diff --git a/src/js/ContextMenu.js b/src/js/ContextMenu.js
index 683c614..338f40a 100644
--- a/src/js/ContextMenu.js
+++ b/src/js/ContextMenu.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var util = require('./util');
 
 /**
diff --git a/src/js/Highlighter.js b/src/js/Highlighter.js
index ece99e6..09b03f6 100644
--- a/src/js/Highlighter.js
+++ b/src/js/Highlighter.js
@@ -1,3 +1,5 @@
+'use strict';
+
 /**
  * The highlighter can highlight/unhighlight a node, and
  * animate the visibility of a context menu.
diff --git a/src/js/History.js b/src/js/History.js
index 6d22aa0..1a8b0bf 100644
--- a/src/js/History.js
+++ b/src/js/History.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var util = require('./util');
 
 /**
diff --git a/src/js/JSONEditor.js b/src/js/JSONEditor.js
index 8b13ac7..eb35df8 100644
--- a/src/js/JSONEditor.js
+++ b/src/js/JSONEditor.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var Ajv;
 try {
   Ajv = require('ajv');
diff --git a/src/js/ModeSwitcher.js b/src/js/ModeSwitcher.js
index 478b76b..0c99557 100644
--- a/src/js/ModeSwitcher.js
+++ b/src/js/ModeSwitcher.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var ContextMenu = require('./ContextMenu');
 
 /**
diff --git a/src/js/Node.js b/src/js/Node.js
index f354dbb..8f0eae0 100644
--- a/src/js/Node.js
+++ b/src/js/Node.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var naturalSort = require('javascript-natural-sort');
 var ContextMenu = require('./ContextMenu');
 var appendNodeFactory = require('./appendNodeFactory');
diff --git a/src/js/SearchBox.js b/src/js/SearchBox.js
index ecd1b88..43ff961 100644
--- a/src/js/SearchBox.js
+++ b/src/js/SearchBox.js
@@ -1,3 +1,5 @@
+'use strict';
+
 /**
  * @constructor SearchBox
  * Create a search box in given HTML container
diff --git a/src/js/appendNodeFactory.js b/src/js/appendNodeFactory.js
index a2f219e..48da1d3 100644
--- a/src/js/appendNodeFactory.js
+++ b/src/js/appendNodeFactory.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var util = require('./util');
 var ContextMenu = require('./ContextMenu');
 
diff --git a/src/js/textmode.js b/src/js/textmode.js
index b3d1c1f..b5c185e 100644
--- a/src/js/textmode.js
+++ b/src/js/textmode.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var ace;
 try {
   ace = require('./ace');
diff --git a/src/js/treemode.js b/src/js/treemode.js
index 6d63cdc..ad215e6 100644
--- a/src/js/treemode.js
+++ b/src/js/treemode.js
@@ -1,3 +1,6 @@
+'use strict';
+
+
 var Highlighter = require('./Highlighter');
 var History = require('./History');
 var SearchBox = require('./SearchBox');
diff --git a/src/js/util.js b/src/js/util.js
index aa384ea..1c72bda 100644
--- a/src/js/util.js
+++ b/src/js/util.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var jsonlint = require('./assets/jsonlint/jsonlint');
 
 /**