diff --git a/js/equality-explorer-two-variables-main.js b/js/equality-explorer-two-variables-main.js
index a431ff9..0efb44d 100644
--- a/js/equality-explorer-two-variables-main.js
+++ b/js/equality-explorer-two-variables-main.js
@@ -5,17 +5,17 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var EqualityExplorerConstants = require( 'EQUALITY_EXPLORER/common/EqualityExplorerConstants' );
-  var Sim = require( 'JOIST/Sim' );
-  var SimLauncher = require( 'JOIST/SimLauncher' );
-  var TwoVariablesScreen = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/TwoVariablesScreen' );
+  const EqualityExplorerConstants = require( 'EQUALITY_EXPLORER/common/EqualityExplorerConstants' );
+  const Sim = require( 'JOIST/Sim' );
+  const SimLauncher = require( 'JOIST/SimLauncher' );
+  const TwoVariablesScreen = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/TwoVariablesScreen' );
 
   // strings
-  var equalityExplorerTwoVariablesTitleString = require( 'string!EQUALITY_EXPLORER_TWO_VARIABLES/equality-explorer-two-variables.title' );
+  const equalityExplorerTwoVariablesTitleString = require( 'string!EQUALITY_EXPLORER_TWO_VARIABLES/equality-explorer-two-variables.title' );
 
   SimLauncher.launch( function() {
     var sim = new Sim( equalityExplorerTwoVariablesTitleString, [ new TwoVariablesScreen() ], {
diff --git a/js/equalityExplorerTwoVariables.js b/js/equalityExplorerTwoVariables.js
index c7a1aaa..f7ef966 100644
--- a/js/equalityExplorerTwoVariables.js
+++ b/js/equalityExplorerTwoVariables.js
@@ -5,11 +5,11 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var Namespace = require( 'PHET_CORE/Namespace' );
+  const Namespace = require( 'PHET_CORE/Namespace' );
 
   return new Namespace( 'equalityExplorerTwoVariables' );
 } );
\ No newline at end of file
diff --git a/js/twovariables/TwoVariablesScreen.js b/js/twovariables/TwoVariablesScreen.js
index d77c395..9ad2b26 100644
--- a/js/twovariables/TwoVariablesScreen.js
+++ b/js/twovariables/TwoVariablesScreen.js
@@ -5,16 +5,16 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var EqualityExplorerScreen = require( 'EQUALITY_EXPLORER/common/EqualityExplorerScreen' );
-  var equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
-  var inherit = require( 'PHET_CORE/inherit' );
-  var Property = require( 'AXON/Property' );
-  var TwoVariablesModel = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/model/TwoVariablesModel' );
-  var TwoVariablesScreenView = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/view/TwoVariablesScreenView' );
+  const EqualityExplorerScreen = require( 'EQUALITY_EXPLORER/common/EqualityExplorerScreen' );
+  const equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
+  const inherit = require( 'PHET_CORE/inherit' );
+  const Property = require( 'AXON/Property' );
+  const TwoVariablesModel = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/model/TwoVariablesModel' );
+  const TwoVariablesScreenView = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/view/TwoVariablesScreenView' );
 
   /**
    * @param {Object} [options]
diff --git a/js/twovariables/model/TwoVariablesModel.js b/js/twovariables/model/TwoVariablesModel.js
index 952d660..5b02539 100644
--- a/js/twovariables/model/TwoVariablesModel.js
+++ b/js/twovariables/model/TwoVariablesModel.js
@@ -5,14 +5,14 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var EqualityExplorerModel = require( 'EQUALITY_EXPLORER/common/model/EqualityExplorerModel' );
-  var equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
-  var inherit = require( 'PHET_CORE/inherit' );
-  var TwoVariablesScene = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/model/TwoVariablesScene' );
+  const EqualityExplorerModel = require( 'EQUALITY_EXPLORER/common/model/EqualityExplorerModel' );
+  const equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
+  const inherit = require( 'PHET_CORE/inherit' );
+  const TwoVariablesScene = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/model/TwoVariablesScene' );
 
   /**
    * @constructor
diff --git a/js/twovariables/model/TwoVariablesScene.js b/js/twovariables/model/TwoVariablesScene.js
index 54a9f4a..7054a0b 100644
--- a/js/twovariables/model/TwoVariablesScene.js
+++ b/js/twovariables/model/TwoVariablesScene.js
@@ -5,21 +5,21 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var ConstantTermCreator = require( 'EQUALITY_EXPLORER/common/model/ConstantTermCreator' );
-  var EqualityExplorerColors = require( 'EQUALITY_EXPLORER/common/EqualityExplorerColors' );
-  var EqualityExplorerScene = require( 'EQUALITY_EXPLORER/common/model/EqualityExplorerScene' );
-  var equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
-  var inherit = require( 'PHET_CORE/inherit' );
-  var Variable = require( 'EQUALITY_EXPLORER/common/model/Variable' );
-  var VariableTermCreator = require( 'EQUALITY_EXPLORER/common/model/VariableTermCreator' );
+  const ConstantTermCreator = require( 'EQUALITY_EXPLORER/common/model/ConstantTermCreator' );
+  const EqualityExplorerColors = require( 'EQUALITY_EXPLORER/common/EqualityExplorerColors' );
+  const EqualityExplorerScene = require( 'EQUALITY_EXPLORER/common/model/EqualityExplorerScene' );
+  const equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
+  const inherit = require( 'PHET_CORE/inherit' );
+  const Variable = require( 'EQUALITY_EXPLORER/common/model/Variable' );
+  const VariableTermCreator = require( 'EQUALITY_EXPLORER/common/model/VariableTermCreator' );
 
   // string
-  var xString = require( 'string!EQUALITY_EXPLORER/x' );
-  var yString = require( 'string!EQUALITY_EXPLORER_TWO_VARIABLES/y' );
+  const xString = require( 'string!EQUALITY_EXPLORER/x' );
+  const yString = require( 'string!EQUALITY_EXPLORER_TWO_VARIABLES/y' );
 
   /**
    * @constructor
diff --git a/js/twovariables/view/TwoVariablesSceneNode.js b/js/twovariables/view/TwoVariablesSceneNode.js
index a84bdf9..6462a39 100644
--- a/js/twovariables/view/TwoVariablesSceneNode.js
+++ b/js/twovariables/view/TwoVariablesSceneNode.js
@@ -5,14 +5,14 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var Dimension2 = require( 'DOT/Dimension2' );
-  var equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
-  var inherit = require( 'PHET_CORE/inherit' );
-  var VariablesSceneNode = require( 'EQUALITY_EXPLORER/variables/view/VariablesSceneNode' );
+  const Dimension2 = require( 'DOT/Dimension2' );
+  const equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
+  const inherit = require( 'PHET_CORE/inherit' );
+  const VariablesSceneNode = require( 'EQUALITY_EXPLORER/variables/view/VariablesSceneNode' );
 
   /**
    * @param {TwoVariablesScene} scene
diff --git a/js/twovariables/view/TwoVariablesScreenView.js b/js/twovariables/view/TwoVariablesScreenView.js
index 51add3d..dfd3acc 100644
--- a/js/twovariables/view/TwoVariablesScreenView.js
+++ b/js/twovariables/view/TwoVariablesScreenView.js
@@ -5,14 +5,14 @@
  *
  * @author Chris Malley (PixelZoom, Inc.)
  */
-define( function( require ) {
+define( require => {
   'use strict';
 
   // modules
-  var EqualityExplorerScreenView = require( 'EQUALITY_EXPLORER/common/view/EqualityExplorerScreenView' );
-  var equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
-  var inherit = require( 'PHET_CORE/inherit' );
-  var TwoVariablesSceneNode = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/view/TwoVariablesSceneNode' );
+  const EqualityExplorerScreenView = require( 'EQUALITY_EXPLORER/common/view/EqualityExplorerScreenView' );
+  const equalityExplorerTwoVariables = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/equalityExplorerTwoVariables' );
+  const inherit = require( 'PHET_CORE/inherit' );
+  const TwoVariablesSceneNode = require( 'EQUALITY_EXPLORER_TWO_VARIABLES/twovariables/view/TwoVariablesSceneNode' );
 
   /**
    * @param {TwoVariablesModel} model