Chrome no longer allows to trigger install from extension msg response callback, so we have to check if extension exists before user clicks "share desktop" button(on startup). Fixes jshint warnings.

index.html

     <script src="libs/popover.js?v=1"></script><!-- bootstrap tooltip lib -->
     <script src="libs/toastr.js?v=1"></script><!-- notifications lib -->
     <script src="interface_config.js?v=5"></script>
-    <script src="libs/app.bundle.js?v=40"></script>
+    <script src="libs/app.bundle.js?v=41"></script>
     <script src="analytics.js?v=1"></script><!-- google analytics plugin -->
     <link rel="stylesheet" href="css/font.css?v=6"/>
     <link rel="stylesheet" href="css/toastr.css?v=1">

modules/desktopsharing/desktopsharing.js

-/* global $, alert, changeLocalVideo, chrome, config, getConferenceHandler, getUserMediaWithConstraints */
+/* global $, alert, APP, changeLocalVideo, chrome, config, getConferenceHandler,
+ getUserMediaWithConstraints */
 /**
  * Indicates that desktop stream is currently in use(for toggle purpose).
  * @type {boolean}
  */
 var isUsingScreenStream = false;
 /**
- * Indicates that switch stream operation is in progress and prevent from triggering new events.
+ * Indicates that switch stream operation is in progress and prevent from
+ * triggering new events.
  * @type {boolean}
  */
 var switchInProgress = false;
 var obtainDesktopStream = null;
 
 /**
- * Flag used to cache desktop sharing enabled state. Do not use directly as it can be <tt>null</tt>.
+ * Indicates whether desktop sharing extension is installed.
+ * @type {boolean}
+ */
+var extInstalled = false;
+
+/**
+ * Indicates whether update of desktop sharing extension is required.
+ * @type {boolean}
+ */
+var extUpdateRequired = false;
+
+/**
+ * Flag used to cache desktop sharing enabled state. Do not use directly as
+ * it can be <tt>null</tt>.
+ *
  * @type {null|boolean}
  */
 var _desktopSharingEnabled = null;
 
 var eventEmitter = new EventEmitter();
 
-var DesktopSharingEventTypes = require("../../service/desktopsharing/DesktopSharingEventTypes");
+var DesktopSharingEventTypes
+    = require("../../service/desktopsharing/DesktopSharingEventTypes");
 
 /**
  * Method obtains desktop stream from WebRTC 'screen' source.
  */
 function getWebStoreInstallUrl()
 {
-    return "https://chrome.google.com/webstore/detail/" + config.chromeExtensionId;
+    return "https://chrome.google.com/webstore/detail/" +
+        config.chromeExtensionId;
 }
 
 /**
     }
 }
 
-
-function checkExtInstalled(isInstalledCallback) {
+function checkExtInstalled(callback) {
     if (!chrome.runtime) {
         // No API, so no extension for sure
-        isInstalledCallback(false);
+        callback(false, false);
         return;
     }
     chrome.runtime.sendMessage(
         function (response) {
             if (!response || !response.version) {
                 // Communication failure - assume that no endpoint exists
-                console.warn("Extension not installed?: " + chrome.runtime.lastError);
-                isInstalledCallback(false);
-            } else {
-                // Check installed extension version
-                var extVersion = response.version;
-                console.log('Extension version is: ' + extVersion);
-                var updateRequired = isUpdateRequired(config.minChromeExtVersion, extVersion);
-                if (updateRequired) {
-                    alert(
-                        'Jitsi Desktop Streamer requires update. ' +
-                        'Changes will take effect after next Chrome restart.');
-                }
-                isInstalledCallback(!updateRequired);
+                console.warn(
+                    "Extension not installed?: ", chrome.runtime.lastError);
+                callback(false, false);
+                return;
             }
+            // Check installed extension version
+            var extVersion = response.version;
+            console.log('Extension version is: ' + extVersion);
+            var updateRequired
+                = isUpdateRequired(config.minChromeExtVersion, extVersion);
+            callback(!updateRequired, updateRequired);
         }
     );
 }
 
 function doGetStreamFromExtension(streamCallback, failCallback) {
-    // Sends 'getStream' msg to the extension. Extension id must be defined in the config.
+    // Sends 'getStream' msg to the extension.
+    // Extension id must be defined in the config.
     chrome.runtime.sendMessage(
         config.chromeExtensionId,
         { getStream: true, sources: config.desktopSharingSources },
     );
 }
 /**
- * Asks Chrome extension to call chooseDesktopMedia and gets chrome 'desktop' stream for returned stream token.
+ * Asks Chrome extension to call chooseDesktopMedia and gets chrome 'desktop'
+ * stream for returned stream token.
  */
 function obtainScreenFromExtension(streamCallback, failCallback) {
-    checkExtInstalled(
-        function (isInstalled) {
-            if (isInstalled) {
-                doGetStreamFromExtension(streamCallback, failCallback);
-            } else {
-                chrome.webstore.install(
-                    getWebStoreInstallUrl(),
-                    function (arg) {
-                        console.log("Extension installed successfully", arg);
-                        // We need to reload the page in order to get the access to chrome.runtime
-                        window.location.reload(false);
-                    },
-                    function (arg) {
-                        console.log("Failed to install the extension", arg);
-                        failCallback(arg);
-                        APP.UI.messageHandler.showError("dialog.error",
-                            "dialog.failtoinstall");
-                    }
-                );
-            }
+    if (extInstalled) {
+        doGetStreamFromExtension(streamCallback, failCallback);
+    } else {
+        if (extUpdateRequired) {
+            alert(
+                'Jitsi Desktop Streamer requires update. ' +
+                'Changes will take effect after next Chrome restart.');
         }
-    );
+
+        chrome.webstore.install(
+            getWebStoreInstallUrl(),
+            function (arg) {
+                console.log("Extension installed successfully", arg);
+                // We need to reload the page in order to get the access to
+                // chrome.runtime
+                window.location.reload(false);
+            },
+            function (arg) {
+                console.log("Failed to install the extension", arg);
+                failCallback(arg);
+                APP.UI.messageHandler.showError("dialog.error",
+                    "dialog.failtoinstall");
+            }
+        );
+    }
 }
 
 /**
  * Call this method to toggle desktop sharing feature.
- * @param method pass "ext" to use chrome extension for desktop capture(chrome extension required),
- *        pass "webrtc" to use WebRTC "screen" desktop source('chrome://flags/#enable-usermedia-screen-capture'
- *        must be enabled), pass any other string or nothing in order to disable this feature completely.
+ * @param method pass "ext" to use chrome extension for desktop capture(chrome
+ *        extension required), pass "webrtc" to use WebRTC "screen" desktop
+ *        source('chrome://flags/#enable-usermedia-screen-capture' must be
+ *        enabled), pass any other string or nothing in order to disable this
+ *        feature completely.
  */
 function setDesktopSharing(method) {
     // Check if we are running chrome
 }
 
 /**
- * Initializes <link rel=chrome-webstore-item /> with extension id set in config.js to support inline installs.
- * Host site must be selected as main website of published extension.
+ * Initializes <link rel=chrome-webstore-item /> with extension id set in
+ * config.js to support inline installs. Host site must be selected as main
+ * website of published extension.
  */
 function initInlineInstalls()
 {
     },
 
     /**
-     * @returns {boolean} <tt>true</tt> if desktop sharing feature is available and enabled.
+     * @returns {boolean} <tt>true</tt> if desktop sharing feature is available
+     *          and enabled.
      */
     isDesktopSharingEnabled: function () {
         if (_desktopSharingEnabled === null) {
             if (obtainDesktopStream === obtainScreenFromExtension) {
                 // Parse chrome version
                 var userAgent = navigator.userAgent.toLowerCase();
-                // We can assume that user agent is chrome, because it's enforced when 'ext' streaming method is set
+                // We can assume that user agent is chrome, because it's
+                // enforced when 'ext' streaming method is set
                 var ver = parseInt(userAgent.match(/chrome\/(\d+)\./)[1], 10);
                 console.log("Chrome version" + userAgent, ver);
                 _desktopSharingEnabled = ver >= 34;
             } else {
-                _desktopSharingEnabled = obtainDesktopStream === obtainWebRTCScreen;
+                _desktopSharingEnabled =
+                    obtainDesktopStream === obtainWebRTCScreen;
             }
         }
         return _desktopSharingEnabled;
 
         // Initialize Chrome extension inline installs
         if (config.chromeExtensionId) {
+
             initInlineInstalls();
+
+            // Check if extension is installed
+            checkExtInstalled(function (installed, updateRequired) {
+                extInstalled = installed;
+                extUpdateRequired = updateRequired;
+                console.info(
+                    "Chrome extension installed: " + extInstalled +
+                    " updateRequired: " + extUpdateRequired);
+            });
         }
 
         eventEmitter.emit(DesktopSharingEventTypes.INIT);
     },
 
-    addListener: function(listener, type)
+    addListener: function (listener, type)
     {
         eventEmitter.on(type, listener);
     },
 
-    removeListener: function (listener,type) {
+    removeListener: function (listener, type) {
         eventEmitter.removeListener(type, listener);
     },
 
                 function (stream) {
                     // We now use screen stream
                     isUsingScreenStream = true;
-                    // Hook 'ended' event to restore camera when screen stream stops
+                    // Hook 'ended' event to restore camera
+                    // when screen stream stops
                     stream.addEventListener('ended',
                         function (e) {
                             if (!switchInProgress && isUsingScreenStream) {
-                                toggleScreenSharing();
+                                APP.desktopsharing.toggleScreenSharing();
                             }
                         }
                     );