diff --git a/assets/javascripts/app/app.js b/assets/javascripts/app/app.js
index 6ff8a6a7..91d380b9 100644
--- a/assets/javascripts/app/app.js
+++ b/assets/javascripts/app/app.js
@@ -119,7 +119,7 @@ class App extends Events {
   bootAll() {
     const docs = this.settings.getDocs();
     for (var doc of this.DOCS) {
-      (docs.indexOf(doc.slug) >= 0 ? this.docs : this.disabledDocs).add(doc);
+      (docs.includes(doc.slug) ? this.docs : this.disabledDocs).add(doc);
     }
     this.migrateDocs();
     this.docs.load(this.start.bind(this), this.onBootError.bind(this), {
@@ -267,7 +267,7 @@ class App extends Events {
       return;
     }
     const tips = this.settings.getTips();
-    if (tips.indexOf(tip) === -1) {
+    if (!tips.includes(tip)) {
       tips.push(tip);
       this.settings.setTips(tips);
       new app.views.Tip(tip);
@@ -358,11 +358,7 @@ Please check your browser extensions/addons. `);
 
   isAppError(error, file) {
     // Ignore errors from external scripts.
-    return (
-      file &&
-      file.indexOf("devdocs") !== -1 &&
-      file.indexOf(".js") === file.length - 3
-    );
+    return file && file.includes("devdocs") && file.endsWith(".js");
   }
 
   isSupportedBrowser() {
@@ -414,7 +410,7 @@ Please check your browser extensions/addons. `);
   isInvalidLocation() {
     return (
       this.config.env === "production" &&
-      location.host.indexOf(app.config.production_host) !== 0
+      !location.host.startsWith(app.config.production_host)
     );
   }
 }
diff --git a/assets/javascripts/app/searcher.js b/assets/javascripts/app/searcher.js
index f2f88cb1..47a0b820 100644
--- a/assets/javascripts/app/searcher.js
+++ b/assets/javascripts/app/searcher.js
@@ -106,7 +106,7 @@ function scoreExactMatch() {
 }
 
 function fuzzyMatch() {
-  if (valueLength <= queryLength || value.indexOf(query) >= 0) {
+  if (valueLength <= queryLength || value.includes(query)) {
     return;
   }
   if (!(match = fuzzyRegexp.exec(value))) {
diff --git a/assets/javascripts/app/settings.js b/assets/javascripts/app/settings.js
index 7337f7b7..1dbe3440 100644
--- a/assets/javascripts/app/settings.js
+++ b/assets/javascripts/app/settings.js
@@ -109,7 +109,7 @@ app.Settings = class Settings {
     $.arrayDelete(layout, "");
 
     if (enable) {
-      if (layout.indexOf(name) === -1) {
+      if (!layout.includes(name)) {
         layout.push(name);
       }
     } else {
@@ -125,7 +125,7 @@ app.Settings = class Settings {
 
   hasLayout(name) {
     const layout = (this.store.get("layout") || "").split(" ");
-    return layout.indexOf(name) !== -1;
+    return layout.includes(name);
   }
 
   setSize(value) {
@@ -155,7 +155,7 @@ app.Settings = class Settings {
     }
     for (key in data) {
       value = data[key];
-      if (Settings.PREFERENCE_KEYS.indexOf(key) !== -1) {
+      if (Settings.PREFERENCE_KEYS.includes(key)) {
         this.set(key, value);
       }
     }
diff --git a/assets/javascripts/debug.js b/assets/javascripts/debug.js
index 4539445e..8fcba75b 100644
--- a/assets/javascripts/debug.js
+++ b/assets/javascripts/debug.js
@@ -75,7 +75,7 @@ this.viewTree = function (view, level, visited) {
   if (visited == null) {
     visited = [];
   }
-  if (visited.indexOf(view) >= 0) {
+  if (visited.includes(view)) {
     return;
   }
   visited.push(view);
diff --git a/assets/javascripts/lib/events.js b/assets/javascripts/lib/events.js
index 381df7de..33d338b8 100644
--- a/assets/javascripts/lib/events.js
+++ b/assets/javascripts/lib/events.js
@@ -1,6 +1,6 @@
 class Events {
   on(event, callback) {
-    if (event.indexOf(" ") >= 0) {
+    if (event.includes(" ")) {
       for (var name of event.split(" ")) {
         this.on(name, callback);
       }
@@ -14,7 +14,7 @@ class Events {
 
   off(event, callback) {
     let callbacks, index;
-    if (event.indexOf(" ") >= 0) {
+    if (event.includes(" ")) {
       for (var name of event.split(" ")) {
         this.off(name, callback);
       }
diff --git a/assets/javascripts/lib/page.js b/assets/javascripts/lib/page.js
index 88f1600d..f5fde7e1 100644
--- a/assets/javascripts/lib/page.js
+++ b/assets/javascripts/lib/page.js
@@ -284,7 +284,7 @@ var onclick = function (event) {
 };
 
 var isSameOrigin = (url) =>
-  url.indexOf(`${location.protocol}//${location.hostname}`) === 0;
+  url.startsWith(`${location.protocol}//${location.hostname}`);
 
 var updateCanonicalLink = function () {
   if (!this.canonicalLink) {
diff --git a/assets/javascripts/lib/util.js b/assets/javascripts/lib/util.js
index 17740a42..4da077ba 100644
--- a/assets/javascripts/lib/util.js
+++ b/assets/javascripts/lib/util.js
@@ -61,7 +61,7 @@ $.on = function (el, event, callback, useCapture) {
   if (useCapture == null) {
     useCapture = false;
   }
-  if (event.indexOf(" ") >= 0) {
+  if (event.includes(" ")) {
     for (var name of event.split(" ")) {
       $.on(el, name, callback);
     }
@@ -74,7 +74,7 @@ $.off = function (el, event, callback, useCapture) {
   if (useCapture == null) {
     useCapture = false;
   }
-  if (event.indexOf(" ") >= 0) {
+  if (event.includes(" ")) {
     for (var name of event.split(" ")) {
       $.off(el, name, callback);
     }
@@ -520,55 +520,37 @@ $.popup = function (value) {
 
 let isMac = null;
 $.isMac = () =>
-  isMac != null
-    ? isMac
-    : (isMac =
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("Mac")
-          : undefined) >= 0);
+  isMac != null ? isMac : (isMac = navigator.userAgent.includes("Mac"));
 
 let isIE = null;
 $.isIE = () =>
   isIE != null
     ? isIE
     : (isIE =
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("MSIE")
-          : undefined) >= 0 ||
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("rv:11.0")
-          : undefined) >= 0);
+        navigator.userAgent.includes("MSIE") ||
+        navigator.userAgent.includes("rv:11.0"));
 
 let isChromeForAndroid = null;
 $.isChromeForAndroid = () =>
   isChromeForAndroid != null
     ? isChromeForAndroid
     : (isChromeForAndroid =
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("Android")
-          : undefined) >= 0 &&
+        navigator.userAgent.includes("Android") &&
         /Chrome\/([.0-9])+ Mobile/.test(navigator.userAgent));
 
 let isAndroid = null;
 $.isAndroid = () =>
   isAndroid != null
     ? isAndroid
-    : (isAndroid =
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("Android")
-          : undefined) >= 0);
+    : (isAndroid = navigator.userAgent.includes("Android"));
 
 let isIOS = null;
 $.isIOS = () =>
   isIOS != null
     ? isIOS
     : (isIOS =
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("iPhone")
-          : undefined) >= 0 ||
-        (navigator.userAgent != null
-          ? navigator.userAgent.indexOf("iPad")
-          : undefined) >= 0);
+        navigator.userAgent.includes("iPhone") ||
+        navigator.userAgent.includes("iPad"));
 
 $.overlayScrollbarsEnabled = function () {
   if (!$.isMac()) {
diff --git a/assets/javascripts/views/layout/mobile.js b/assets/javascripts/views/layout/mobile.js
index 1d993a73..4ac7dff4 100644
--- a/assets/javascripts/views/layout/mobile.js
+++ b/assets/javascripts/views/layout/mobile.js
@@ -24,9 +24,9 @@ app.views.Mobile = class Mobile extends app.View {
           .matches ||
         // Need to sniff the user agent because some Android and Windows Phone devices don't take
         // resolution (dpi) into account when reporting device width/height.
-        (navigator.userAgent.indexOf("Android") !== -1 &&
-          navigator.userAgent.indexOf("Mobile") !== -1) ||
-        navigator.userAgent.indexOf("IEMobile") !== -1
+        (navigator.userAgent.includes("Android") &&
+          navigator.userAgent.includes("Mobile")) ||
+        navigator.userAgent.includes("IEMobile")
       );
     } catch (error) {
       return false;
diff --git a/assets/javascripts/views/layout/settings.js b/assets/javascripts/views/layout/settings.js
index 201de0b8..68733106 100644
--- a/assets/javascripts/views/layout/settings.js
+++ b/assets/javascripts/views/layout/settings.js
@@ -65,7 +65,7 @@ app.views.Settings = class Settings extends app.View {
         (() => {
           const result = [];
           for (var doc of app.docs.all()) {
-            if (docs.indexOf(doc.slug) === -1) {
+            if (!docs.includes(doc.slug)) {
               result.push(doc);
             }
           }
diff --git a/assets/javascripts/views/list/list_focus.js b/assets/javascripts/views/list/list_focus.js
index 63fd232a..394aec99 100644
--- a/assets/javascripts/views/list/list_focus.js
+++ b/assets/javascripts/views/list/list_focus.js
@@ -56,7 +56,7 @@ app.views.ListFocus = class ListFocus extends app.View {
         return this.findNext(cursor);
       } else if (next.tagName === "DIV") {
         // sub-list
-        if (cursor.className.indexOf(" open") >= 0) {
+        if (cursor.className.includes(" open")) {
           return this.findFirst(next) || this.findNext(next);
         } else {
           return this.findNext(next);
@@ -96,7 +96,7 @@ app.views.ListFocus = class ListFocus extends app.View {
         return this.findPrev(cursor);
       } else if (prev.tagName === "DIV") {
         // sub-list
-        if (prev.previousSibling.className.indexOf("open") >= 0) {
+        if (prev.previousSibling.className.includes("open")) {
           return this.findLast(prev) || this.findPrev(prev);
         } else {
           return this.findPrev(prev);