diff --git a/web/pdf_history.js b/web/pdf_history.js
index 214186487..756a4de54 100644
--- a/web/pdf_history.js
+++ b/web/pdf_history.js
@@ -581,7 +581,7 @@ function isDestArraysEqual(firstDest, secondDest) {
       if (Object.keys(first).length !== Object.keys(second).length) {
         return false;
       }
-      for (var key in first) {
+      for (let key in first) {
         if (!isEntryEqual(first[key], second[key])) {
           return false;
         }
diff --git a/web/pdf_rendering_queue.js b/web/pdf_rendering_queue.js
index e3a9e2fc9..b8aa60ee4 100644
--- a/web/pdf_rendering_queue.js
+++ b/web/pdf_rendering_queue.js
@@ -103,14 +103,14 @@ class PDFRenderingQueue {
      * 2. if last scrolled down, the page after the visible pages, or
      *    if last scrolled up, the page before the visible pages
      */
-    var visibleViews = visible.views;
+    let visibleViews = visible.views;
 
-    var numVisible = visibleViews.length;
+    let numVisible = visibleViews.length;
     if (numVisible === 0) {
       return false;
     }
-    for (var i = 0; i < numVisible; ++i) {
-      var view = visibleViews[i].view;
+    for (let i = 0; i < numVisible; ++i) {
+      let view = visibleViews[i].view;
       if (!this.isViewFinished(view)) {
         return view;
       }
@@ -118,13 +118,13 @@ class PDFRenderingQueue {
 
     // All the visible views have rendered; try to render next/previous pages.
     if (scrolledDown) {
-      var nextPageIndex = visible.last.id;
+      let nextPageIndex = visible.last.id;
       // IDs start at 1, so no need to add 1.
       if (views[nextPageIndex] && !this.isViewFinished(views[nextPageIndex])) {
         return views[nextPageIndex];
       }
     } else {
-      var previousPageIndex = visible.first.id - 2;
+      let previousPageIndex = visible.first.id - 2;
       if (views[previousPageIndex] &&
           !this.isViewFinished(views[previousPageIndex])) {
         return views[previousPageIndex];
@@ -162,7 +162,7 @@ class PDFRenderingQueue {
         break;
       case RenderingStates.INITIAL:
         this.highestPriorityPage = view.renderingId;
-        var continueRendering = () => {
+        let continueRendering = () => {
           this.renderHighestPriority();
         };
         view.draw().then(continueRendering, continueRendering);
diff --git a/web/preferences.js b/web/preferences.js
index 8f537e0a4..0e6fcf104 100644
--- a/web/preferences.js
+++ b/web/preferences.js
@@ -15,7 +15,7 @@
 
 import { cloneObj } from './ui_utils';
 
-var defaultPreferences = null;
+let defaultPreferences = null;
 function getDefaultPreferences() {
   if (!defaultPreferences) {
     if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('PRODUCTION')) {
@@ -23,7 +23,7 @@ function getDefaultPreferences() {
         PDFJSDev.json('$ROOT/web/default_preferences.json'));
     } else {
       defaultPreferences = new Promise(function (resolve) {
-        var xhr = new XMLHttpRequest();
+        let xhr = new XMLHttpRequest();
         xhr.open('GET', 'default_preferences.json');
         xhr.onload = xhr.onerror = function loaded() {
           try {
@@ -130,8 +130,8 @@ class BasePreferences {
       } else if (value === undefined) {
         throw new Error('Set preference: no value is specified.');
       }
-      var valueType = typeof value;
-      var defaultType = typeof this.defaults[name];
+      let valueType = typeof value;
+      let defaultType = typeof this.defaults[name];
 
       if (valueType !== defaultType) {
         if (valueType === 'number' && defaultType === 'string') {
@@ -158,12 +158,12 @@ class BasePreferences {
    */
   get(name) {
     return this._initializedPromise.then(() => {
-      var defaultValue = this.defaults[name];
+      let defaultValue = this.defaults[name];
 
       if (defaultValue === undefined) {
         throw new Error(`Get preference: "${name}" is undefined.`);
       } else {
-        var prefValue = this.prefs[name];
+        let prefValue = this.prefs[name];
 
         if (prefValue !== undefined) {
           return prefValue;