diff --git a/js/extensionPrefs/main.js b/js/extensionPrefs/main.js
index 43efa95e9..24ecdc165 100644
--- a/js/extensionPrefs/main.js
+++ b/js/extensionPrefs/main.js
@@ -582,12 +582,12 @@ class ExtensionRow extends Gtk.ListBoxRow {
 
     _isEnabled() {
         let extensions = this._settings.get_strv('enabled-extensions');
-        return extensions.indexOf(this.uuid) != -1;
+        return extensions.includes(this.uuid);
     }
 
     _enable() {
         let extensions = this._settings.get_strv('enabled-extensions');
-        if (extensions.indexOf(this.uuid) != -1)
+        if (extensions.includes(this.uuid))
             return;
 
         extensions.push(this.uuid);
diff --git a/js/misc/extensionUtils.js b/js/misc/extensionUtils.js
index cf308b31f..89a085d68 100644
--- a/js/misc/extensionUtils.js
+++ b/js/misc/extensionUtils.js
@@ -31,7 +31,7 @@ function getCurrentExtension() {
     // Search for an occurrence of an extension stack frame
     // Start at 1 because 0 is the stack frame of this function
     for (let i = 1; i < stack.length; i++) {
-        if (stack[i].indexOf('/gnome-shell/extensions/') > -1) {
+        if (stack[i].includes('/gnome-shell/extensions/')) {
             extensionStackLine = stack[i];
             break;
         }
diff --git a/js/misc/keyboardManager.js b/js/misc/keyboardManager.js
index 99e100231..ae71b646e 100644
--- a/js/misc/keyboardManager.js
+++ b/js/misc/keyboardManager.js
@@ -125,7 +125,7 @@ var KeyboardManager = class {
 
     _getLocaleLayout() {
         let locale = GLib.get_language_names()[0];
-        if (locale.indexOf('_') == -1)
+        if (!locale.includes('_'))
             locale = DEFAULT_LOCALE;
 
         let [found, , id] = GnomeDesktop.get_input_source_from_locale(locale);
diff --git a/js/ui/appDisplay.js b/js/ui/appDisplay.js
index ea86c81b9..00019772a 100644
--- a/js/ui/appDisplay.js
+++ b/js/ui/appDisplay.js
@@ -69,7 +69,7 @@ function _getCategories(info) {
 
 function _listsIntersect(a, b) {
     for (let itemA of a)
-        if (b.indexOf(itemA) >= 0)
+        if (b.includes(itemA))
             return true;
     return false;
 }
@@ -1165,7 +1165,7 @@ var FolderIcon = class FolderIcon {
         let excludedApps = this._folder.get_strv('excluded-apps');
         let appSys = Shell.AppSystem.get_default();
         let addAppId = appId => {
-            if (excludedApps.indexOf(appId) >= 0)
+            if (excludedApps.includes(appId))
                 return;
 
             let app = appSys.lookup_app(appId);
@@ -1728,7 +1728,7 @@ var AppIconMenu = class AppIconMenu extends PopupMenu.PopupMenu {
             let appInfo = this._source.app.get_app_info();
             let actions = appInfo.list_actions();
             if (this._source.app.can_open_new_window() &&
-                actions.indexOf('new-window') == -1) {
+                actions.includes('new-window')) {
                 this._newWindowMenuItem = this._appendMenuItem(_("New Window"));
                 this._newWindowMenuItem.connect('activate', () => {
                     if (this._source.app.state == Shell.AppState.STOPPED)
@@ -1742,7 +1742,7 @@ var AppIconMenu = class AppIconMenu extends PopupMenu.PopupMenu {
 
             if (discreteGpuAvailable &&
                 this._source.app.state == Shell.AppState.STOPPED &&
-                actions.indexOf('activate-discrete-gpu') == -1) {
+                actions.includes('activate-discrete-gpu')) {
                 this._onDiscreteGpuMenuItem = this._appendMenuItem(_("Launch using Dedicated Graphics Card"));
                 this._onDiscreteGpuMenuItem.connect('activate', () => {
                     if (this._source.app.state == Shell.AppState.STOPPED)
diff --git a/js/ui/calendar.js b/js/ui/calendar.js
index cd3e879c4..d7c5bc805 100644
--- a/js/ui/calendar.js
+++ b/js/ui/calendar.js
@@ -38,7 +38,7 @@ function isToday(date) {
 function _isWorkDay(date) {
     /* Translators: Enter 0-6 (Sunday-Saturday) for non-work days. Examples: "0" (Sunday) "6" (Saturday) "06" (Sunday and Saturday). */
     let days = C_('calendar-no-work', "06");
-    return days.indexOf(date.getDay().toString()) == -1;
+    return !days.includes(date.getDay().toString());
 }
 
 function _getBeginningOfDay(date) {
diff --git a/js/ui/components/__init__.js b/js/ui/components/__init__.js
index 6d6cc9d69..652f6951c 100644
--- a/js/ui/components/__init__.js
+++ b/js/ui/components/__init__.js
@@ -13,13 +13,13 @@ var ComponentManager = class {
         let newEnabledComponents = Main.sessionMode.components;
 
         newEnabledComponents.filter(
-            name => this._enabledComponents.indexOf(name) == -1
+            name => !this._enabledComponents.includes(name)
         ).forEach(name => {
             this._enableComponent(name);
         });
 
         this._enabledComponents.filter(
-            name => newEnabledComponents.indexOf(name) == -1
+            name => !newEnabledComponents.includes(name)
         ).forEach(name => {
             this._disableComponent(name);
         });
diff --git a/js/ui/components/autorunManager.js b/js/ui/components/autorunManager.js
index 34c7d6250..4891e5d95 100644
--- a/js/ui/components/autorunManager.js
+++ b/js/ui/components/autorunManager.js
@@ -40,7 +40,7 @@ function isMountRootHidden(root) {
     let path = root.get_path();
 
     // skip any mounts in hidden directory hierarchies
-    return (path.indexOf('/.') != -1);
+    return (path.includes('/.'));
 }
 
 function isMountNonLocal(mount) {
@@ -192,15 +192,15 @@ var AutorunDispatcher = class {
 
     _getAutorunSettingForType(contentType) {
         let runApp = this._settings.get_strv(SETTING_START_APP);
-        if (runApp.indexOf(contentType) != -1)
+        if (runApp.includes(contentType))
             return AutorunSetting.RUN;
 
         let ignore = this._settings.get_strv(SETTING_IGNORE);
-        if (ignore.indexOf(contentType) != -1)
+        if (ignore.includes(contentType))
             return AutorunSetting.IGNORE;
 
         let openFiles = this._settings.get_strv(SETTING_OPEN_FOLDER);
-        if (openFiles.indexOf(contentType) != -1)
+        if (openFiles.includes(contentType))
             return AutorunSetting.FILES;
 
         return AutorunSetting.ASK;
diff --git a/js/ui/components/networkAgent.js b/js/ui/components/networkAgent.js
index 48bbd308f..dca8318ed 100644
--- a/js/ui/components/networkAgent.js
+++ b/js/ui/components/networkAgent.js
@@ -327,7 +327,7 @@ class NetworkSecretDialog extends ModalDialog.ModalDialog {
             this._getPPPoESecrets(content.secrets);
             break;
         case 'gsm':
-            if (this._hints.indexOf('pin') != -1) {
+            if (this._hints.includes('pin')) {
                 let gsmSetting = this._connection.get_setting_gsm();
                 content.title = _("PIN code required");
                 content.message = _("PIN code is needed for the mobile broadband device");
@@ -695,7 +695,7 @@ var NetworkAgent = class {
             body = _("A password is required to connect to “%s”.".format(connection.get_id()));
             break;
         case 'gsm':
-            if (hints.indexOf('pin') != -1) {
+            if (hints.includes('pin')) {
                 let gsmSetting = connection.get_setting_gsm();
                 title = _("PIN code required");
                 body = _("PIN code is needed for the mobile broadband device");
diff --git a/js/ui/components/polkitAgent.js b/js/ui/components/polkitAgent.js
index 27566531f..a44165df9 100644
--- a/js/ui/components/polkitAgent.js
+++ b/js/ui/components/polkitAgent.js
@@ -45,9 +45,9 @@ var AuthenticationDialog = GObject.registerClass({
         }
 
         let userName = GLib.get_user_name();
-        if (userNames.indexOf(userName) < 0)
+        if (!userNames.includes(userName))
             userName = 'root';
-        if (userNames.indexOf(userName) < 0)
+        if (!userNames.includes(userName))
             userName = userNames[0];
 
         this._user = AccountsService.UserManager.get_default().get_user(userName);
diff --git a/js/ui/dash.js b/js/ui/dash.js
index 0032c912e..878045410 100644
--- a/js/ui/dash.js
+++ b/js/ui/dash.js
@@ -700,14 +700,14 @@ var Dash = class Dash {
             }
 
             // App removed at oldIndex
-            if (oldApp && newApps.indexOf(oldApp) == -1) {
+            if (oldApp && !newApps.includes(oldApp)) {
                 removedActors.push(children[oldIndex]);
                 oldIndex++;
                 continue;
             }
 
             // App added at newIndex
-            if (newApp && oldApps.indexOf(newApp) == -1) {
+            if (newApp && !oldApps.includes(newApp)) {
                 addedItems.push({ app: newApp,
                                   item: this._createAppItem(newApp),
                                   pos: newIndex });
diff --git a/js/ui/endSessionDialog.js b/js/ui/endSessionDialog.js
index d83578fb9..3c7729eb3 100644
--- a/js/ui/endSessionDialog.js
+++ b/js/ui/endSessionDialog.js
@@ -585,7 +585,7 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
     }
 
     _onInhibitorLoaded(inhibitor) {
-        if (this._applications.indexOf(inhibitor) < 0) {
+        if (!this._applications.includes(inhibitor)) {
             // Stale inhibitor
             return;
         }
diff --git a/js/ui/extensionDownloader.js b/js/ui/extensionDownloader.js
index 184db2464..fb36ac478 100644
--- a/js/ui/extensionDownloader.js
+++ b/js/ui/extensionDownloader.js
@@ -228,7 +228,7 @@ class InstallExtensionDialog extends ModalDialog.ModalDialog {
         function callback() {
             // Add extension to 'enabled-extensions' for the user, always...
             let enabledExtensions = global.settings.get_strv(ExtensionSystem.ENABLED_EXTENSIONS_KEY);
-            if (enabledExtensions.indexOf(uuid) == -1) {
+            if (!enabledExtensions.includes(uuid)) {
                 enabledExtensions.push(uuid);
                 global.settings.set_strv(ExtensionSystem.ENABLED_EXTENSIONS_KEY, enabledExtensions);
             }
diff --git a/js/ui/extensionSystem.js b/js/ui/extensionSystem.js
index 6244c39b4..744f5e7ac 100644
--- a/js/ui/extensionSystem.js
+++ b/js/ui/extensionSystem.js
@@ -169,7 +169,7 @@ function loadExtension(extension) {
     if (checkVersion && ExtensionUtils.isOutOfDate(extension)) {
         extension.state = ExtensionState.OUT_OF_DATE;
     } else {
-        let enabled = enabledExtensions.indexOf(extension.uuid) != -1;
+        let enabled = enabledExtensions.includes(extension.uuid);
         if (enabled) {
             if (!initExtension(extension.uuid))
                 return;
diff --git a/js/ui/grabHelper.js b/js/ui/grabHelper.js
index 509915019..39873e16f 100644
--- a/js/ui/grabHelper.js
+++ b/js/ui/grabHelper.js
@@ -87,7 +87,7 @@ var GrabHelper = class GrabHelper {
     _isWithinGrabbedActor(actor) {
         let currentActor = this.currentGrab.actor;
         while (actor) {
-            if (this._actors.indexOf(actor) != -1)
+            if (this._actors.includes(actor))
                 return true;
             if (actor == currentActor)
                 return true;
diff --git a/js/ui/inhibitShortcutsDialog.js b/js/ui/inhibitShortcutsDialog.js
index 170cd1a33..6c988577c 100644
--- a/js/ui/inhibitShortcutsDialog.js
+++ b/js/ui/inhibitShortcutsDialog.js
@@ -111,7 +111,7 @@ var InhibitShortcutsDialog = GObject.registerClass({
     }
 
     vfunc_show() {
-        if (this._app && APP_WHITELIST.indexOf(this._app.get_id()) != -1) {
+        if (this._app && APP_WHITELIST.includes(this._app.get_id())) {
             this._emitResponse(DialogResponse.ALLOW);
             return;
         }
diff --git a/js/ui/keyboard.js b/js/ui/keyboard.js
index b6a26663a..c30f90a76 100644
--- a/js/ui/keyboard.js
+++ b/js/ui/keyboard.js
@@ -1074,7 +1074,7 @@ var Keyboard = class Keyboard {
                 let manager = Clutter.DeviceManager.get_default();
                 let device = manager.get_device(deviceId);
 
-                if (device.get_device_name().indexOf('XTEST') < 0) {
+                if (!device.get_device_name().includes('XTEST')) {
                     this._lastDeviceId = deviceId;
                     this._syncEnabled();
                 }
diff --git a/js/ui/main.js b/js/ui/main.js
index 04f333ea1..9b6986001 100644
--- a/js/ui/main.js
+++ b/js/ui/main.js
@@ -663,7 +663,7 @@ function initializeDeferredWork(actor, callback, props) {
     _deferredWorkData[workId] = { 'actor': actor,
                                   'callback': callback };
     actor.connect('notify::mapped', () => {
-        if (!(actor.mapped && _deferredWorkQueue.indexOf(workId) >= 0))
+        if (!(actor.mapped && _deferredWorkQueue.includes(workId)))
             return;
         _queueBeforeRedraw(workId);
     });
@@ -693,7 +693,7 @@ function queueDeferredWork(workId) {
         logError(new Error(message), message);
         return;
     }
-    if (_deferredWorkQueue.indexOf(workId) < 0)
+    if (!_deferredWorkQueue.includes(workId))
         _deferredWorkQueue.push(workId);
     if (data.actor.mapped) {
         _queueBeforeRedraw(workId);
diff --git a/js/ui/messageList.js b/js/ui/messageList.js
index bd97ffcb0..93c3fc3ac 100644
--- a/js/ui/messageList.js
+++ b/js/ui/messageList.js
@@ -72,7 +72,7 @@ var URLHighlighter = class URLHighlighter {
             let urlId = this._findUrlAtPos(event);
             if (urlId != -1) {
                 let url = this._urls[urlId].url;
-                if (url.indexOf(':') == -1)
+                if (!url.includes(':'))
                     url = 'http://' + url;
 
                 Gio.app_info_launch_default_for_uri(url, global.create_app_launch_context(0, -1));
diff --git a/js/ui/messageTray.js b/js/ui/messageTray.js
index f1f79d8c2..4007f1b00 100644
--- a/js/ui/messageTray.js
+++ b/js/ui/messageTray.js
@@ -228,7 +228,7 @@ class NotificationApplicationPolicy extends NotificationPolicy {
         this._settings.set_string('application-id', this.id + '.desktop');
 
         let apps = this._masterSettings.get_strv('application-children');
-        if (apps.indexOf(this._canonicalId) < 0) {
+        if (!apps.includes(this._canonicalId)) {
             apps.push(this._canonicalId);
             this._masterSettings.set_strv('application-children', apps);
         }
@@ -772,7 +772,7 @@ var Source = class Source {
     }
 
     pushNotification(notification) {
-        if (this.notifications.indexOf(notification) >= 0)
+        if (this.notifications.includes(notification))
             return;
 
         while (this.notifications.length >= MAX_NOTIFICATIONS_PER_SOURCE)
@@ -1069,7 +1069,7 @@ var MessageTray = class MessageTray {
             // If a new notification is updated while it is being hidden,
             // we stop hiding it and show it again.
             this._updateShowingNotification();
-        } else if (this._notificationQueue.indexOf(notification) < 0) {
+        } else if (!this._notificationQueue.includes(notification)) {
             // If the queue is "full", we skip banner mode and just show a small
             // indicator in the panel; however do make an exception for CRITICAL
             // notifications, as only banner mode allows expansion.
diff --git a/js/ui/padOsd.js b/js/ui/padOsd.js
index a4af47297..903b967c4 100644
--- a/js/ui/padOsd.js
+++ b/js/ui/padOsd.js
@@ -630,7 +630,7 @@ var PadOsd = class {
             // If the device is being removed, destroy the padOsd.
             if (device == this.padDevice) {
                 this.destroy();
-            } else if (this._groupPads.indexOf(device) != -1) {
+            } else if (this._groupPads.includes(device)) {
                 // Or update the pad chooser if the device belongs to
                 // the same group.
                 this._groupPads.splice(this._groupPads.indexOf(device), 1);
@@ -749,8 +749,7 @@ var PadOsd = class {
     }
 
     _requestForOtherPad(pad) {
-        if (pad == this.padDevice ||
-            this._groupPads.indexOf(pad) == -1)
+        if (pad == this.padDevice || !this._groupPads.includes(pad))
             return;
 
         let editionMode = this._editionMode;
@@ -801,7 +800,7 @@ var PadOsd = class {
 
         // If the event comes from another pad in the same group,
         // show the OSD for it.
-        if (this._groupPads.indexOf(event.get_source_device()) != -1) {
+        if (this._groupPads.includes(event.get_source_device())) {
             this._requestForOtherPad(event.get_source_device());
             return Clutter.EVENT_STOP;
         }
diff --git a/js/ui/panel.js b/js/ui/panel.js
index be02785a8..fbd65a263 100644
--- a/js/ui/panel.js
+++ b/js/ui/panel.js
@@ -1056,9 +1056,9 @@ class Panel extends St.Widget {
         this._updateBox(panel.center, this._centerBox);
         this._updateBox(panel.right, this._rightBox);
 
-        if (panel.left.indexOf('dateMenu') != -1)
+        if (panel.left.includes('dateMenu'))
             Main.messageTray.bannerAlignment = Clutter.ActorAlign.START;
-        else if (panel.right.indexOf('dateMenu') != -1)
+        else if (panel.right.includes('dateMenu'))
             Main.messageTray.bannerAlignment = Clutter.ActorAlign.END;
         // Default to center if there is no dateMenu
         else
diff --git a/js/ui/remoteSearch.js b/js/ui/remoteSearch.js
index ce8c4d09b..ae6c6dab7 100644
--- a/js/ui/remoteSearch.js
+++ b/js/ui/remoteSearch.js
@@ -146,10 +146,10 @@ function loadRemoteSearchProviders(searchSettings, callback) {
 
         if (provider.defaultEnabled) {
             let disabled = searchSettings.get_strv('disabled');
-            return disabled.indexOf(appId) == -1;
+            return !disabled.includes(appId);
         } else {
             let enabled = searchSettings.get_strv('enabled');
-            return enabled.indexOf(appId) != -1;
+            return enabled.includes(appId);
         }
     });
 
diff --git a/js/ui/runDialog.js b/js/ui/runDialog.js
index 9186ff7bb..21bd04c3e 100644
--- a/js/ui/runDialog.js
+++ b/js/ui/runDialog.js
@@ -182,7 +182,7 @@ class RunDialog extends ModalDialog.ModalDialog {
     }
 
     _getCompletion(text) {
-        if (text.indexOf('/') != -1) {
+        if (text.includes('/')) {
             return this._pathCompleter.get_completion_suffix(text);
         } else {
             return this._getCommandCompletion(text);
diff --git a/js/ui/sessionMode.js b/js/ui/sessionMode.js
index ae6c64dd3..70a6e8a15 100644
--- a/js/ui/sessionMode.js
+++ b/js/ui/sessionMode.js
@@ -128,7 +128,7 @@ function _loadMode(file, info) {
     let propBlacklist = ['unlockDialog'];
     for (let prop in _modes[DEFAULT_MODE]) {
         if (newMode[prop] !== undefined &&
-            propBlacklist.indexOf(prop) == -1)
+            !propBlacklist.includes(prop))
             _modes[modeName][prop] = newMode[prop];
     }
     _modes[modeName]['isPrimary'] = true;
diff --git a/js/ui/status/network.js b/js/ui/status/network.js
index 7c2499d0e..8ad451347 100644
--- a/js/ui/status/network.js
+++ b/js/ui/status/network.js
@@ -1040,7 +1040,7 @@ class NMWirelessDialog extends ModalDialog.ModalDialog {
     _checkConnections(network, accessPoint) {
         this._connections.forEach(connection => {
             if (accessPoint.connection_valid(connection) &&
-                network.connections.indexOf(connection) == -1) {
+                !network.connections.includes(connection)) {
                 network.connections.push(connection);
             }
         });
@@ -1059,7 +1059,7 @@ class NMWirelessDialog extends ModalDialog.ModalDialog {
 
         if (pos != -1) {
             network = this._networks[pos];
-            if (network.accessPoints.indexOf(accessPoint) != -1) {
+            if (network.accessPoints.includes(accessPoint)) {
                 log('Access point was already seen, not adding again');
                 return;
             }
diff --git a/js/ui/status/volume.js b/js/ui/status/volume.js
index ae2497a93..cc270500b 100644
--- a/js/ui/status/volume.js
+++ b/js/ui/status/volume.js
@@ -215,7 +215,7 @@ var OutputStreamSlider = class extends StreamSlider {
         // of different identifiers for headphones, and I could
         // not find the complete list
         if (sink.get_ports().length > 0)
-            return sink.get_port().port.indexOf('headphone') >= 0;
+            return sink.get_port().port.includes('headphone');
 
         return false;
     }
diff --git a/js/ui/windowManager.js b/js/ui/windowManager.js
index 19dc263cd..d016f3371 100644
--- a/js/ui/windowManager.js
+++ b/js/ui/windowManager.js
@@ -1288,7 +1288,7 @@ var WindowManager = class {
             return false;
 
         let type = actor.meta_window.get_window_type();
-        return types.indexOf(type) >= 0;
+        return types.includes(type);
     }
 
     _removeEffect(list, actor) {
@@ -1501,7 +1501,7 @@ var WindowManager = class {
     _sizeChangedWindow(shellwm, actor) {
         if (!actor.__animationInfo)
             return;
-        if (this._resizing.indexOf(actor) != -1)
+        if (this._resizing.includes(actor))
             return;
 
         let actorClone = actor.__animationInfo.clone;
diff --git a/js/ui/workspace.js b/js/ui/workspace.js
index 50ac141d9..1b6c63eb3 100644
--- a/js/ui/workspace.js
+++ b/js/ui/workspace.js
@@ -536,7 +536,7 @@ var WindowOverlay = class {
         let [cloneX, cloneY, cloneWidth, cloneHeight] = this._windowClone.slot;
 
         let layout = Meta.prefs_get_button_layout();
-        let side = layout.left_buttons.indexOf(Meta.ButtonFunction.CLOSE) > -1 ? St.Side.LEFT : St.Side.RIGHT;
+        let side = layout.left_buttons.includes(Meta.ButtonFunction.CLOSE) ? St.Side.LEFT : St.Side.RIGHT;
 
         let buttonX;
         let buttonY = cloneY - (button.height - button._overlap);
diff --git a/js/ui/workspaceThumbnail.js b/js/ui/workspaceThumbnail.js
index 969167178..be196c04b 100644
--- a/js/ui/workspaceThumbnail.js
+++ b/js/ui/workspaceThumbnail.js
@@ -378,7 +378,7 @@ var WorkspaceThumbnail = class {
             return;
         }
 
-        if (this._allWindows.indexOf(metaWin) == -1) {
+        if (!this._allWindows.includes(metaWin)) {
             let minimizedChangedId = metaWin.connect('notify::minimized',
                                                      this._updateMinimized.bind(this));
             this._allWindows.push(metaWin);