', { expression, returnByValue: true });
-    var frameManager = await FrameManager.create(client);
-    var page = new Page(client, frameManager, screenDPI);
+    let expression = Page._evaluationString(() => window.devicePixelRatio);
+    let {result:{value: screenDPI}} = await client.send('Runtime.evaluate', { expression, returnByValue: true });
+    let frameManager = await FrameManager.create(client);
+    let page = new Page(client, frameManager, screenDPI);
     // Initialize default page size.
     await page.setViewportSize({width: 400, height: 300});
     return page;
@@ -100,7 +100,7 @@ class Page extends EventEmitter {
    * @param {!Object} event
    */
   _onRequestIntercepted(event) {
-    var request = new Request(this._client, event.InterceptionId, event.request);
+    let request = new Request(this._client, event.InterceptionId, event.request);
     this._requestInterceptor(request);
   }
 
@@ -115,9 +115,9 @@ class Page extends EventEmitter {
        * @param {string} url
        */
     function addScriptTag(url) {
-      var script = document.createElement('script');
+      let script = document.createElement('script');
       script.src = url;
-      var promise = new Promise(x => script.onload = x);
+      let promise = new Promise(x => script.onload = x);
       document.head.appendChild(script);
       return promise;
     }
@@ -128,9 +128,9 @@ class Page extends EventEmitter {
    * @return {!Promise}
    */
   async injectFile(filePath) {
-    var callback;
-    var promise = new Promise(fulfill => callback = fulfill);
-    var expression = fs.readFile(filePath, 'utf8', (err, data) => callback({err, data}));
+    let callback;
+    let promise = new Promise(fulfill => callback = fulfill);
+    let expression = fs.readFile(filePath, 'utf8', (err, data) => callback({err, data}));
     await promise;
     return this._client.send('Runtime.evaluate', { expression, returnByValue: true });
   }
@@ -144,7 +144,7 @@ class Page extends EventEmitter {
       throw new Error(`Failed to set in-page callback with name ${name}: window['${name}'] already exists!`);
     this._inPageCallbacks[name] = callback;
 
-    var expression = Page._evaluationString(inPageCallback, name);
+    let expression = Page._evaluationString(inPageCallback, name);
     await this._client.send('Page.addScriptToEvaluateOnLoad', { scriptSource: expression });
     await this._client.send('Runtime.evaluate', { expression, returnByValue: true });
 
@@ -173,7 +173,7 @@ class Page extends EventEmitter {
   async setExtraHTTPHeaders(headers) {
     this._extraHeaders = {};
     // Note: header names are case-insensitive.
-    for (var key of Object.keys(headers))
+    for (let key of Object.keys(headers))
       this._extraHeaders[key.toLowerCase()] = headers[key];
     return this._client.send('Network.setExtraHTTPHeaders', { headers });
   }
@@ -205,15 +205,15 @@ class Page extends EventEmitter {
    * @param {!Object} exceptionDetails
    */
   async _handleException(exceptionDetails) {
-    var message = await this._getExceptionMessage(exceptionDetails);
+    let message = await this._getExceptionMessage(exceptionDetails);
     this.emit(Page.Events.Error, new Error(message));
   }
 
   async _onConsoleAPI(event) {
     if (event.type === 'debug' && event.args.length && event.args[0].value === 'driver:InPageCallback') {
-      var {name, seq, args} = JSON.parse(event.args[1].value);
-      var result = await this._inPageCallbacks[name](...args);
-      var expression = Page._evaluationString(deliverResult, name, seq, result);
+      let {name, seq, args} = JSON.parse(event.args[1].value);
+      let result = await this._inPageCallbacks[name](...args);
+      let expression = Page._evaluationString(deliverResult, name, seq, result);
       this._client.send('Runtime.evaluate', { expression });
 
       function deliverResult(name, seq, result) {
@@ -222,12 +222,12 @@ class Page extends EventEmitter {
       }
       return;
     }
-    var values = event.args.map(arg => arg.value || arg.description || '');
+    let values = event.args.map(arg => arg.value || arg.description || '');
     this.emit(Page.Events.ConsoleMessage, values.join(' '));
   }
 
   _onDialog(event) {
-    var dialogType = null;
+    let dialogType = null;
     if (event.type === 'alert')
       dialogType = Dialog.Type.Alert;
     else if (event.type === 'confirm')
@@ -237,7 +237,7 @@ class Page extends EventEmitter {
     else if (event.type === 'beforeunload')
       dialogType = Dialog.Type.BeforeUnload;
     console.assert(dialogType, 'Unknown javascript dialog type: ' + event.type);
-    var dialog = new Dialog(this._client, dialogType, event.message);
+    let dialog = new Dialog(this._client, dialogType, event.message);
     this.emit(Page.Events.Dialog, dialog);
   }
 
@@ -275,9 +275,9 @@ class Page extends EventEmitter {
    */
   async setViewportSize(size) {
     this._viewportSize = size;
-    var width = size.width;
-    var height = size.height;
-    var zoom = this._screenDPI;
+    let width = size.width;
+    let height = size.height;
+    let zoom = this._screenDPI;
     return Promise.all([
       this._client.send('Emulation.setDeviceMetricsOverride', {
         width,
@@ -307,11 +307,11 @@ class Page extends EventEmitter {
    * @return {!Promise<(!Object|undefined)>}
    */
   async evaluate(fun, ...args) {
-    var syncExpression = Page._evaluationString(fun, ...args);
-    var expression = `Promise.resolve(${syncExpression})`;
-    var { exceptionDetails, result: remoteObject }  = await this._client.send('Runtime.evaluate', { expression, returnByValue: true, awaitPromise: true });
+    let syncExpression = Page._evaluationString(fun, ...args);
+    let expression = `Promise.resolve(${syncExpression})`;
+    let { exceptionDetails, result: remoteObject }  = await this._client.send('Runtime.evaluate', { expression, returnByValue: true, awaitPromise: true });
     if (exceptionDetails) {
-      var message = await this._getExceptionMessage(exceptionDetails);
+      let message = await this._getExceptionMessage(exceptionDetails);
       throw new Error('Evaluation failed: ' + message);
     }
     return remoteObject.value;
@@ -322,10 +322,10 @@ class Page extends EventEmitter {
    * @return {string}
    */
   async _getExceptionMessage(exceptionDetails) {
-    var message = '';
-    var exception = exceptionDetails.exception;
+    let message = '';
+    let exception = exceptionDetails.exception;
     if (exception) {
-      var response = await this._client.send('Runtime.callFunctionOn', {
+      let response = await this._client.send('Runtime.callFunctionOn', {
         objectId: exception.objectId,
         functionDeclaration: 'function() { return this.message; }',
         returnByValue: true,
@@ -336,9 +336,9 @@ class Page extends EventEmitter {
     }
 
     if (exceptionDetails.stackTrace) {
-      for (var callframe of exceptionDetails.stackTrace.callFrames) {
-        var location = callframe.url + ':' + callframe.lineNumber + ':' + callframe.columnNumber;
-        var functionName = callframe.functionName || '<anonymous>';
+      for (let callframe of exceptionDetails.stackTrace.callFrames) {
+        let location = callframe.url + ':' + callframe.lineNumber + ':' + callframe.columnNumber;
+        let functionName = callframe.functionName || '<anonymous>';
         message += `\n    at ${functionName} (${location})`;
       }
     }
@@ -351,7 +351,7 @@ class Page extends EventEmitter {
    * @return {!Promise}
    */
   async evaluateOnInitialized(fun, ...args) {
-    var scriptSource = Page._evaluationString(fun, ...args);
+    let scriptSource = Page._evaluationString(fun, ...args);
     await this._client.send('Page.addScriptToEvaluateOnLoad', { scriptSource });
   }
 
@@ -370,9 +370,9 @@ class Page extends EventEmitter {
    */
   async screenshot(options) {
     options = options || {};
-    var screenshotType = null;
+    let screenshotType = null;
     if (options.path) {
-      var mimeType = mime.lookup(options.path);
+      let mimeType = mime.lookup(options.path);
       if (mimeType === 'image/png')
         screenshotType = 'png';
       else if (mimeType === 'image/jpeg')
@@ -423,7 +423,7 @@ class Page extends EventEmitter {
         })
       ]);
     } else if (options.fullPage) {
-      var response = await this._client.send('Page.getLayoutMetrics');
+      let response = await this._client.send('Page.getLayoutMetrics');
       await Promise.all([
         this._client.send('Emulation.setVisibleSize', {
           width: Math.ceil(response.contentSize.width / this._screenDPI),
@@ -436,7 +436,7 @@ class Page extends EventEmitter {
         })
       ]);
     }
-    var result = await this._client.send('Page.captureScreenshot', {
+    let result = await this._client.send('Page.captureScreenshot', {
       fromSurface: true,
       format: screenshotType,
       quality: options.quality
@@ -447,7 +447,7 @@ class Page extends EventEmitter {
         this._client.send('Emulation.resetViewport')
       ]);
     }
-    var buffer = new Buffer(result.data, 'base64');
+    let buffer = new Buffer(result.data, 'base64');
     if (options.path)
       fs.writeFileSync(options.path, buffer);
     return buffer;
@@ -461,16 +461,16 @@ class Page extends EventEmitter {
   async printToPDF(filePath, options) {
     options = options || {};
 
-    var scale = options.scale || 1;
-    var displayHeaderFooter = options.displayHeaderFooter || false;
-    var printBackground = options.printBackground || true;
-    var landscape = options.landscape || false;
-    var pageRanges = options.pageRanges || '';
+    let scale = options.scale || 1;
+    let displayHeaderFooter = options.displayHeaderFooter || false;
+    let printBackground = options.printBackground || true;
+    let landscape = options.landscape || false;
+    let pageRanges = options.pageRanges || '';
 
-    var paperWidth = 8.5;
-    var paperHeight = 11;
+    let paperWidth = 8.5;
+    let paperHeight = 11;
     if (options.format) {
-      var format = Page.PaperFormats[options.format];
+      let format = Page.PaperFormats[options.format];
       console.assert(format, 'Unknown paper format: ' + options.format);
       paperWidth = format.width;
       paperHeight = format.height;
@@ -479,13 +479,13 @@ class Page extends EventEmitter {
       paperHeight = convertPrintParameterToInches(options.height) || paperHeight;
     }
 
-    var marginOptions = options.margin || {};
-    var marginTop = convertPrintParameterToInches(marginOptions.top) || 0;
-    var marginLeft = convertPrintParameterToInches(marginOptions.left) || 0;
-    var marginBottom = convertPrintParameterToInches(marginOptions.bottom) || 0;
-    var marginRight = convertPrintParameterToInches(marginOptions.right) || 0;
+    let marginOptions = options.margin || {};
+    let marginTop = convertPrintParameterToInches(marginOptions.top) || 0;
+    let marginLeft = convertPrintParameterToInches(marginOptions.left) || 0;
+    let marginBottom = convertPrintParameterToInches(marginOptions.bottom) || 0;
+    let marginRight = convertPrintParameterToInches(marginOptions.right) || 0;
 
-    var result = await this._client.send('Page.printToPDF', {
+    let result = await this._client.send('Page.printToPDF', {
       landscape: landscape,
       displayHeaderFooter: displayHeaderFooter,
       printBackground: printBackground,
@@ -498,7 +498,7 @@ class Page extends EventEmitter {
       marginRight: marginRight,
       pageRanges: pageRanges
     });
-    var buffer = new Buffer(result.data, 'base64');
+    let buffer = new Buffer(result.data, 'base64');
     fs.writeFileSync(filePath, buffer);
   }
 
@@ -538,7 +538,7 @@ Page.PaperFormats = {
   A5: {width: 5.83, height: 8.27 },
 };
 
-var unitToPixels = {
+let unitToPixels = {
   'px': 1,
   'in': 96,
   'cm': 37.8,
@@ -552,14 +552,14 @@ var unitToPixels = {
 function convertPrintParameterToInches(parameter) {
   if (typeof parameter === 'undefined')
     return undefined;
-  var pixels;
+  let pixels;
   if (typeof parameter === 'number') {
     // Treat numbers as pixel values to be aligned with phantom's paperSize.
     pixels = /** @type {number} */ (parameter);
   } else if (typeof parameter === 'string') {
-    var text = parameter;
-    var unit = text.substring(text.length - 2).toLowerCase();
-    var valueText = '';
+    let text = parameter;
+    let unit = text.substring(text.length - 2).toLowerCase();
+    let valueText = '';
     if (unitToPixels.hasOwnProperty(unit)) {
       valueText = text.substring(0, text.length - 2);
     } else {
@@ -568,7 +568,7 @@ function convertPrintParameterToInches(parameter) {
       unit = 'px';
       valueText = text;
     }
-    var value = Number(valueText);
+    let value = Number(valueText);
     console.assert(!isNaN(value), 'Failed to parse parameter value: ' + text);
     pixels = value * unitToPixels[unit];
   } else {
diff --git a/phantom_shim/FileSystem.js b/phantom_shim/FileSystem.js
index 6ddda7877b37c..30ec4888fa330 100644
--- a/phantom_shim/FileSystem.js
+++ b/phantom_shim/FileSystem.js
@@ -14,11 +14,11 @@
  * limitations under the License.
  */
 
-var path = require('path');
-var fs = require('fs');
-var deasync = require('deasync');
-var removeRecursive = require('rimraf').sync;
-var copyRecursive = deasync(require('ncp').ncp);
+let path = require('path');
+let fs = require('fs');
+let deasync = require('deasync');
+let removeRecursive = require('rimraf').sync;
+let copyRecursive = deasync(require('ncp').ncp);
 
 class FileSystem {
   constructor() {
@@ -68,7 +68,7 @@ class FileSystem {
      * @param {string} toPath
      */
   copy(fromPath, toPath) {
-    var content = fs.readFileSync(fromPath);
+    let content = fs.readFileSync(fromPath);
     fs.writeFileSync(toPath, content);
   }
 
@@ -77,7 +77,7 @@ class FileSystem {
      * @param {string} toPath
      */
   move(fromPath, toPath) {
-    var content = fs.readFileSync(fromPath);
+    let content = fs.readFileSync(fromPath);
     fs.writeFileSync(toPath, content);
     fs.unlinkSync(fromPath);
   }
@@ -170,7 +170,7 @@ class FileSystem {
      * @param {string} mode
      */
   write(filePath, data, mode) {
-    var fd = new FileDescriptor(filePath, mode, 'utf8');
+    let fd = new FileDescriptor(filePath, mode, 'utf8');
     fd.write(data);
     fd.close();
   }
@@ -288,8 +288,8 @@ class FileSystem {
   }
 }
 
-var fdwrite = deasync(fs.write);
-var fdread = deasync(fs.read);
+let fdwrite = deasync(fs.write);
+let fdread = deasync(fs.read);
 
 class FileDescriptor {
   /**
@@ -318,8 +318,8 @@ class FileDescriptor {
      * @param {string} data
      */
   write(data) {
-    var buffer = Buffer.from(data, this._encoding);
-    var written = fdwrite(this._fd, buffer, 0, buffer.length, this._position);
+    let buffer = Buffer.from(data, this._encoding);
+    let written = fdwrite(this._fd, buffer, 0, buffer.length, this._position);
     this._position += written;
   }
 
@@ -339,13 +339,13 @@ class FileDescriptor {
      * @return {string}
      */
   read(size) {
-    var position = this._position;
+    let position = this._position;
     if (!size) {
       size = fs.fstatSync(this._fd).size;
       position = 0;
     }
-    var buffer = new Buffer(size);
-    var bytesRead = fdread(this._fd, buffer, 0, size, position);
+    let buffer = new Buffer(size);
+    let bytesRead = fdread(this._fd, buffer, 0, size, position);
     this._position += bytesRead;
     return buffer.toString(this._encoding);
   }
diff --git a/phantom_shim/Phantom.js b/phantom_shim/Phantom.js
index fa7c9165632ca..61566b4ed5113 100644
--- a/phantom_shim/Phantom.js
+++ b/phantom_shim/Phantom.js
@@ -14,17 +14,17 @@
  * limitations under the License.
  */
 
-var fs = require('fs');
-var path = require('path');
-var vm = require('vm');
-var url = require('url');
+let fs = require('fs');
+let path = require('path');
+let vm = require('vm');
+let url = require('url');
 
 /**
  * @param {!Object} context
  * @param {string} scriptPath
  */
 module.exports.create = function(context, scriptPath) {
-  var phantom = {
+  const phantom = {
     page: {
       onConsoleMessage: null,
     },
@@ -82,7 +82,7 @@ module.exports.create = function(context, scriptPath) {
          * @return {!{major: number, minor: number, patch: number}}
          */
     get version() {
-      var versionParts = require('../package.json').version.split('.');
+      let versionParts = require('../package.json').version.split('.');
       return {
         major: parseInt(versionParts[0], 10),
         minor: parseInt(versionParts[1], 10),
@@ -105,7 +105,7 @@ module.exports.create = function(context, scriptPath) {
       filePath = path.resolve(phantom.libraryPath, filePath);
       if (!fs.existsSync(filePath))
         return false;
-      var code = fs.readFileSync(filePath, 'utf8');
+      let code = fs.readFileSync(filePath, 'utf8');
       if (code.startsWith('#!'))
         code = code.substring(code.indexOf('\n'));
       vm.runInContext(code, context, {
@@ -120,7 +120,7 @@ module.exports.create = function(context, scriptPath) {
          * @param {string} filename
          */
     loadModule: function(moduleSource, filename) {
-      var code = [
+      let code = [
         '(function(require, exports, module) {\n',
         moduleSource,
         '\n}.call({},',
diff --git a/phantom_shim/System.js b/phantom_shim/System.js
index 88c49dc12d208..06286e2e0c019 100644
--- a/phantom_shim/System.js
+++ b/phantom_shim/System.js
@@ -14,9 +14,9 @@
  * limitations under the License.
  */
 
-var readline = require('readline');
-var await = require('./utilities').await;
-var os = require('os');
+let readline = require('readline');
+let await = require('./utilities').await;
+let os = require('os');
 
 class System {
   /**
@@ -61,7 +61,7 @@ class StandardInput {
     if (this._closed && !this._lines.length)
       return '';
     if (!this._lines.length) {
-      var linePromise = new Promise(fulfill => this._readline.once('line', fulfill));
+      let linePromise = new Promise(fulfill => this._readline.once('line', fulfill));
       await(linePromise);
     }
     return this._lines.shift();
@@ -72,10 +72,10 @@ class StandardInput {
      */
   read() {
     if (!this._closed) {
-      var closePromise = new Promise(fulfill => this._readline.once('close', fulfill));
+      let closePromise = new Promise(fulfill => this._readline.once('close', fulfill));
       await(closePromise);
     }
-    var text = this._lines.join('\n');
+    let text = this._lines.join('\n');
     this._lines = [];
     return text;
   }
diff --git a/phantom_shim/WebPage.js b/phantom_shim/WebPage.js
index 31cbcbc50a346..e37f5d49159ac 100644
--- a/phantom_shim/WebPage.js
+++ b/phantom_shim/WebPage.js
@@ -14,13 +14,13 @@
  * limitations under the License.
  */
 
-var await = require('./utilities').await;
-var EventEmitter = require('events');
-var fs = require('fs');
-var path = require('path');
-var PageEvents = require('../lib/Page').Events;
+let await = require('./utilities').await;
+let EventEmitter = require('events');
+let fs = require('fs');
+let path = require('path');
+let PageEvents = require('../lib/Page').Events;
 
-var noop = function() { };
+let noop = function() { };
 
 class WebPage {
   /**
@@ -82,7 +82,7 @@ class WebPage {
          * @param {!Request} request
          */
     function resourceInterceptor(request) {
-      var requestData = {
+      let requestData = {
         url: request.url(),
         headers: request.headers()
       };
@@ -95,8 +95,8 @@ class WebPage {
   _onResponseReceived(response) {
     if (!this.onResourceReceived)
       return;
-    var headers = [];
-    for (var key in response.headers) {
+    let headers = [];
+    for (let key in response.headers) {
       headers.push({
         name: key,
         value: response.headers[key]
@@ -217,7 +217,7 @@ class WebPage {
       this._onAlertCallback.call(null, dialog.message());
       await(dialog.accept());
     } else if (dialog.type === 'confirm' && this._onConfirmCallback) {
-      var result = this._onConfirmCallback.call(null, dialog.message());
+      let result = this._onConfirmCallback.call(null, dialog.message());
       await(result ? dialog.accept() : dialog.dismiss());
     }
   }
@@ -247,7 +247,7 @@ class WebPage {
       this.onInitialized();
     this._deferEvaluate = false;
     this._page.navigate(url).then(result => {
-      var status = result ? 'success' : 'fail';
+      let status = result ? 'success' : 'fail';
       if (!result) {
         this.onResourceError.call(null, {
           url,
@@ -283,8 +283,8 @@ class WebPage {
      */
   render(fileName) {
     if (fileName.endsWith('pdf')) {
-      var options = {};
-      var paperSize = this.paperSize || {};
+      let options = {};
+      let paperSize = this.paperSize || {};
       options.margin = paperSize.margin;
       options.format = paperSize.format;
       options.landscape = paperSize.orientation === 'landscape';
@@ -292,7 +292,7 @@ class WebPage {
       options.height = paperSize.height;
       await(this._page.printToPDF(fileName, options));
     } else {
-      var options = {};
+      let options = {};
       if (this.clipRect && (this.clipRect.left || this.clipRect.top || this.clipRect.width || this.clipRect.height)) {
         options.clip = {
           x: this.clipRect.left,
@@ -362,7 +362,7 @@ class AsyncEmitter extends EventEmitter {
 
   _onListenerAdded(event, listener) {
     // Async listener calls original listener on next tick.
-    var asyncListener = (...args) => {
+    let asyncListener = (...args) => {
       process.nextTick(() => listener.apply(null, args));
     };
     listener[this._symbol] = asyncListener;
diff --git a/phantom_shim/WebServer.js b/phantom_shim/WebServer.js
index ac95d396bf9f6..05c3168ca9b49 100644
--- a/phantom_shim/WebServer.js
+++ b/phantom_shim/WebServer.js
@@ -14,8 +14,8 @@
  * limitations under the License.
  */
 
-var http = require('http');
-var await = require('./utilities').await;
+let http = require('http');
+let await = require('./utilities').await;
 
 class WebServer {
   constructor() {
@@ -47,17 +47,17 @@ class WebServer {
       return false;
     this.newRequest = callback;
     this._server.listen(port);
-    var errorPromise = new Promise(x => this._server.once('error', x));
-    var successPromise = new Promise(x => this._server.once('listening', x));
+    let errorPromise = new Promise(x => this._server.once('error', x));
+    let successPromise = new Promise(x => this._server.once('listening', x));
     await(Promise.race([errorPromise, successPromise]));
     if (!this._server.listening)
       return false;
 
     this._server.on('request', (req, res) => {
       res.close = res.end.bind(res);
-      var headers = res.getHeaders();
+      let headers = res.getHeaders();
       res.headers = [];
-      for (var key in headers) {
+      for (let key in headers) {
         res.headers.push({
           name: key,
           value: headers[key]
@@ -65,7 +65,7 @@ class WebServer {
       }
       res.header = res.getHeader;
       res.setHeaders = headers => {
-        for (var key in headers)
+        for (let key in headers)
           res.setHeader(key, headers[key]);
       };
       Object.defineProperty(res, 'statusCode', {
diff --git a/phantom_shim/runner.js b/phantom_shim/runner.js
index fc83eacb52b16..9d35aec842989 100755
--- a/phantom_shim/runner.js
+++ b/phantom_shim/runner.js
@@ -15,18 +15,18 @@
  * limitations under the License.
  */
 
-var vm = require('vm');
-var path = require('path');
-var fs = require('fs');
-var Phantom = require('./Phantom');
-var FileSystem = require('./FileSystem');
-var System = require('./System');
-var WebPage = require('./WebPage');
-var WebServer = require('./WebServer');
-var child_process = require('child_process');
-var Browser = require('..').Browser;
-var version = require('../package.json').version;
-var argv = require('minimist')(process.argv.slice(2), {
+let vm = require('vm');
+let path = require('path');
+let fs = require('fs');
+let Phantom = require('./Phantom');
+let FileSystem = require('./FileSystem');
+let System = require('./System');
+let WebPage = require('./WebPage');
+let WebServer = require('./WebServer');
+let child_process = require('child_process');
+let Browser = require('..').Browser;
+let version = require('../package.json').version;
+let argv = require('minimist')(process.argv.slice(2), {
   alias: { v: 'version' },
   boolean: ['headless'],
   default: {'headless': true },
@@ -43,27 +43,27 @@ if (argv['ssl-certificates-path']) {
   return;
 }
 
-var scriptArguments = argv._;
+let scriptArguments = argv._;
 if (!scriptArguments.length) {
   console.log(__filename.split('/').pop() + ' [scriptfile]');
   return;
 }
 
-var scriptPath = path.resolve(process.cwd(), scriptArguments[0]);
+let scriptPath = path.resolve(process.cwd(), scriptArguments[0]);
 if (!fs.existsSync(scriptPath)) {
   console.error(`script not found: ${scriptPath}`);
   process.exit(1);
   return;
 }
 
-var browser = new Browser({
+let browser = new Browser({
   remoteDebuggingPort: 9229,
   headless: argv.headless,
   args: ['--no-sandbox']
 });
 
-var context = createPhantomContext(browser, scriptPath, argv);
-var scriptContent = fs.readFileSync(scriptPath, 'utf8');
+let context = createPhantomContext(browser, scriptPath, argv);
+let scriptContent = fs.readFileSync(scriptPath, 'utf8');
 vm.runInContext(scriptContent, context);
 
 /**
@@ -73,7 +73,7 @@ vm.runInContext(scriptContent, context);
  * @return {!Object}
  */
 function createPhantomContext(browser, scriptPath, argv) {
-  var context = {};
+  let context = {};
   context.setInterval = setInterval;
   context.setTimeout = setTimeout;
   context.clearInterval = clearInterval;
@@ -86,7 +86,7 @@ function createPhantomContext(browser, scriptPath, argv) {
 
   vm.createContext(context);
 
-  var nativeExports = {
+  let nativeExports = {
     fs: new FileSystem(),
     system: new System(argv._),
     webpage: {
@@ -100,8 +100,8 @@ function createPhantomContext(browser, scriptPath, argv) {
     },
     child_process: child_process
   };
-  var bootstrapPath = path.join(__dirname, '..', 'third_party', 'phantomjs', 'bootstrap.js');
-  var bootstrapCode = fs.readFileSync(bootstrapPath, 'utf8');
+  let bootstrapPath = path.join(__dirname, '..', 'third_party', 'phantomjs', 'bootstrap.js');
+  let bootstrapCode = fs.readFileSync(bootstrapPath, 'utf8');
   vm.runInContext(bootstrapCode, context, {
     filename: 'bootstrap.js'
   })(nativeExports);
diff --git a/phantom_shim/utilities.js b/phantom_shim/utilities.js
index 487ef4be71b0a..3f6c7453bc007 100644
--- a/phantom_shim/utilities.js
+++ b/phantom_shim/utilities.js
@@ -14,13 +14,13 @@
  * limitations under the License.
  */
 
-var loopWhile = require('deasync').loopWhile;
+let loopWhile = require('deasync').loopWhile;
 
 module.exports = {
   await: function(promise) {
-    var error;
-    var result;
-    var done = false;
+    let error;
+    let result;
+    let done = false;
     promise.then(r => result = r)
         .catch(err => error = err)
         .then(() => done = true);
diff --git a/test/StaticServer.js b/test/StaticServer.js
index 8cdf930c98c4c..c37e7bbd355a2 100644
--- a/test/StaticServer.js
+++ b/test/StaticServer.js
@@ -14,11 +14,11 @@
  * limitations under the License.
  */
 
-var http = require('http');
-var url = require('url');
-var fs = require('fs');
-var path = require('path');
-var mime = require('mime');
+let http = require('http');
+let url = require('url');
+let fs = require('fs');
+let path = require('path');
+let mime = require('mime');
 
 class StaticServer {
   /**
@@ -36,7 +36,7 @@ class StaticServer {
   }
 
   _onRequest(request, response) {
-    var pathName = url.parse(request.url).path;
+    let pathName = url.parse(request.url).path;
     if (pathName === '/')
       pathName = '/index.html';
     pathName = path.join(this._dirPath, pathName.substring(1));
diff --git a/test/frame-utils.js b/test/frame-utils.js
index 2dfc3105b94d1..ee2064b32724e 100644
--- a/test/frame-utils.js
+++ b/test/frame-utils.js
@@ -1,4 +1,4 @@
-var utils = module.exports = {
+const utils = module.exports = {
   /**
    * @param {!Page} page
    * @param {string} frameId
@@ -9,7 +9,7 @@ var utils = module.exports = {
     await page.evaluate(attachFrame, frameId, url);
 
     function attachFrame(frameId, url) {
-      var frame = document.createElement('iframe');
+      let frame = document.createElement('iframe');
       frame.src = url;
       frame.id = frameId;
       document.body.appendChild(frame);
@@ -26,7 +26,7 @@ var utils = module.exports = {
     await page.evaluate(detachFrame, frameId);
 
     function detachFrame(frameId) {
-      var frame = document.getElementById(frameId);
+      let frame = document.getElementById(frameId);
       frame.remove();
     }
   },
@@ -41,7 +41,7 @@ var utils = module.exports = {
     await page.evaluate(navigateFrame, frameId, url);
 
     function navigateFrame(frameId, url) {
-      var frame = document.getElementById(frameId);
+      let frame = document.getElementById(frameId);
       frame.src = url;
       return new Promise(x => frame.onload = x);
     }
@@ -54,8 +54,8 @@ var utils = module.exports = {
    */
   dumpFrames: function(frame, indentation) {
     indentation = indentation || '';
-    var result = indentation + frame.url();
-    for (var child of frame.childFrames())
+    let result = indentation + frame.url();
+    for (let child of frame.childFrames())
       result += '\n' + utils.dumpFrames(child, '    ' + indentation);
     return result;
   },
diff --git a/test/golden-utils.js b/test/golden-utils.js
index 7859c4847e7aa..f0d81fd5027b2 100644
--- a/test/golden-utils.js
+++ b/test/golden-utils.js
@@ -13,16 +13,16 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-var path = require('path');
-var fs = require('fs');
-var Diff = require('text-diff');
-var mime = require('mime');
-var PNG = require('pngjs').PNG;
-var pixelmatch = require('pixelmatch');
-var rm = require('rimraf').sync;
+let path = require('path');
+let fs = require('fs');
+let Diff = require('text-diff');
+let mime = require('mime');
+let PNG = require('pngjs').PNG;
+let pixelmatch = require('pixelmatch');
+let rm = require('rimraf').sync;
 
-var GOLDEN_DIR = path.join(__dirname, 'golden');
-var OUTPUT_DIR = path.join(__dirname, 'output');
+let GOLDEN_DIR = path.join(__dirname, 'golden');
+let OUTPUT_DIR = path.join(__dirname, 'output');
 
 module.exports = {
   addMatchers: function(jasmine) {
@@ -35,7 +35,7 @@ module.exports = {
   },
 };
 
-var GoldenComparators = {
+let GoldenComparators = {
   'image/png': compareImages,
   'text/plain': compareText
 };
@@ -49,15 +49,15 @@ function compareImages(actualBuffer, expectedBuffer) {
   if (!actualBuffer || !(actualBuffer instanceof Buffer))
     return { errorMessage: 'Actual result should be Buffer.' };
 
-  var actual = PNG.sync.read(actualBuffer);
-  var expected = PNG.sync.read(expectedBuffer);
+  let actual = PNG.sync.read(actualBuffer);
+  let expected = PNG.sync.read(expectedBuffer);
   if (expected.width !== actual.width || expected.height !== actual.height) {
     return {
       errorMessage: `Sizes differ: expected image ${expected.width}px X ${expected.height}px, but got ${actual.width}px X ${actual.height}px. `
     };
   }
-  var diff = new PNG({width: expected.width, height: expected.height});
-  var count = pixelmatch(expected.data, actual.data, diff.data, expected.width, expected.height, {threshold: 0.1});
+  let diff = new PNG({width: expected.width, height: expected.height});
+  let count = pixelmatch(expected.data, actual.data, diff.data, expected.width, expected.height, {threshold: 0.1});
   return count > 0 ? { diff: PNG.sync.write(diff) } : null;
 }
 
@@ -69,14 +69,14 @@ function compareImages(actualBuffer, expectedBuffer) {
 function compareText(actual, expectedBuffer) {
   if (typeof actual !== 'string')
     return { errorMessage: 'Actual result should be string' };
-  var expected = expectedBuffer.toString('utf-8');
+  let expected = expectedBuffer.toString('utf-8');
   if (expected === actual)
     return null;
-  var diff = new Diff();
-  var result = diff.main(expected, actual);
+  let diff = new Diff();
+  let result = diff.main(expected, actual);
   diff.cleanupSemantic(result);
-  var html = diff.prettyHtml(result);
-  var diffStylePath = path.join(__dirname, 'diffstyle.css');
+  let html = diff.prettyHtml(result);
+  let diffStylePath = path.join(__dirname, 'diffstyle.css');
   html = `<link rel="stylesheet" href="file://${diffStylePath}">` + html;
   return {
     diff: html,
@@ -84,7 +84,7 @@ function compareText(actual, expectedBuffer) {
   };
 }
 
-var customMatchers = {
+let customMatchers = {
   toBeGolden: function(util, customEqualityTesters) {
     return {
       /**
@@ -93,10 +93,10 @@ var customMatchers = {
              * @return {!{pass: boolean, message: (undefined|string)}}
              */
       compare: function(actual, goldenName) {
-        var expectedPath = path.join(GOLDEN_DIR, goldenName);
-        var actualPath = path.join(OUTPUT_DIR, goldenName);
+        let expectedPath = path.join(GOLDEN_DIR, goldenName);
+        let actualPath = path.join(OUTPUT_DIR, goldenName);
 
-        var messageSuffix = 'Output is saved in "' + path.basename(OUTPUT_DIR + '" directory');
+        let messageSuffix = 'Output is saved in "' + path.basename(OUTPUT_DIR + '" directory');
 
         if (!fs.existsSync(expectedPath)) {
           ensureOutputDir();
@@ -106,15 +106,15 @@ var customMatchers = {
             message: goldenName + ' is missing in golden results. ' + messageSuffix
           };
         }
-        var expected = fs.readFileSync(expectedPath);
-        var comparator = GoldenComparators[mime.lookup(goldenName)];
+        let expected = fs.readFileSync(expectedPath);
+        let comparator = GoldenComparators[mime.lookup(goldenName)];
         if (!comparator) {
           return {
             pass: false,
             message: 'Failed to find comparator with type ' + mime.lookup(goldenName) + ': '  + goldenName
           };
         }
-        var result = comparator(actual, expected);
+        let result = comparator(actual, expected);
         if (!result)
           return { pass: true };
         ensureOutputDir();
@@ -122,11 +122,11 @@ var customMatchers = {
         // Copy expected to the output/ folder for convenience.
         fs.writeFileSync(addSuffix(actualPath, '-expected'), expected);
         if (result.diff) {
-          var diffPath = addSuffix(actualPath, '-diff', result.diffExtension);
+          let diffPath = addSuffix(actualPath, '-diff', result.diffExtension);
           fs.writeFileSync(diffPath, result.diff);
         }
 
-        var message = goldenName + ' mismatch!';
+        let message = goldenName + ' mismatch!';
         if (result.errorMessage)
           message += ' ' + result.errorMessage;
         return {
@@ -150,8 +150,8 @@ var customMatchers = {
  * @return {string}
  */
 function addSuffix(filePath, suffix, customExtension) {
-  var dirname = path.dirname(filePath);
-  var ext = path.extname(filePath);
-  var name = path.basename(filePath, ext);
+  let dirname = path.dirname(filePath);
+  let ext = path.extname(filePath);
+  let name = path.basename(filePath, ext);
   return path.join(dirname, name + suffix + (customExtension || ext));
 }
diff --git a/test/test.js b/test/test.js
index c0154b6ae5c65..fffd055f213ce 100644
--- a/test/test.js
+++ b/test/test.js
@@ -14,21 +14,21 @@
  * limitations under the License.
  */
 
-var path = require('path');
-var Browser = require('../lib/Browser');
-var StaticServer = require('./StaticServer');
-var GoldenUtils = require('./golden-utils');
+let path = require('path');
+let Browser = require('../lib/Browser');
+let StaticServer = require('./StaticServer');
+let GoldenUtils = require('./golden-utils');
 
-var PORT = 8907;
-var STATIC_PREFIX = 'http://localhost:' + PORT;
-var EMPTY_PAGE = STATIC_PREFIX + '/empty.html';
+let PORT = 8907;
+let STATIC_PREFIX = 'http://localhost:' + PORT;
+let EMPTY_PAGE = STATIC_PREFIX + '/empty.html';
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 10 * 1000;
 
 describe('Puppeteer', function() {
-  var browser;
-  var staticServer;
-  var page;
+  let browser;
+  let staticServer;
+  let page;
 
   beforeAll(function() {
     browser = new Browser({args: ['--no-sandbox']});
@@ -52,11 +52,11 @@ describe('Puppeteer', function() {
 
   describe('Page.evaluate', function() {
     it('should work', SX(async function() {
-      var result = await page.evaluate(() => 7 * 3);
+      let result = await page.evaluate(() => 7 * 3);
       expect(result).toBe(21);
     }));
     it('should await promise', SX(async function() {
-      var result = await page.evaluate(() => Promise.resolve(8 * 7));
+      let result = await page.evaluate(() => Promise.resolve(8 * 7));
       expect(result).toBe(56);
     }));
     it('should work from-inside inPageCallback', SX(async function() {
@@ -64,13 +64,13 @@ describe('Puppeteer', function() {
       await page.setInPageCallback('callController', async function(a, b) {
         return await page.evaluate((a, b) => a * b, a, b);
       });
-      var result = await page.evaluate(async function() {
+      let result = await page.evaluate(async function() {
         return await callController(9, 3);
       });
       expect(result).toBe(27);
     }));
     it('should reject promise with exception', SX(async function() {
-      var error = null;
+      let error = null;
       try {
         await page.evaluate(() => not.existing.object.property);
       } catch (e) {
@@ -82,7 +82,7 @@ describe('Puppeteer', function() {
   });
 
   it('Page Events: ConsoleMessage', SX(async function() {
-    var msgs = [];
+    let msgs = [];
     page.on('consolemessage', msg => msgs.push(msg));
     await page.evaluate(() => console.log('Message!'));
     expect(msgs).toEqual(['Message!']);
@@ -90,11 +90,11 @@ describe('Puppeteer', function() {
 
   describe('Page.navigate', function() {
     it('should fail when navigating to bad url', SX(async function() {
-      var success = await page.navigate('asdfasdf');
+      let success = await page.navigate('asdfasdf');
       expect(success).toBe(false);
     }));
     it('should succeed when navigating to good url', SX(async function() {
-      var success = await page.navigate(EMPTY_PAGE);
+      let success = await page.navigate(EMPTY_PAGE);
       expect(success).toBe(true);
     }));
   });
@@ -104,7 +104,7 @@ describe('Puppeteer', function() {
       await page.setInPageCallback('callController', function(a, b) {
         return a * b;
       });
-      var result = await page.evaluate(async function() {
+      let result = await page.evaluate(async function() {
         return await callController(9, 4);
       });
       expect(result).toBe(36);
@@ -115,7 +115,7 @@ describe('Puppeteer', function() {
       });
 
       await page.navigate(EMPTY_PAGE);
-      var result = await page.evaluate(async function() {
+      let result = await page.evaluate(async function() {
         return await callController(9, 4);
       });
       expect(result).toBe(36);
@@ -125,7 +125,7 @@ describe('Puppeteer', function() {
         return Promise.resolve(a * b);
       });
 
-      var result = await page.evaluate(async function() {
+      let result = await page.evaluate(async function() {
         return await callController(3, 5);
       });
       expect(result).toBe(15);
@@ -141,7 +141,7 @@ describe('Puppeteer', function() {
         expect(request.postData()).toBe(undefined);
         request.continue();
       });
-      var success = await page.navigate(EMPTY_PAGE);
+      let success = await page.navigate(EMPTY_PAGE);
       expect(success).toBe(true);
     }));
     it('should show extraHTTPHeaders', SX(async function() {
@@ -152,7 +152,7 @@ describe('Puppeteer', function() {
         expect(request.headers()['foo']).toBe('bar');
         request.continue();
       });
-      var success = await page.navigate(EMPTY_PAGE);
+      let success = await page.navigate(EMPTY_PAGE);
       expect(success).toBe(true);
     }));
     it('should be abortable', SX(async function() {
@@ -162,9 +162,9 @@ describe('Puppeteer', function() {
         else
           request.continue();
       });
-      var failedResources = 0;
+      let failedResources = 0;
       page.on('resourceloadingfailed', event => ++failedResources);
-      var success = await page.navigate(STATIC_PREFIX + '/one-style.html');
+      let success = await page.navigate(STATIC_PREFIX + '/one-style.html');
       expect(success).toBe(true);
       expect(failedResources).toBe(1);
     }));
@@ -186,7 +186,7 @@ describe('Puppeteer', function() {
         expect(dialog.message()).toBe('question?');
         dialog.accept('answer!');
       });
-      var result = await page.evaluate(() => prompt('question?'));
+      let result = await page.evaluate(() => prompt('question?'));
       expect(result).toBe('answer!');
     }));
   });
@@ -205,13 +205,13 @@ describe('Puppeteer', function() {
     it('should work', SX(async function() {
       await page.setViewportSize({width: 500, height: 500});
       await page.navigate(STATIC_PREFIX + '/grid.html');
-      var screenshot = await page.screenshot();
+      let screenshot = await page.screenshot();
       expect(screenshot).toBeGolden('screenshot-sanity.png');
     }));
     it('should clip rect', SX(async function() {
       await page.setViewportSize({width: 500, height: 500});
       await page.navigate(STATIC_PREFIX + '/grid.html');
-      var screenshot = await page.screenshot({
+      let screenshot = await page.screenshot({
         clip: {
           x: 50,
           y: 100,
@@ -224,7 +224,7 @@ describe('Puppeteer', function() {
     it('should work for offscreen clip', SX(async function() {
       await page.setViewportSize({width: 500, height: 500});
       await page.navigate(STATIC_PREFIX + '/grid.html');
-      var screenshot = await page.screenshot({
+      let screenshot = await page.screenshot({
         clip: {
           x: 50,
           y: 600,
@@ -237,8 +237,8 @@ describe('Puppeteer', function() {
     it('should run in parallel', SX(async function() {
       await page.setViewportSize({width: 500, height: 500});
       await page.navigate(STATIC_PREFIX + '/grid.html');
-      var promises = [];
-      for (var i = 0; i < 3; ++i) {
+      let promises = [];
+      for (let i = 0; i < 3; ++i) {
         promises.push(page.screenshot({
           clip: {
             x: 50 * i,
@@ -248,13 +248,13 @@ describe('Puppeteer', function() {
           }
         }));
       }
-      var screenshot = await promises[1];
+      let screenshot = await promises[1];
       expect(screenshot).toBeGolden('screenshot-parallel-calls.png');
     }));
     it('should take fullPage screenshots', SX(async function() {
       await page.setViewportSize({width: 500, height: 500});
       await page.navigate(STATIC_PREFIX + '/grid.html');
-      var screenshot = await page.screenshot({
+      let screenshot = await page.screenshot({
         fullPage: true
       });
       expect(screenshot).toBeGolden('screenshot-grid-fullpage.png');
@@ -262,7 +262,7 @@ describe('Puppeteer', function() {
   });
 
   describe('Frame Management', function() {
-    var FrameUtils = require('./frame-utils');
+    let FrameUtils = require('./frame-utils');
     it('should handle nested frames', SX(async function() {
       await page.navigate(STATIC_PREFIX + '/frames/nested-frames.html');
       expect(FrameUtils.dumpFrames(page.mainFrame())).toBeGolden('nested-frames.txt');
@@ -270,21 +270,21 @@ describe('Puppeteer', function() {
     it('should send events when frames are manipulated dynamically', SX(async function() {
       await page.navigate(EMPTY_PAGE);
       // validate frameattached events
-      var attachedFrames = [];
+      let attachedFrames = [];
       page.on('frameattached', frame => attachedFrames.push(frame));
       await FrameUtils.attachFrame(page, 'frame1', './assets/frame.html');
       expect(attachedFrames.length).toBe(1);
       expect(attachedFrames[0].url()).toContain('/assets/frame.html');
 
       // validate framenavigated events
-      var navigatedFrames = [];
+      let navigatedFrames = [];
       page.on('framenavigated', frame => navigatedFrames.push(frame));
       await FrameUtils.navigateFrame(page, 'frame1', './empty.html');
       expect(navigatedFrames.length).toBe(1);
       expect(navigatedFrames[0].url()).toContain('/empty.html');
 
       // validate framedetached events
-      var detachedFrames = [];
+      let detachedFrames = [];
       page.on('framedetached', frame => detachedFrames.push(frame));
       await FrameUtils.detachFrame(page, 'frame1');
       expect(detachedFrames.length).toBe(1);
@@ -292,21 +292,21 @@ describe('Puppeteer', function() {
     }));
     it('should persist mainFrame on cross-process navigation', SX(async function() {
       await page.navigate(EMPTY_PAGE);
-      var mainFrame = page.mainFrame();
+      let mainFrame = page.mainFrame();
       await page.navigate('http://127.0.0.1:' + PORT + '/empty.html');
       expect(page.mainFrame() === mainFrame).toBeTruthy();
     }));
     it('should not send attach/detach events for main frame', SX(async function() {
-      var hasEvents = false;
+      let hasEvents = false;
       page.on('frameattached', frame => hasEvents = true);
       page.on('framedetached', frame => hasEvents = true);
       await page.navigate(EMPTY_PAGE);
       expect(hasEvents).toBe(false);
     }));
     it('should detach child frames on navigation', SX(async function() {
-      var attachedFrames = [];
-      var detachedFrames = [];
-      var navigatedFrames = [];
+      let attachedFrames = [];
+      let detachedFrames = [];
+      let navigatedFrames = [];
       page.on('frameattached', frame => attachedFrames.push(frame));
       page.on('framedetached', frame => detachedFrames.push(frame));
       page.on('framenavigated', frame => navigatedFrames.push(frame));
@@ -315,9 +315,9 @@ describe('Puppeteer', function() {
       expect(detachedFrames.length).toBe(0);
       expect(navigatedFrames.length).toBe(5);
 
-      var attachedFrames = [];
-      var detachedFrames = [];
-      var navigatedFrames = [];
+      attachedFrames = [];
+      detachedFrames = [];
+      navigatedFrames = [];
       await page.navigate(EMPTY_PAGE);
       expect(attachedFrames.length).toBe(0);
       expect(detachedFrames.length).toBe(4);
diff --git a/utils/ChromiumDownloader.js b/utils/ChromiumDownloader.js
index 3fc42b1c15189..55e75af32c878 100644
--- a/utils/ChromiumDownloader.js
+++ b/utils/ChromiumDownloader.js
@@ -14,17 +14,17 @@
  * limitations under the License.
  */
 
-var os = require('os');
-var https = require('https');
-var fs = require('fs');
-var path = require('path');
-var extract = require('extract-zip');
-var util = require('util');
-var URL = require('url');
+let os = require('os');
+let https = require('https');
+let fs = require('fs');
+let path = require('path');
+let extract = require('extract-zip');
+let util = require('util');
+let URL = require('url');
 
-var CHROMIUM_PATH = path.join(__dirname, '..', '.local-chromium');
+let CHROMIUM_PATH = path.join(__dirname, '..', '.local-chromium');
 
-var downloadURLs = {
+let downloadURLs = {
   linux: 'https://storage.googleapis.com/chromium-browser-snapshots/Linux_x64/%d/chrome-linux.zip',
   mac: 'https://storage.googleapis.com/chromium-browser-snapshots/Mac/%d/chrome-mac.zip',
   win32: 'https://storage.googleapis.com/chromium-browser-snapshots/Win/%d/chrome-win32.zip',
@@ -43,7 +43,7 @@ module.exports = {
      * @return {string}
      */
   currentPlatform: function() {
-    var platform = os.platform();
+    let platform = os.platform();
     if (platform === 'darwin')
       return 'mac';
     if (platform === 'linux')
@@ -60,15 +60,15 @@ module.exports = {
      */
   canDownloadRevision: function(platform, revision) {
     console.assert(downloadURLs[platform], 'Unknown platform: ' + platform);
-    var url = URL.parse(util.format(downloadURLs[platform], revision));
-    var options = {
+    let url = URL.parse(util.format(downloadURLs[platform], revision));
+    let options = {
       method: 'HEAD',
       host: url.host,
       path: url.pathname,
     };
-    var resolve;
-    var promise = new Promise(x => resolve = x);
-    var request = https.request(options, response => {
+    let resolve;
+    let promise = new Promise(x => resolve = x);
+    let request = https.request(options, response => {
       resolve(response.statusCode === 200);
     });
     request.on('error', error => {
@@ -86,11 +86,11 @@ module.exports = {
      * @return {!Promise}
      */
   downloadRevision: async function(platform, revision, progressCallback) {
-    var url = downloadURLs[platform];
+    let url = downloadURLs[platform];
     console.assert(url, `Unsupported platform: ${platform}`);
     url = util.format(url, revision);
-    var zipPath = path.join(CHROMIUM_PATH, `download-${platform}-${revision}.zip`);
-    var folderPath = getFolderPath(platform, revision);
+    let zipPath = path.join(CHROMIUM_PATH, `download-${platform}-${revision}.zip`);
+    let folderPath = getFolderPath(platform, revision);
     if (fs.existsSync(folderPath))
       return;
     try {
@@ -110,10 +110,10 @@ module.exports = {
      * @return {?{executablePath: string}}
      */
   revisionInfo: function(platform, revision) {
-    var folderPath = getFolderPath(platform, revision);
+    let folderPath = getFolderPath(platform, revision);
     if (!fs.existsSync(folderPath))
       return null;
-    var executablePath = '';
+    let executablePath = '';
     if (platform === 'mac')
       executablePath = path.join(folderPath, 'chrome-mac', 'Chromium.app', 'Contents', 'MacOS', 'Chromium');
     else if (platform === 'linux')
@@ -144,21 +144,21 @@ function getFolderPath(platform, revision) {
  * @return {!Promise}
  */
 function downloadFile(url, destinationPath, progressCallback) {
-  var fulfill, reject;
-  var promise = new Promise((x, y) => { fulfill = x; reject = y; });
-  var request = https.get(url, response => {
+  let fulfill, reject;
+  let promise = new Promise((x, y) => { fulfill = x; reject = y; });
+  let request = https.get(url, response => {
     if (response.statusCode !== 200) {
-      var error = new Error(`Download failed: server returned code ${response.statusCode}. URL: ${url}`);
+      let error = new Error(`Download failed: server returned code ${response.statusCode}. URL: ${url}`);
       // consume response data to free up memory
       response.resume();
       reject(error);
       return;
     }
-    var file = fs.createWriteStream(destinationPath);
+    let file = fs.createWriteStream(destinationPath);
     file.on('finish', () => fulfill());
     file.on('error', error => reject(error));
     response.pipe(file);
-    var totalBytes = parseInt(response.headers['content-length'], 10);
+    let totalBytes = parseInt(response.headers['content-length'], 10);
     if (progressCallback)
       response.on('data', onData.bind(null, totalBytes));
   });
diff --git a/utils/check_availability.js b/utils/check_availability.js
index df496885247aa..9f5614a3ed625 100755
--- a/utils/check_availability.js
+++ b/utils/check_availability.js
@@ -15,11 +15,11 @@
  * limitations under the License.
  */
 
-var Downloader = require('./ChromiumDownloader');
-var https = require('https');
-var OMAHA_PROXY = 'https://omahaproxy.appspot.com/all.json';
+let Downloader = require('./ChromiumDownloader');
+let https = require('https');
+let OMAHA_PROXY = 'https://omahaproxy.appspot.com/all.json';
 
-var colors = {
+let colors = {
   reset: '\x1b[0m',
   red: '\x1b[31m',
   green: '\x1b[32m',
@@ -39,8 +39,8 @@ class Table {
      */
   drawRow(values) {
     console.assert(values.length === this.widths.length);
-    var row = '';
-    for (var i = 0; i < values.length; ++i)
+    let row = '';
+    for (let i = 0; i < values.length; ++i)
       row += padCenter(values[i], this.widths[i]);
     console.log(row);
   }
@@ -59,8 +59,8 @@ Running command without arguments will check against omahaproxy revisions.`);
   return;
 }
 
-var fromRevision = parseInt(process.argv[2], 10);
-var toRevision = parseInt(process.argv[3], 10);
+let fromRevision = parseInt(process.argv[2], 10);
+let toRevision = parseInt(process.argv[3], 10);
 checkRangeAvailability(fromRevision, toRevision);
 
 /**
@@ -68,23 +68,23 @@ checkRangeAvailability(fromRevision, toRevision);
  */
 async function checkOmahaProxyAvailability() {
   console.log('Fetching revisions from ' + OMAHA_PROXY);
-  var platforms = await loadJSON(OMAHA_PROXY);
+  let platforms = await loadJSON(OMAHA_PROXY);
   if (!platforms) {
     console.error('ERROR: failed to fetch chromium revisions from omahaproxy.');
     return;
   }
-  var table = new Table([27, 7, 7, 7, 7]);
+  let table = new Table([27, 7, 7, 7, 7]);
   table.drawRow([''].concat(Downloader.supportedPlatforms()));
-  for (var platform of platforms) {
+  for (let platform of platforms) {
     // Trust only to the main platforms.
     if (platform.os !== 'mac' && platform.os !== 'win' && platform.os !== 'win64' && platform.os !== 'linux')
       continue;
-    var osName = platform.os === 'win' ? 'win32' : platform.os;
-    for (var version of platform.versions) {
+    let osName = platform.os === 'win' ? 'win32' : platform.os;
+    for (let version of platform.versions) {
       if (version.channel !== 'dev' && version.channel !== 'beta' && version.channel !== 'canary' && version.channel !== 'stable')
         continue;
-      var revisionName = padLeft('[' + osName + ' ' + version.channel + ']', 15);
-      var revision = parseInt(version.branch_base_position, 10);
+      let revisionName = padLeft('[' + osName + ' ' + version.channel + ']', 15);
+      let revision = parseInt(version.branch_base_position, 10);
       await checkAndDrawRevisionAvailability(table, revisionName, revision);
     }
   }
@@ -96,10 +96,10 @@ async function checkOmahaProxyAvailability() {
  * @return {!Promise}
  */
 async function checkRangeAvailability(fromRevision, toRevision) {
-  var table = new Table([10, 7, 7, 7, 7]);
+  let table = new Table([10, 7, 7, 7, 7]);
   table.drawRow([''].concat(Downloader.supportedPlatforms()));
-  var inc = fromRevision < toRevision ? 1 : -1;
-  for (var revision = fromRevision; revision !== toRevision; revision += inc)
+  let inc = fromRevision < toRevision ? 1 : -1;
+  for (let revision = fromRevision; revision !== toRevision; revision += inc)
     await checkAndDrawRevisionAvailability(table, '', revision);
 }
 
@@ -110,15 +110,15 @@ async function checkRangeAvailability(fromRevision, toRevision) {
  * @return {!Promise}
  */
 async function checkAndDrawRevisionAvailability(table, name, revision) {
-  var promises = [];
-  for (var platform of Downloader.supportedPlatforms())
+  let promises = [];
+  for (let platform of Downloader.supportedPlatforms())
     promises.push(Downloader.canDownloadRevision(platform, revision));
-  var availability = await Promise.all(promises);
-  var allAvailable = availability.every(e => !!e);
-  var values = [name + ' ' + (allAvailable ? colors.green + revision + colors.reset : revision)];
-  for (var i = 0; i < availability.length; ++i) {
-    var decoration = availability[i] ? '+' : '-';
-    var color = availability[i] ? colors.green : colors.red;
+  let availability = await Promise.all(promises);
+  let allAvailable = availability.every(e => !!e);
+  let values = [name + ' ' + (allAvailable ? colors.green + revision + colors.reset : revision)];
+  for (let i = 0; i < availability.length; ++i) {
+    let decoration = availability[i] ? '+' : '-';
+    let color = availability[i] ? colors.green : colors.red;
     values.push(color + decoration + colors.reset);
   }
   table.drawRow(values);
@@ -129,19 +129,19 @@ async function checkAndDrawRevisionAvailability(table, name, revision) {
  * @return {!Promise<?Object>}
  */
 function loadJSON(url) {
-  var resolve;
-  var promise = new Promise(x => resolve = x);
+  let resolve;
+  let promise = new Promise(x => resolve = x);
   https.get(url, response => {
     if (response.statusCode !== 200) {
       resolve(null);
       return;
     }
-    var body = '';
+    let body = '';
     response.on('data', function(chunk){
       body += chunk;
     });
     response.on('end', function(){
-      var json = JSON.parse(body);
+      let json = JSON.parse(body);
       resolve(json);
     });
   }).on('error', function(e){
@@ -164,8 +164,8 @@ function spaceString(size) {
  * @return {string}
  */
 function filterOutColors(text) {
-  for (var colorName in colors) {
-    var color = colors[colorName];
+  for (let colorName in colors) {
+    let color = colors[colorName];
     text = text.replace(color, '');
   }
   return text;
@@ -177,7 +177,7 @@ function filterOutColors(text) {
  * @return {string}
  */
 function padLeft(text, length) {
-  var printableCharacters = filterOutColors(text);
+  let printableCharacters = filterOutColors(text);
   return printableCharacters.length >= length ? text : spaceString(length - text.length) + text;
 }
 
@@ -187,10 +187,10 @@ function padLeft(text, length) {
  * @return {string}
  */
 function padCenter(text, length) {
-  var printableCharacters = filterOutColors(text);
+  let printableCharacters = filterOutColors(text);
   if (printableCharacters.length >= length)
     return text;
-  var left = Math.floor((length - printableCharacters.length) / 2);
-  var right = Math.ceil((length - printableCharacters.length) / 2);
+  let left = Math.floor((length - printableCharacters.length) / 2);
+  let right = Math.ceil((length - printableCharacters.length) / 2);
   return spaceString(left) + text + spaceString(right);
 }