Use strict mode for JavaScript

source/javascripts/app/lang.js

@@ -14,6 +14,8 @@ License for the specific language governing permissions and limitations
 under the License.
 */
 (function (global) {
+  'use strict';
+
   var languages = [];
 
   global.setupLanguages = setupLanguages;

source/javascripts/app/search.js

 (function (global) {
+  'use strict';
 
   var $global = $(global);
   var content, darkBox, searchResults;

source/javascripts/app/toc.js

 (function (global) {
+  'use strict';
 
   var closeToc = function() {
     $(".tocify-wrapper").removeClass('open');