diff --git a/IPython/html/static/edit/js/editor.js b/IPython/html/static/edit/js/editor.js
index 9026ab5..24a52de 100644
--- a/IPython/html/static/edit/js/editor.js
+++ b/IPython/html/static/edit/js/editor.js
@@ -31,6 +31,7 @@ function($,
         this.file_path = options.file_path;
         this.config = options.config;
         this.codemirror = new CodeMirror($(this.selector)[0]);
+        this.generation = -1;
         
         // It appears we have to set commands on the CodeMirror class, not the
         // instance. I'd like to be wrong, but since there should only be one CM
@@ -79,6 +80,7 @@ function($,
                     cm.setOption('mode', mode);
                 });
                 that.save_enabled = true;
+                that.generation = cm.changeGeneration();
             },
             function(error) {
                 cm.setValue("Error! " + error.message +
@@ -101,6 +103,8 @@ function($,
             content: this.codemirror.getValue(),
         };
         var that = this;
+        // record change generation for isClean
+        this.generation = this.codemirror.changeGeneration();
         return this.contents.save(this.file_path, model).then(function() {
             that.events.trigger("save_succeeded.TextEditor");
         });
diff --git a/IPython/html/static/edit/js/main.js b/IPython/html/static/edit/js/main.js
index e172d5f..2bcaf6a 100644
--- a/IPython/html/static/edit/js/main.js
+++ b/IPython/html/static/edit/js/main.js
@@ -19,7 +19,7 @@ require([
     events,
     contents,
     configmod,
-    editor,
+    editmod,
     menubar,
     notificationarea
     ){
@@ -28,10 +28,10 @@ require([
     var base_url = utils.get_body_data('baseUrl');
     var file_path = utils.get_body_data('filePath');
     contents = new contents.Contents({base_url: base_url});
-    var config = new configmod.ConfigSection('edit', {base_url: base_url})
+    var config = new configmod.ConfigSection('edit', {base_url: base_url});
     config.load();
     
-    var editor = new editor.Editor('#texteditor-container', {
+    var editor = new editmod.Editor('#texteditor-container', {
         base_url: base_url,
         events: events,
         contents: contents,
@@ -63,4 +63,11 @@ require([
     });
     editor.load();
     page.show();
+
+    window.onbeforeunload = function () {
+        if (!editor.codemirror.isClean(editor.generation)) {
+            return "Unsaved changes will be lost. Close anyway?";
+        }
+    };
+
 });