diff --git a/.eslintrc.js b/.eslintrc.js
new file mode 100644
index 000000000..ecd04c2f8
--- /dev/null
+++ b/.eslintrc.js
@@ -0,0 +1,6 @@
+module.exports = {
+    extends: ['scratch', 'scratch/node'],
+    env: {
+        node: true
+    }
+};
diff --git a/package.json b/package.json
index ac5ee4071..948e7130a 100644
--- a/package.json
+++ b/package.json
@@ -24,14 +24,11 @@
     "watch": "./node_modules/.bin/webpack --progress --colors --watch",
     "version": "./node_modules/.bin/json -f package.json -I -e \"this.repository.sha = '$(git log -n1 --pretty=format:%H)'\""
   },
-  "eslintConfig": {
-    "extends": ["scratch"]
-  },
   "devDependencies": {
     "babel-eslint": "7.0.0",
     "copy-webpack-plugin": "3.0.1",
-    "eslint": "2.7.0",
-    "eslint-config-scratch": "1.0.0",
+    "eslint": "3.8.1",
+    "eslint-config-scratch": "1.1.0",
     "expose-loader": "0.7.1",
     "gh-pages": "0.11.0",
     "highlightjs": "8.7.0",
diff --git a/src/.eslintrc.js b/src/.eslintrc.js
new file mode 100644
index 000000000..154e590c2
--- /dev/null
+++ b/src/.eslintrc.js
@@ -0,0 +1,11 @@
+module.exports = {
+    root: true,
+    extends: 'scratch',
+    env: {
+        node: false,
+        browser: true
+    },
+    globals: {
+        Promise: true
+    }
+};
diff --git a/src/blocks/scratch3_control.js b/src/blocks/scratch3_control.js
index 934500755..09f52a427 100644
--- a/src/blocks/scratch3_control.js
+++ b/src/blocks/scratch3_control.js
@@ -1,13 +1,13 @@
 var Cast = require('../util/cast');
 var Timer = require('../util/timer');
 
-function Scratch3ControlBlocks (runtime) {
+var Scratch3ControlBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -39,7 +39,7 @@ Scratch3ControlBlocks.prototype.getHats = function () {
 Scratch3ControlBlocks.prototype.repeat = function (args, util) {
     var times = Math.floor(Cast.toNumber(args.TIMES));
     // Initialize loop
-    if (util.stackFrame.loopCounter === undefined) {
+    if (typeof util.stackFrame.loopCounter === 'undefined') {
         util.stackFrame.loopCounter = times;
     }
     // Only execute once per frame.
@@ -104,19 +104,19 @@ Scratch3ControlBlocks.prototype.ifElse = function (args, util) {
 
 Scratch3ControlBlocks.prototype.stop = function (args, util) {
     var option = args.STOP_OPTION;
-    if (option == 'all') {
+    if (option === 'all') {
         util.stopAll();
-    } else if (option == 'other scripts in sprite' ||
-        option == 'other scripts in stage') {
+    } else if (option === 'other scripts in sprite' ||
+        option === 'other scripts in stage') {
         util.stopOtherTargetThreads();
-    } else if (option == 'this script') {
+    } else if (option === 'this script') {
         util.stopThread();
     }
 };
 
 Scratch3ControlBlocks.prototype.createClone = function (args, util) {
     var cloneTarget;
-    if (args.CLONE_OPTION == '_myself_') {
+    if (args.CLONE_OPTION === '_myself_') {
         cloneTarget = util.target;
     } else {
         cloneTarget = this.runtime.getSpriteTargetByName(args.CLONE_OPTION);
diff --git a/src/blocks/scratch3_data.js b/src/blocks/scratch3_data.js
index fa3bb379a..6b2d339f2 100644
--- a/src/blocks/scratch3_data.js
+++ b/src/blocks/scratch3_data.js
@@ -1,12 +1,12 @@
 var Cast = require('../util/cast');
 
-function Scratch3DataBlocks (runtime) {
+var Scratch3DataBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -54,7 +54,7 @@ Scratch3DataBlocks.prototype.getListContents = function (args, util) {
     for (var i = 0; i < list.contents.length; i++) {
         var listItem = list.contents[i];
         if (!((typeof listItem === 'string') &&
-              (listItem.length == 1))) {
+              (listItem.length === 1))) {
             allSingleLetters = false;
             break;
         }
@@ -126,7 +126,7 @@ Scratch3DataBlocks.prototype.listContainsItem = function (args, util) {
     // Try using Scratch comparison operator on each item.
     // (Scratch considers the string '123' equal to the number 123).
     for (var i = 0; i < list.contents.length; i++) {
-        if (Cast.compare(list.contents[i], item) == 0) {
+        if (Cast.compare(list.contents[i], item) === 0) {
             return true;
         }
     }
diff --git a/src/blocks/scratch3_event.js b/src/blocks/scratch3_event.js
index ad763ce4f..a2eaded7d 100644
--- a/src/blocks/scratch3_event.js
+++ b/src/blocks/scratch3_event.js
@@ -1,12 +1,12 @@
 var Cast = require('../util/cast');
 
-function Scratch3EventBlocks (runtime) {
+var Scratch3EventBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -48,7 +48,7 @@ Scratch3EventBlocks.prototype.hatGreaterThanPredicate = function (args, util) {
     var option = Cast.toString(args.WHENGREATERTHANMENU).toLowerCase();
     var value = Cast.toNumber(args.VALUE);
     // @todo: Other cases :)
-    if (option == 'timer') {
+    if (option === 'timer') {
         return util.ioQuery('clock', 'projectTimer') > value;
     }
     return false;
@@ -71,7 +71,7 @@ Scratch3EventBlocks.prototype.broadcastAndWait = function (args, util) {
                 'BROADCAST_OPTION': broadcastOption
             }
         );
-        if (util.stackFrame.startedThreads.length == 0) {
+        if (util.stackFrame.startedThreads.length === 0) {
             // Nothing was started.
             return;
         }
diff --git a/src/blocks/scratch3_looks.js b/src/blocks/scratch3_looks.js
index 46310b190..b440551d0 100644
--- a/src/blocks/scratch3_looks.js
+++ b/src/blocks/scratch3_looks.js
@@ -1,12 +1,12 @@
 var Cast = require('../util/cast');
 
-function Scratch3LooksBlocks (runtime) {
+var Scratch3LooksBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -82,33 +82,33 @@ Scratch3LooksBlocks.prototype.hide = function (args, util) {
  * Matches the behavior of Scratch 2.0 for different types of arguments.
  * @param {!Target} target Target to set costume/backdrop to.
  * @param {Any} requestedCostume Costume requested, e.g., 0, 'name', etc.
- * @param {boolean=} opt_zeroIndex Set to zero-index the requestedCostume.
+ * @param {boolean=} optZeroIndex Set to zero-index the requestedCostume.
  * @return {Array.<!Thread>} Any threads started by this switch.
  */
 Scratch3LooksBlocks.prototype._setCostumeOrBackdrop = function (target,
-        requestedCostume, opt_zeroIndex) {
+        requestedCostume, optZeroIndex) {
     if (typeof requestedCostume === 'number') {
-        target.setCostume(opt_zeroIndex ?
+        target.setCostume(optZeroIndex ?
             requestedCostume : requestedCostume - 1);
     } else {
         var costumeIndex = target.getCostumeIndexByName(requestedCostume);
         if (costumeIndex > -1) {
             target.setCostume(costumeIndex);
-        } else if (costumeIndex == 'previous costume' ||
-                   costumeIndex == 'previous backdrop') {
+        } else if (costumeIndex === 'previous costume' ||
+                   costumeIndex === 'previous backdrop') {
             target.setCostume(target.currentCostume - 1);
-        } else if (costumeIndex == 'next costume' ||
-                   costumeIndex == 'next backdrop') {
+        } else if (costumeIndex === 'next costume' ||
+                   costumeIndex === 'next backdrop') {
             target.setCostume(target.currentCostume + 1);
         } else {
             var forcedNumber = Cast.toNumber(requestedCostume);
             if (!isNaN(forcedNumber)) {
-                target.setCostume(opt_zeroIndex ?
+                target.setCostume(optZeroIndex ?
                     forcedNumber : forcedNumber - 1);
             }
         }
     }
-    if (target == this.runtime.getTargetForStage()) {
+    if (target === this.runtime.getTargetForStage()) {
         // Target is the stage - start hats.
         var newName = target.sprite.costumes[target.currentCostume].name;
         return this.runtime.startHats('event_whenbackdropswitchesto', {
@@ -142,7 +142,7 @@ Scratch3LooksBlocks.prototype.switchBackdropAndWait = function (args, util) {
                 args.BACKDROP
             )
         );
-        if (util.stackFrame.startedThreads.length == 0) {
+        if (util.stackFrame.startedThreads.length === 0) {
             // Nothing was started.
             return;
         }
diff --git a/src/blocks/scratch3_motion.js b/src/blocks/scratch3_motion.js
index 463d2b25c..3b53c02ec 100644
--- a/src/blocks/scratch3_motion.js
+++ b/src/blocks/scratch3_motion.js
@@ -2,13 +2,13 @@ var Cast = require('../util/cast');
 var MathUtil = require('../util/math-util');
 var Timer = require('../util/timer');
 
-function Scratch3MotionBlocks (runtime) {
+var Scratch3MotionBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -149,10 +149,10 @@ Scratch3MotionBlocks.prototype.ifOnEdgeBounce = function (args, util) {
     // and clamped to zero when the sprite is beyond.
     var stageWidth = this.runtime.constructor.STAGE_WIDTH;
     var stageHeight = this.runtime.constructor.STAGE_HEIGHT;
-    var distLeft = Math.max(0, stageWidth / 2 + bounds.left);
-    var distTop = Math.max(0, stageHeight / 2 - bounds.top);
-    var distRight = Math.max(0, stageWidth / 2 - bounds.right);
-    var distBottom = Math.max(0, stageHeight / 2 + bounds.bottom);
+    var distLeft = Math.max(0, (stageWidth / 2) + bounds.left);
+    var distTop = Math.max(0, (stageHeight / 2) - bounds.top);
+    var distRight = Math.max(0, (stageWidth / 2) - bounds.right);
+    var distBottom = Math.max(0, (stageHeight / 2) + bounds.bottom);
     // Find the nearest edge.
     var nearestEdge = '';
     var minDist = Infinity;
@@ -179,13 +179,13 @@ Scratch3MotionBlocks.prototype.ifOnEdgeBounce = function (args, util) {
     var radians = MathUtil.degToRad(90 - util.target.direction);
     var dx = Math.cos(radians);
     var dy = -Math.sin(radians);
-    if (nearestEdge == 'left') {
+    if (nearestEdge === 'left') {
         dx = Math.max(0.2, Math.abs(dx));
-    } else if (nearestEdge == 'top') {
+    } else if (nearestEdge === 'top') {
         dy = Math.max(0.2, Math.abs(dy));
-    } else if (nearestEdge == 'right') {
+    } else if (nearestEdge === 'right') {
         dx = 0 - Math.max(0.2, Math.abs(dx));
-    } else if (nearestEdge == 'bottom') {
+    } else if (nearestEdge === 'bottom') {
         dy = 0 - Math.max(0.2, Math.abs(dy));
     }
     var newDirection = MathUtil.radToDeg(Math.atan2(dy, dx)) + 90;
diff --git a/src/blocks/scratch3_operators.js b/src/blocks/scratch3_operators.js
index 2502cc1a8..4af4533cd 100644
--- a/src/blocks/scratch3_operators.js
+++ b/src/blocks/scratch3_operators.js
@@ -1,12 +1,12 @@
 var Cast = require('../util/cast.js');
 
-function Scratch3OperatorsBlocks (runtime) {
+var Scratch3OperatorsBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -55,7 +55,7 @@ Scratch3OperatorsBlocks.prototype.lt = function (args) {
 };
 
 Scratch3OperatorsBlocks.prototype.equals = function (args) {
-    return Cast.compare(args.OPERAND1, args.OPERAND2) == 0;
+    return Cast.compare(args.OPERAND1, args.OPERAND2) === 0;
 };
 
 Scratch3OperatorsBlocks.prototype.gt = function (args) {
@@ -79,10 +79,10 @@ Scratch3OperatorsBlocks.prototype.random = function (args) {
     var nTo = Cast.toNumber(args.TO);
     var low = nFrom <= nTo ? nFrom : nTo;
     var high = nFrom <= nTo ? nTo : nFrom;
-    if (low == high) return low;
+    if (low === high) return low;
     // If both arguments are ints, truncate the result to an int.
     if (Cast.isInt(args.FROM) && Cast.isInt(args.TO)) {
-        return low + parseInt(Math.random() * ((high + 1) - low));
+        return low + parseInt(Math.random() * ((high + 1) - low), 10);
     }
     return (Math.random() * (high - low)) + low;
 };
diff --git a/src/blocks/scratch3_procedures.js b/src/blocks/scratch3_procedures.js
index c094dc47d..d35bb9ee0 100644
--- a/src/blocks/scratch3_procedures.js
+++ b/src/blocks/scratch3_procedures.js
@@ -1,10 +1,10 @@
-function Scratch3ProcedureBlocks (runtime) {
+var Scratch3ProcedureBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
diff --git a/src/blocks/scratch3_sensing.js b/src/blocks/scratch3_sensing.js
index bd77a3e91..6ee1d0813 100644
--- a/src/blocks/scratch3_sensing.js
+++ b/src/blocks/scratch3_sensing.js
@@ -1,12 +1,12 @@
 var Cast = require('../util/cast');
 
-function Scratch3SensingBlocks (runtime) {
+var Scratch3SensingBlocks = function (runtime) {
     /**
      * The runtime instantiating this block package.
      * @type {Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Retrieve the block primitives implemented by this package.
@@ -31,11 +31,11 @@ Scratch3SensingBlocks.prototype.getPrimitives = function () {
 
 Scratch3SensingBlocks.prototype.touchingObject = function (args, util) {
     var requestedObject = args.TOUCHINGOBJECTMENU;
-    if (requestedObject == '_mouse_') {
+    if (requestedObject === '_mouse_') {
         var mouseX = util.ioQuery('mouse', 'getX');
         var mouseY = util.ioQuery('mouse', 'getY');
         return util.target.isTouchingPoint(mouseX, mouseY);
-    } else if (requestedObject == '_edge_') {
+    } else if (requestedObject === '_edge_') {
         return util.target.isTouchingEdge();
     } else {
         return util.target.isTouchingSprite(requestedObject);
@@ -114,8 +114,7 @@ Scratch3SensingBlocks.prototype.getKeyPressed = function (args, util) {
     return util.ioQuery('keyboard', 'getKeyIsDown', args.KEY_OPTION);
 };
 
-Scratch3SensingBlocks.prototype.daysSince2000 = function ()
-{
+Scratch3SensingBlocks.prototype.daysSince2000 = function () {
     var msPerDay = 24 * 60 * 60 * 1000;
     var start = new Date(2000, 1-1, 1);
     var today = new Date();
diff --git a/src/engine/adapter.js b/src/engine/adapter.js
index 327aae5e3..027d63665 100644
--- a/src/engine/adapter.js
+++ b/src/engine/adapter.js
@@ -1,6 +1,34 @@
 var mutationAdapter = require('./mutation-adapter');
 var html = require('htmlparser2');
 
+/**
+ * Convert outer blocks DOM from a Blockly CREATE event
+ * to a usable form for the Scratch runtime.
+ * This structure is based on Blockly xml.js:`domToWorkspace` and `domToBlock`.
+ * @param {Element} blocksDOM DOM tree for this event.
+ * @return {Array.<Object>} Usable list of blocks from this CREATE event.
+ */
+var domToBlocks = function (blocksDOM) {
+    // At this level, there could be multiple blocks adjacent in the DOM tree.
+    var blocks = {};
+    for (var i = 0; i < blocksDOM.length; i++) {
+        var block = blocksDOM[i];
+        if (!block.name || !block.attribs) {
+            continue;
+        }
+        var tagName = block.name.toLowerCase();
+        if (tagName === 'block' || tagName === 'shadow') {
+            domToBlock(block, blocks, true, null);
+        }
+    }
+    // Flatten blocks object into a list.
+    var blocksList = [];
+    for (var b in blocks) {
+        blocksList.push(blocks[b]);
+    }
+    return blocksList;
+};
+
 /**
  * Adapter between block creation events and block representation which can be
  * used by the Scratch runtime.
@@ -15,34 +43,6 @@ module.exports = function (e) {
     return domToBlocks(html.parseDOM(e.xml.outerHTML));
 };
 
-/**
- * Convert outer blocks DOM from a Blockly CREATE event
- * to a usable form for the Scratch runtime.
- * This structure is based on Blockly xml.js:`domToWorkspace` and `domToBlock`.
- * @param {Element} blocksDOM DOM tree for this event.
- * @return {Array.<Object>} Usable list of blocks from this CREATE event.
- */
-function domToBlocks (blocksDOM) {
-    // At this level, there could be multiple blocks adjacent in the DOM tree.
-    var blocks = {};
-    for (var i = 0; i < blocksDOM.length; i++) {
-        var block = blocksDOM[i];
-        if (!block.name || !block.attribs) {
-            continue;
-        }
-        var tagName = block.name.toLowerCase();
-        if (tagName == 'block' || tagName == 'shadow') {
-            domToBlock(block, blocks, true, null);
-        }
-    }
-    // Flatten blocks object into a list.
-    var blocksList = [];
-    for (var b in blocks) {
-        blocksList.push(blocks[b]);
-    }
-    return blocksList;
-}
-
 /**
  * Convert and an individual block DOM to the representation tree.
  * Based on Blockly's `domToBlockHeadless_`.
@@ -50,8 +50,9 @@ function domToBlocks (blocksDOM) {
  * @param {Object} blocks Collection of blocks to add to.
  * @param {Boolean} isTopBlock Whether blocks at this level are "top blocks."
  * @param {?string} parent Parent block ID.
+ * @return {undefined}
  */
-function domToBlock (blockDOM, blocks, isTopBlock, parent) {
+var domToBlock = function (blockDOM, blocks, isTopBlock, parent) {
     // Block skeleton.
     var block = {
         id: blockDOM.attribs.id, // Block ID
@@ -61,7 +62,7 @@ function domToBlock (blockDOM, blocks, isTopBlock, parent) {
         next: null, // Next block in the stack, if one exists.
         topLevel: isTopBlock, // If this block starts a stack.
         parent: parent, // Parent block ID, if available.
-        shadow: blockDOM.name == 'shadow', // If this represents a shadow/slot.
+        shadow: blockDOM.name === 'shadow', // If this represents a shadow/slot.
         x: blockDOM.attribs.x, // X position of script, if top-level.
         y: blockDOM.attribs.y // Y position of script, if top-level.
     };
@@ -82,9 +83,9 @@ function domToBlock (blockDOM, blocks, isTopBlock, parent) {
                 continue;
             }
             var grandChildNodeName = grandChildNode.name.toLowerCase();
-            if (grandChildNodeName == 'block') {
+            if (grandChildNodeName === 'block') {
                 childBlockNode = grandChildNode;
-            } else if (grandChildNodeName == 'shadow') {
+            } else if (grandChildNodeName === 'shadow') {
                 childShadowNode = grandChildNode;
             }
         }
@@ -117,7 +118,7 @@ function domToBlock (blockDOM, blocks, isTopBlock, parent) {
         case 'statement':
             // Recursively generate block structure for input block.
             domToBlock(childBlockNode, blocks, false, block.id);
-            if (childShadowNode && childBlockNode != childShadowNode) {
+            if (childShadowNode && childBlockNode !== childShadowNode) {
                 // Also generate the shadow block.
                 domToBlock(childShadowNode, blocks, false, block.id);
             }
@@ -144,4 +145,4 @@ function domToBlock (blockDOM, blocks, isTopBlock, parent) {
             break;
         }
     }
-}
+};
diff --git a/src/engine/blocks.js b/src/engine/blocks.js
index 0e89025e9..736d1778e 100644
--- a/src/engine/blocks.js
+++ b/src/engine/blocks.js
@@ -8,7 +8,7 @@ var xmlEscape = require('../util/xml-escape');
  * and handle updates from Scratch Blocks events.
  */
 
-function Blocks () {
+var Blocks = function () {
     /**
      * All blocks in the workspace.
      * Keys are block IDs, values are metadata about the block.
@@ -22,7 +22,7 @@ function Blocks () {
      * @type {Array.<String>}
      */
     this._scripts = [];
-}
+};
 
 /**
  * Blockly inputs that represent statements/branch.
@@ -109,8 +109,8 @@ Blocks.prototype.getInputs = function (id) {
     var inputs = {};
     for (var input in this._blocks[id].inputs) {
         // Ignore blocks prefixed with branch prefix.
-        if (input.substring(0, Blocks.BRANCH_INPUT_PREFIX.length)
-            != Blocks.BRANCH_INPUT_PREFIX) {
+        if (input.substring(0, Blocks.BRANCH_INPUT_PREFIX.length) !==
+            Blocks.BRANCH_INPUT_PREFIX) {
             inputs[input] = this._blocks[id].inputs[input];
         }
     }
@@ -149,9 +149,9 @@ Blocks.prototype.getTopLevelScript = function (id) {
 Blocks.prototype.getProcedureDefinition = function (name) {
     for (var id in this._blocks) {
         var block = this._blocks[id];
-        if ((block.opcode == 'procedures_defnoreturn' ||
-            block.opcode == 'procedures_defreturn') &&
-            block.mutation.proccode == name) {
+        if ((block.opcode === 'procedures_defnoreturn' ||
+            block.opcode === 'procedures_defreturn') &&
+            block.mutation.proccode === name) {
             return id;
         }
     }
@@ -166,9 +166,9 @@ Blocks.prototype.getProcedureDefinition = function (name) {
 Blocks.prototype.getProcedureParamNames = function (name) {
     for (var id in this._blocks) {
         var block = this._blocks[id];
-        if ((block.opcode == 'procedures_defnoreturn' ||
-            block.opcode == 'procedures_defreturn') &&
-            block.mutation.proccode == name) {
+        if ((block.opcode === 'procedures_defnoreturn' ||
+            block.opcode === 'procedures_defreturn') &&
+            block.mutation.proccode === name) {
             return JSON.parse(block.mutation.argumentnames);
         }
     }
@@ -181,18 +181,18 @@ Blocks.prototype.getProcedureParamNames = function (name) {
  * Create event listener for blocks. Handles validation and serves as a generic
  * adapter between the blocks and the runtime interface.
  * @param {Object} e Blockly "block" event
- * @param {?Runtime} opt_runtime Optional runtime to forward click events to.
+ * @param {?Runtime} optRuntime Optional runtime to forward click events to.
  */
 
-Blocks.prototype.blocklyListen = function (e, opt_runtime) {
+Blocks.prototype.blocklyListen = function (e, optRuntime) {
     // Validate event
     if (typeof e !== 'object') return;
     if (typeof e.blockId !== 'string') return;
 
     // UI event: clicked scripts toggle in the runtime.
     if (e.element === 'stackclick') {
-        if (opt_runtime) {
-            opt_runtime.toggleScript(e.blockId);
+        if (optRuntime) {
+            optRuntime.toggleScript(e.blockId);
         }
         return;
     }
@@ -232,8 +232,8 @@ Blocks.prototype.blocklyListen = function (e, opt_runtime) {
             return;
         }
         // Inform any runtime to forget about glows on this script.
-        if (opt_runtime && this._blocks[e.blockId].topLevel) {
-            opt_runtime.quietGlow(e.blockId);
+        if (optRuntime && this._blocks[e.blockId].topLevel) {
+            optRuntime.quietGlow(e.blockId);
         }
         this.deleteBlock({
             id: e.blockId
@@ -273,11 +273,11 @@ Blocks.prototype.changeBlock = function (args) {
     if (args.element !== 'field' && args.element !== 'mutation') return;
     if (typeof this._blocks[args.id] === 'undefined') return;
 
-    if (args.element == 'field') {
+    if (args.element === 'field') {
         // Update block value
         if (!this._blocks[args.id].fields[args.name]) return;
         this._blocks[args.id].fields[args.name].value = args.value;
-    } else if (args.element == 'mutation') {
+    } else if (args.element === 'mutation') {
         this._blocks[args.id].mutation = mutationAdapter(args.value);
     }
 };
@@ -298,9 +298,9 @@ Blocks.prototype.moveBlock = function (e) {
     }
 
     // Remove from any old parent.
-    if (e.oldParent !== undefined) {
+    if (typeof e.oldParent !== 'undefined') {
         var oldParent = this._blocks[e.oldParent];
-        if (e.oldInput !== undefined &&
+        if (typeof e.oldInput !== 'undefined' &&
             oldParent.inputs[e.oldInput].block === e.id) {
             // This block was connected to the old parent's input.
             oldParent.inputs[e.oldInput].block = null;
@@ -312,13 +312,16 @@ Blocks.prototype.moveBlock = function (e) {
     }
 
     // Has the block become a top-level block?
-    if (e.newParent === undefined) {
+    if (typeof e.newParent === 'undefined') {
         this._addScript(e.id);
     } else {
         // Remove script, if one exists.
         this._deleteScript(e.id);
         // Otherwise, try to connect it in its new place.
-        if (e.newInput !== undefined) {
+        if (typeof e.newInput === 'undefined') {
+            // Moved to the new parent's next connection.
+            this._blocks[e.newParent].next = e.id;
+        } else {
             // Moved to the new parent's input.
             // Don't obscure the shadow block.
             var oldShadow = null;
@@ -330,9 +333,6 @@ Blocks.prototype.moveBlock = function (e) {
                 block: e.id,
                 shadow: oldShadow
             };
-        } else {
-            // Moved to the new parent's next connection.
-            this._blocks[e.newParent].next = e.id;
         }
         this._blocks[e.id].parent = e.newParent;
     }
@@ -399,7 +399,7 @@ Blocks.prototype.blockToXML = function (blockId) {
     // Encode properties of this block.
     var tagName = (block.shadow) ? 'shadow' : 'block';
     var xy = (block.topLevel) ?
-        ' x="' + block.x +'"' + ' y="' + block.y +'"' :
+        ' x="' + block.x +'" y="' + block.y +'"' :
         '';
     var xmlString = '';
     xmlString += '<' + tagName +
@@ -420,7 +420,7 @@ Blocks.prototype.blockToXML = function (blockId) {
             if (blockInput.block) {
                 xmlString += this.blockToXML(blockInput.block);
             }
-            if (blockInput.shadow && blockInput.shadow != blockInput.block) {
+            if (blockInput.shadow && blockInput.shadow !== blockInput.block) {
                 // Obscured shadow.
                 xmlString += this.blockToXML(blockInput.shadow);
             }
@@ -453,7 +453,7 @@ Blocks.prototype.blockToXML = function (blockId) {
 Blocks.prototype.mutationToXML = function (mutation) {
     var mutationString = '<' + mutation.tagName;
     for (var prop in mutation) {
-        if (prop == 'children' || prop == 'tagName') continue;
+        if (prop === 'children' || prop === 'tagName') continue;
         var mutationValue = (typeof mutation[prop] === 'string') ?
             xmlEscape(mutation[prop]) : mutation[prop];
         mutationString += ' ' + prop + '="' + mutationValue + '"';
diff --git a/src/engine/execute.js b/src/engine/execute.js
index 70ee2e2eb..9017ff240 100644
--- a/src/engine/execute.js
+++ b/src/engine/execute.js
@@ -76,12 +76,10 @@ var execute = function (sequencer, thread) {
                 if (!edgeWasActivated) {
                     sequencer.retireThread(thread);
                 }
-            } else {
+            } else if (!resolvedValue) {
                 // Not an edge-activated hat: retire the thread
                 // if predicate was false.
-                if (!resolvedValue) {
-                    sequencer.retireThread(thread);
-                }
+                sequencer.retireThread(thread);
             }
         } else {
             // In a non-hat, report the value visually if necessary if
@@ -105,8 +103,8 @@ var execute = function (sequencer, thread) {
             // Skip through the block (hat with no predicate).
             return;
         } else {
-            if (Object.keys(fields).length == 1 &&
-                Object.keys(inputs).length == 0) {
+            if (Object.keys(fields).length === 1 &&
+                Object.keys(inputs).length === 0) {
                 // One field and no inputs - treat as arg.
                 for (var fieldKey in fields) { // One iteration.
                     handleReport(fields[fieldKey].value);
@@ -133,8 +131,8 @@ var execute = function (sequencer, thread) {
         var input = inputs[inputName];
         var inputBlockId = input.block;
         // Is there no value for this input waiting in the stack frame?
-        if (typeof currentStackFrame.reported[inputName] === 'undefined'
-            && inputBlockId) {
+        if (typeof currentStackFrame.reported[inputName] === 'undefined' &&
+            inputBlockId) {
             // If there's not, we need to evaluate the block.
             // Push to the stack to evaluate the reporter block.
             thread.pushStack(inputBlockId);
@@ -197,16 +195,16 @@ var execute = function (sequencer, thread) {
         getParam: function (paramName) {
             return thread.getParam(paramName);
         },
-        startHats: function (requestedHat, opt_matchFields, opt_target) {
+        startHats: function (requestedHat, optMatchFields, optTarget) {
             return (
-                runtime.startHats(requestedHat, opt_matchFields, opt_target)
+                runtime.startHats(requestedHat, optMatchFields, optTarget)
             );
         },
         ioQuery: function (device, func, args) {
             // Find the I/O device and execute the query/function call.
             if (runtime.ioDevices[device] && runtime.ioDevices[device][func]) {
                 var devObject = runtime.ioDevices[device];
-                return devObject[func].call(devObject, args);
+                return devObject[func](args);
             }
         }
     });
@@ -226,12 +224,12 @@ var execute = function (sequencer, thread) {
         // Promise handlers
         primitiveReportedValue.then(function (resolvedValue) {
             handleReport(resolvedValue);
-            if (typeof resolvedValue !== 'undefined') {
-                thread.popStack();
-            } else {
+            if (typeof resolvedValue === 'undefined') {
                 var popped = thread.popStack();
                 var nextBlockId = thread.target.blocks.getNextBlock(popped);
                 thread.pushStack(nextBlockId);
+            } else {
+                thread.popStack();
             }
         }, function (rejectionReason) {
             // Promise rejected: the primitive had some error.
diff --git a/src/engine/list.js b/src/engine/list.js
index 8ef082cde..de04fbac5 100644
--- a/src/engine/list.js
+++ b/src/engine/list.js
@@ -8,9 +8,7 @@
   * @param {Array} contents Contents of the list, as an array.
   * @constructor
   */
-function List (name, contents) {
+module.exports = function List (name, contents) {
     this.name = name;
     this.contents = contents;
-}
-
-module.exports = List;
+};
diff --git a/src/engine/mutation-adapter.js b/src/engine/mutation-adapter.js
index 12dc123e1..b18dbb8a7 100644
--- a/src/engine/mutation-adapter.js
+++ b/src/engine/mutation-adapter.js
@@ -1,5 +1,26 @@
 var html = require('htmlparser2');
 
+/**
+ * Convert a part of a mutation DOM to a mutation VM object, recursively.
+ * @param {Object} dom DOM object for mutation tag.
+ * @return {Object} Object representing useful parts of this mutation.
+ */
+var mutatorTagToObject = function (dom) {
+    var obj = Object.create(null);
+    obj.tagName = dom.name;
+    obj.children = [];
+    for (var prop in dom.attribs) {
+        if (prop === 'xmlns') continue;
+        obj[prop] = dom.attribs[prop];
+    }
+    for (var i = 0; i < dom.children.length; i++) {
+        obj.children.push(
+            mutatorTagToObject(dom.children[i])
+        );
+    }
+    return obj;
+};
+
 /**
  * Adapter between mutator XML or DOM and block representation which can be
  * used by the Scratch runtime.
@@ -16,24 +37,3 @@ module.exports = function (mutation) {
     }
     return mutatorTagToObject(mutationParsed);
 };
-
-/**
- * Convert a part of a mutation DOM to a mutation VM object, recursively.
- * @param {Object} dom DOM object for mutation tag.
- * @return {Object} Object representing useful parts of this mutation.
- */
-function mutatorTagToObject (dom) {
-    var obj = Object.create(null);
-    obj.tagName = dom.name;
-    obj.children = [];
-    for (var prop in dom.attribs) {
-        if (prop == 'xmlns') continue;
-        obj[prop] = dom.attribs[prop];
-    }
-    for (var i = 0; i < dom.children.length; i++) {
-        obj.children.push(
-            mutatorTagToObject(dom.children[i])
-        );
-    }
-    return obj;
-}
diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 717283d76..7a2363182 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -23,7 +23,7 @@ var defaultBlockPackages = {
 /**
  * Manages targets, scripts, and the sequencer.
  */
-function Runtime () {
+var Runtime = function () {
     // Bind event emitter
     EventEmitter.call(this);
 
@@ -143,7 +143,7 @@ function Runtime () {
         'keyboard': new Keyboard(this),
         'mouse': new Mouse(this)
     };
-}
+};
 
 /**
  * Inherit from EventEmitter
@@ -346,7 +346,7 @@ Runtime.prototype.isActiveThread = function (thread) {
 Runtime.prototype.toggleScript = function (topBlockId) {
     // Remove any existing thread.
     for (var i = 0; i < this.threads.length; i++) {
-        if (this.threads[i].topBlock == topBlockId) {
+        if (this.threads[i].topBlock === topBlockId) {
             this._removeThread(this.threads[i]);
             return;
         }
@@ -361,12 +361,12 @@ Runtime.prototype.toggleScript = function (topBlockId) {
  *  - the top block ID of the script.
  *  - the target that owns the script.
  * @param {!Function} f Function to call for each script.
- * @param {Target=} opt_target Optionally, a target to restrict to.
+ * @param {Target=} optTarget Optionally, a target to restrict to.
  */
-Runtime.prototype.allScriptsDo = function (f, opt_target) {
+Runtime.prototype.allScriptsDo = function (f, optTarget) {
     var targets = this.targets;
-    if (opt_target) {
-        targets = [opt_target];
+    if (optTarget) {
+        targets = [optTarget];
     }
     for (var t = 0; t < targets.length; t++) {
         var target = targets[t];
@@ -381,12 +381,12 @@ Runtime.prototype.allScriptsDo = function (f, opt_target) {
 /**
  * Start all relevant hats.
  * @param {!string} requestedHatOpcode Opcode of hats to start.
- * @param {Object=} opt_matchFields Optionally, fields to match on the hat.
- * @param {Target=} opt_target Optionally, a target to restrict to.
+ * @param {Object=} optMatchFields Optionally, fields to match on the hat.
+ * @param {Target=} optTarget Optionally, a target to restrict to.
  * @return {Array.<Thread>} List of threads started by this function.
  */
 Runtime.prototype.startHats = function (requestedHatOpcode,
-    opt_matchFields, opt_target) {
+    optMatchFields, optTarget) {
     if (!this._hats.hasOwnProperty(requestedHatOpcode)) {
         // No known hat with this opcode.
         return;
@@ -406,10 +406,10 @@ Runtime.prototype.startHats = function (requestedHatOpcode,
         // (i.e., before the predicate can be run) because "broadcast and wait"
         // needs to have a precise collection of started threads.
         var hatFields = target.blocks.getFields(topBlockId);
-        if (opt_matchFields) {
-            for (var matchField in opt_matchFields) {
+        if (optMatchFields) {
+            for (var matchField in optMatchFields) {
                 if (hatFields[matchField].value !==
-                    opt_matchFields[matchField]) {
+                    optMatchFields[matchField]) {
                     // Field mismatch.
                     return;
                 }
@@ -422,7 +422,7 @@ Runtime.prototype.startHats = function (requestedHatOpcode,
             // any existing threads starting with the top block.
             for (var i = 0; i < instance.threads.length; i++) {
                 if (instance.threads[i].topBlock === topBlockId &&
-                    instance.threads[i].target == target) {
+                    instance.threads[i].target === target) {
                     instance._removeThread(instance.threads[i]);
                 }
             }
@@ -431,7 +431,7 @@ Runtime.prototype.startHats = function (requestedHatOpcode,
             // give up if any threads with the top block are running.
             for (var j = 0; j < instance.threads.length; j++) {
                 if (instance.threads[j].topBlock === topBlockId &&
-                    instance.threads[j].target == target) {
+                    instance.threads[j].target === target) {
                     // Some thread is already running.
                     return;
                 }
@@ -439,7 +439,7 @@ Runtime.prototype.startHats = function (requestedHatOpcode,
         }
         // Start the thread with this top block.
         newThreads.push(instance._pushThread(topBlockId, target));
-    }, opt_target);
+    }, optTarget);
     return newThreads;
 };
 
@@ -468,15 +468,15 @@ Runtime.prototype.disposeTarget = function (disposingTarget) {
 /**
  * Stop any threads acting on the target.
  * @param {!Target} target Target to stop threads for.
- * @param {Thread=} opt_threadException Optional thread to skip.
+ * @param {Thread=} optThreadException Optional thread to skip.
  */
-Runtime.prototype.stopForTarget = function (target, opt_threadException) {
+Runtime.prototype.stopForTarget = function (target, optThreadException) {
     // Stop any threads on the target.
     for (var i = 0; i < this.threads.length; i++) {
-        if (this.threads[i] === opt_threadException) {
+        if (this.threads[i] === optThreadException) {
             continue;
         }
-        if (this.threads[i].target == target) {
+        if (this.threads[i].target === target) {
             this._removeThread(this.threads[i]);
         }
     }
@@ -562,13 +562,13 @@ Runtime.prototype.setCompatibilityMode = function (compatibilityModeOn) {
 /**
  * Emit glows/glow clears for scripts after a single tick.
  * Looks at `this.threads` and notices which have turned on/off new glows.
- * @param {Array.<Thread>=} opt_extraThreads Optional list of inactive threads.
+ * @param {Array.<Thread>=} optExtraThreads Optional list of inactive threads.
  */
-Runtime.prototype._updateGlows = function (opt_extraThreads) {
+Runtime.prototype._updateGlows = function (optExtraThreads) {
     var searchThreads = [];
     searchThreads.push.apply(searchThreads, this.threads);
-    if (opt_extraThreads) {
-        searchThreads.push.apply(searchThreads, opt_extraThreads);
+    if (optExtraThreads) {
+        searchThreads.push.apply(searchThreads, optExtraThreads);
     }
     // Set of scripts that request a glow this frame.
     var requestedGlowsThisFrame = [];
@@ -578,7 +578,7 @@ Runtime.prototype._updateGlows = function (opt_extraThreads) {
     for (var i = 0; i < searchThreads.length; i++) {
         var thread = searchThreads[i];
         var target = thread.target;
-        if (target == this._editingTarget) {
+        if (target === this._editingTarget) {
             var blockForThread = thread.blockGlowInFrame;
             if (thread.requestScriptGlowInFrame) {
                 var script = target.blocks.getTopLevelScript(blockForThread);
@@ -672,7 +672,7 @@ Runtime.prototype.visualReport = function (blockId, value) {
 Runtime.prototype.getTargetById = function (targetId) {
     for (var i = 0; i < this.targets.length; i++) {
         var target = this.targets[i];
-        if (target.id == targetId) {
+        if (target.id === targetId) {
             return target;
         }
     }
@@ -686,7 +686,7 @@ Runtime.prototype.getTargetById = function (targetId) {
 Runtime.prototype.getSpriteTargetByName = function (spriteName) {
     for (var i = 0; i < this.targets.length; i++) {
         var target = this.targets[i];
-        if (target.sprite && target.sprite.name == spriteName) {
+        if (target.sprite && target.sprite.name === spriteName) {
             return target;
         }
     }
diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index dbfa1f5f4..46cc3ebcd 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -2,7 +2,7 @@ var Timer = require('../util/timer');
 var Thread = require('./thread');
 var execute = require('./execute.js');
 
-function Sequencer (runtime) {
+var Sequencer = function (runtime) {
     /**
      * A utility timer for timing thread sequencing.
      * @type {!Timer}
@@ -14,7 +14,7 @@ function Sequencer (runtime) {
      * @type {!Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Time to run a warp-mode thread, in ms.
@@ -219,11 +219,9 @@ Sequencer.prototype.stepToProcedure = function (thread, procedureCode) {
         var doWarp = definitionBlock.mutation.warp;
         if (doWarp) {
             thread.peekStackFrame().warpMode = true;
-        } else {
+        } else if (isRecursive) {
             // In normal-mode threads, yield any time we have a recursive call.
-            if (isRecursive) {
-                thread.status = Thread.STATUS_YIELD;
-            }
+            thread.status = Thread.STATUS_YIELD;
         }
     }
 };
diff --git a/src/engine/target.js b/src/engine/target.js
index d5a59b47a..f8f99f47d 100644
--- a/src/engine/target.js
+++ b/src/engine/target.js
@@ -13,7 +13,7 @@ var uid = require('../util/uid');
  * @param {?Blocks} blocks Blocks instance for the blocks owned by this target.
  * @constructor
  */
-function Target (blocks) {
+var Target = function (blocks) {
     if (!blocks) {
         blocks = new Blocks(this);
     }
@@ -39,7 +39,7 @@ function Target (blocks) {
      * @type {Object.<string,*>}
      */
     this.lists = {};
-}
+};
 
 /**
  * Called when the project receives a "green flag."
@@ -109,8 +109,6 @@ Target.prototype.lookupOrCreateList = function (name) {
  * Call to destroy a target.
  * @abstract
  */
-Target.prototype.dispose = function () {
-
-};
+Target.prototype.dispose = function () {};
 
 module.exports = Target;
diff --git a/src/engine/thread.js b/src/engine/thread.js
index 5ad6e72bb..765f48b71 100644
--- a/src/engine/thread.js
+++ b/src/engine/thread.js
@@ -3,7 +3,7 @@
  * @param {?string} firstBlock First block to execute in the thread.
  * @constructor
  */
-function Thread (firstBlock) {
+var Thread = function (firstBlock) {
     /**
      * ID of top block of the thread
      * @type {!string}
@@ -53,7 +53,7 @@ function Thread (firstBlock) {
      * @type {?Timer}
      */
     this.warpTimer = null;
-}
+};
 
 /**
  * Thread status for initialized or running thread.
@@ -225,8 +225,8 @@ Thread.prototype.isRecursiveCall = function (procedureCode) {
     var sp = this.stack.length - 1;
     for (var i = sp - 1; i >= 0; i--) {
         var block = this.target.blocks.getBlock(this.stack[i]);
-        if (block.opcode == 'procedures_callnoreturn' &&
-            block.mutation.proccode == procedureCode) {
+        if (block.opcode === 'procedures_callnoreturn' &&
+            block.mutation.proccode === procedureCode) {
             return true;
         }
         if (--callCount < 0) return false;
diff --git a/src/engine/variable.js b/src/engine/variable.js
index 4e5e5e6e3..e28170d4d 100644
--- a/src/engine/variable.js
+++ b/src/engine/variable.js
@@ -9,10 +9,8 @@
  * @param {boolean} isCloud Whether the variable is stored in the cloud.
  * @constructor
  */
-function Variable (name, value, isCloud) {
+module.exports = function Variable (name, value, isCloud) {
     this.name = name;
     this.value = value;
     this.isCloud = isCloud;
-}
-
-module.exports = Variable;
+};
diff --git a/src/import/sb2import.js b/src/import/sb2import.js
index 927fb650d..a2526c3d0 100644
--- a/src/import/sb2import.js
+++ b/src/import/sb2import.js
@@ -14,22 +14,6 @@ var specMap = require('./sb2specmap');
 var Variable = require('../engine/variable');
 var List = require('../engine/list');
 
-/**
- * Top-level handler. Parse provided JSON,
- * and process the top-level object (the stage object).
- * @param {!string} json SB2-format JSON to load.
- * @param {!Runtime} runtime Runtime object to load all structures into.
- * @param {Boolean=} opt_forceSprite If set, treat as sprite (Sprite2).
- * @return {?Target} Top-level target created (stage or sprite).
- */
-function sb2import (json, runtime, opt_forceSprite) {
-    return parseScratchObject(
-        JSON.parse(json),
-        runtime,
-        !opt_forceSprite
-    );
-}
-
 /**
  * Parse a single "Scratch object" and create all its in-memory VM objects.
  * @param {!Object} object From-JSON "Scratch object:" sprite, stage, watcher.
@@ -37,7 +21,7 @@ function sb2import (json, runtime, opt_forceSprite) {
  * @param {boolean} topLevel Whether this is the top-level object (stage).
  * @return {?Target} Target created (stage or sprite).
  */
-function parseScratchObject (object, runtime, topLevel) {
+var parseScratchObject = function (object, runtime, topLevel) {
     if (!object.hasOwnProperty('objName')) {
         // Watcher/monitor - skip this object until those are implemented in VM.
         // @todo
@@ -57,8 +41,8 @@ function parseScratchObject (object, runtime, topLevel) {
             var costume = object.costumes[i];
             // @todo: Make sure all the relevant metadata is being pulled out.
             sprite.costumes.push({
-                skin: 'https://cdn.assets.scratch.mit.edu/internalapi/asset/'
-                    + costume.baseLayerMD5 + '/get/',
+                skin: 'https://cdn.assets.scratch.mit.edu/internalapi/asset/' +
+                    costume.baseLayerMD5 + '/get/',
                 name: costume.costumeName,
                 bitmapResolution: costume.bitmapResolution,
                 rotationCenterX: costume.rotationCenterX,
@@ -115,11 +99,11 @@ function parseScratchObject (object, runtime, topLevel) {
         target.currentCostume = Math.round(object.currentCostumeIndex);
     }
     if (object.hasOwnProperty('rotationStyle')) {
-        if (object.rotationStyle == 'none') {
+        if (object.rotationStyle === 'none') {
             target.rotationStyle = Clone.ROTATION_STYLE_NONE;
-        } else if (object.rotationStyle == 'leftRight') {
+        } else if (object.rotationStyle === 'leftRight') {
             target.rotationStyle = Clone.ROTATION_STYLE_LEFT_RIGHT;
-        } else if (object.rotationStyle == 'normal') {
+        } else if (object.rotationStyle === 'normal') {
             target.rotationStyle = Clone.ROTATION_STYLE_ALL_AROUND;
         }
     }
@@ -132,7 +116,23 @@ function parseScratchObject (object, runtime, topLevel) {
         }
     }
     return target;
-}
+};
+
+/**
+ * Top-level handler. Parse provided JSON,
+ * and process the top-level object (the stage object).
+ * @param {!string} json SB2-format JSON to load.
+ * @param {!Runtime} runtime Runtime object to load all structures into.
+ * @param {Boolean=} optForceSprite If set, treat as sprite (Sprite2).
+ * @return {?Target} Top-level target created (stage or sprite).
+ */
+var sb2import = function (json, runtime, optForceSprite) {
+    return parseScratchObject(
+        JSON.parse(json),
+        runtime,
+        !optForceSprite
+    );
+};
 
 /**
  * Parse a Scratch object's scripts into VM blocks.
@@ -140,7 +140,7 @@ function parseScratchObject (object, runtime, topLevel) {
  * @param {!Object} scripts Scripts object from SB2 JSON.
  * @param {!Blocks} blocks Blocks object to load parsed blocks into.
  */
-function parseScripts (scripts, blocks) {
+var parseScripts = function (scripts, blocks) {
     for (var i = 0; i < scripts.length; i++) {
         var script = scripts[i];
         var scriptX = script[0];
@@ -162,7 +162,7 @@ function parseScripts (scripts, blocks) {
             blocks.createBlock(convertedBlocks[j]);
         }
     }
-}
+};
 
 /**
  * Parse any list of blocks from SB2 JSON into a list of VM-format blocks.
@@ -172,7 +172,7 @@ function parseScripts (scripts, blocks) {
  * @param {Array.<Object>} blockList SB2 JSON-format block list.
  * @return {Array.<Object>} Scratch VM-format block list.
  */
-function parseBlockList (blockList) {
+var parseBlockList = function (blockList) {
     var resultingList = [];
     var previousBlock = null; // For setting next.
     for (var i = 0; i < blockList.length; i++) {
@@ -186,7 +186,7 @@ function parseBlockList (blockList) {
         resultingList.push(parsedBlock);
     }
     return resultingList;
-}
+};
 
 /**
  * Flatten a block tree into a block list.
@@ -194,7 +194,7 @@ function parseBlockList (blockList) {
  * @param {Array.<Object>} blocks list generated by `parseBlockList`.
  * @return {Array.<Object>} Flattened list to be passed to `blocks.createBlock`.
  */
-function flatten (blocks) {
+var flatten = function (blocks) {
     var finalBlocks = [];
     for (var i = 0; i < blocks.length; i++) {
         var block = blocks[i];
@@ -205,7 +205,7 @@ function flatten (blocks) {
         delete block.children;
     }
     return finalBlocks;
-}
+};
 
 /**
  * Convert a Scratch 2.0 procedure string (e.g., "my_procedure %s %b %n")
@@ -214,39 +214,39 @@ function flatten (blocks) {
  * @param {string} procCode Scratch 2.0 procedure string.
  * @return {Object} Argument map compatible with those in sb2specmap.
  */
-function parseProcedureArgMap (procCode) {
+var parseProcedureArgMap = function (procCode) {
     var argMap = [
         {} // First item in list is op string.
     ];
     var INPUT_PREFIX = 'input';
     var inputCount = 0;
     // Split by %n, %b, %s.
-    var parts = procCode.split(/(?=[^\\]\%[nbs])/);
+    var parts = procCode.split(/(?=[^\\]%[nbs])/);
     for (var i = 0; i < parts.length; i++) {
         var part = parts[i].trim();
-        if (part.substring(0, 1) == '%') {
+        if (part.substring(0, 1) === '%') {
             var argType = part.substring(1, 2);
             var arg = {
                 type: 'input',
                 inputName: INPUT_PREFIX + (inputCount++)
             };
-            if (argType == 'n') {
+            if (argType === 'n') {
                 arg.inputOp = 'math_number';
-            } else if (argType == 's') {
+            } else if (argType === 's') {
                 arg.inputOp = 'text';
             }
             argMap.push(arg);
         }
     }
     return argMap;
-}
+};
 
 /**
  * Parse a single SB2 JSON-formatted block and its children.
  * @param {!Object} sb2block SB2 JSON-formatted block.
  * @return {Object} Scratch VM format block.
  */
-function parseBlock (sb2block) {
+var parseBlock = function (sb2block) {
     // First item in block object is the old opcode (e.g., 'forward:').
     var oldOpcode = sb2block[0];
     // Convert the block using the specMap. See sb2specmap.js.
@@ -266,7 +266,7 @@ function parseBlock (sb2block) {
         children: [] // Store any generated children, flattened in `flatten`.
     };
     // For a procedure call, generate argument map from proc string.
-    if (oldOpcode == 'call') {
+    if (oldOpcode === 'call') {
         blockMetadata.argMap = parseProcedureArgMap(sb2block[1]);
     }
     // Look at the expected arguments in `blockMetadata.argMap.`
@@ -278,7 +278,7 @@ function parseBlock (sb2block) {
         // Whether the input is obscuring a shadow.
         var shadowObscured = false;
         // Positional argument is an input.
-        if (expectedArg.type == 'input') {
+        if (expectedArg.type === 'input') {
             // Create a new block and input metadata.
             var inputUid = uid();
             activeBlock.inputs[expectedArg.inputName] = {
@@ -286,10 +286,10 @@ function parseBlock (sb2block) {
                 block: null,
                 shadow: null
             };
-            if (typeof providedArg == 'object' && providedArg) {
+            if (typeof providedArg === 'object' && providedArg) {
                 // Block or block list occupies the input.
                 var innerBlocks;
-                if (typeof providedArg[0] == 'object' && providedArg[0]) {
+                if (typeof providedArg[0] === 'object' && providedArg[0]) {
                     // Block list occupies the input.
                     innerBlocks = parseBlockList(providedArg);
                 } else {
@@ -298,7 +298,7 @@ function parseBlock (sb2block) {
                 }
                 var previousBlock = null;
                 for (var j = 0; j < innerBlocks.length; j++) {
-                    if (j == 0) {
+                    if (j === 0) {
                         innerBlocks[j].parent = activeBlock.id;
                     } else {
                         innerBlocks[j].parent = previousBlock;
@@ -324,22 +324,22 @@ function parseBlock (sb2block) {
             var fieldValue = providedArg;
             // Shadows' field names match the input name, except for these:
             var fieldName = expectedArg.inputName;
-            if (expectedArg.inputOp == 'math_number' ||
-                expectedArg.inputOp == 'math_whole_number' ||
-                expectedArg.inputOp == 'math_positive_number' ||
-                expectedArg.inputOp == 'math_integer' ||
-                expectedArg.inputOp == 'math_angle') {
+            if (expectedArg.inputOp === 'math_number' ||
+                expectedArg.inputOp === 'math_whole_number' ||
+                expectedArg.inputOp === 'math_positive_number' ||
+                expectedArg.inputOp === 'math_integer' ||
+                expectedArg.inputOp === 'math_angle') {
                 fieldName = 'NUM';
                 // Fields are given Scratch 2.0 default values if obscured.
                 if (shadowObscured) {
                     fieldValue = 10;
                 }
-            } else if (expectedArg.inputOp == 'text') {
+            } else if (expectedArg.inputOp === 'text') {
                 fieldName = 'TEXT';
                 if (shadowObscured) {
                     fieldValue = '';
                 }
-            } else if (expectedArg.inputOp == 'colour_picker') {
+            } else if (expectedArg.inputOp === 'colour_picker') {
                 // Convert SB2 color to hex.
                 fieldValue = Color.decimalToHex(providedArg);
                 fieldName = 'COLOUR';
@@ -367,7 +367,7 @@ function parseBlock (sb2block) {
             if (!activeBlock.inputs[expectedArg.inputName].block) {
                 activeBlock.inputs[expectedArg.inputName].block = inputUid;
             }
-        } else if (expectedArg.type == 'field') {
+        } else if (expectedArg.type === 'field') {
             // Add as a field on this block.
             activeBlock.fields[expectedArg.fieldName] = {
                 name: expectedArg.fieldName,
@@ -376,18 +376,18 @@ function parseBlock (sb2block) {
         }
     }
     // Special cases to generate mutations.
-    if (oldOpcode == 'stopScripts') {
+    if (oldOpcode === 'stopScripts') {
         // Mutation for stop block: if the argument is 'other scripts',
         // the block needs a next connection.
-        if (sb2block[1] == 'other scripts in sprite' ||
-            sb2block[1] == 'other scripts in stage') {
+        if (sb2block[1] === 'other scripts in sprite' ||
+            sb2block[1] === 'other scripts in stage') {
             activeBlock.mutation = {
                 tagName: 'mutation',
                 hasnext: 'true',
                 children: []
             };
         }
-    } else if (oldOpcode == 'procDef') {
+    } else if (oldOpcode === 'procDef') {
         // Mutation for procedure definition:
         // store all 2.0 proc data.
         var procData = sb2block.slice(1);
@@ -399,7 +399,7 @@ function parseBlock (sb2block) {
             warp: procData[3], // Warp mode, e.g., true/false.
             children: []
         };
-    } else if (oldOpcode == 'call') {
+    } else if (oldOpcode === 'call') {
         // Mutation for procedure call:
         // string for proc code (e.g., "abc %n %b %s").
         activeBlock.mutation = {
@@ -407,7 +407,7 @@ function parseBlock (sb2block) {
             children: [],
             proccode: sb2block[1]
         };
-    } else if (oldOpcode == 'getParam') {
+    } else if (oldOpcode === 'getParam') {
         // Mutation for procedure parameter.
         activeBlock.mutation = {
             tagName: 'mutation',
@@ -417,6 +417,6 @@ function parseBlock (sb2block) {
         };
     }
     return activeBlock;
-}
+};
 
 module.exports = sb2import;
diff --git a/src/index.js b/src/index.js
index 06add3b41..786bc383e 100644
--- a/src/index.js
+++ b/src/index.js
@@ -11,7 +11,7 @@ var Blocks = require('./engine/blocks');
  *
  * @author Andrew Sliwinski <ascii@media.mit.edu>
  */
-function VirtualMachine () {
+var VirtualMachine = function () {
     var instance = this;
     // Bind event emitter and runtime to VM instance
     EventEmitter.call(instance);
@@ -45,7 +45,7 @@ function VirtualMachine () {
 
     this.blockListener = this.blockListener.bind(this);
     this.flyoutBlockListener = this.flyoutBlockListener.bind(this);
-}
+};
 
 /**
  * Inherit from EventEmitter
@@ -107,11 +107,11 @@ VirtualMachine.prototype.getPlaygroundData = function () {
     var instance = this;
     // Only send back thread data for the current editingTarget.
     var threadData = this.runtime.threads.filter(function (thread) {
-        return thread.target == instance.editingTarget;
+        return thread.target === instance.editingTarget;
     });
     // Remove the target key, since it's a circular reference.
     var filteredThreadData = JSON.stringify(threadData, function (key, value) {
-        if (key == 'target') return undefined;
+        if (key === 'target') return;
         return value;
     }, 2);
     this.emit('playgroundData', {
@@ -273,7 +273,7 @@ VirtualMachine.prototype.flyoutBlockListener = function (e) {
  */
 VirtualMachine.prototype.setEditingTarget = function (targetId) {
     // Has the target id changed? If not, exit.
-    if (targetId == this.editingTarget.id) {
+    if (targetId === this.editingTarget.id) {
         return;
     }
     var target = this.runtime.getTargetById(targetId);
diff --git a/src/io/clock.js b/src/io/clock.js
index ce4c8f553..aaefa9d5a 100644
--- a/src/io/clock.js
+++ b/src/io/clock.js
@@ -1,6 +1,6 @@
 var Timer = require('../util/timer');
 
-function Clock (runtime) {
+var Clock = function (runtime) {
     this._projectTimer = new Timer();
     this._projectTimer.start();
     this._pausedTime = null;
@@ -10,7 +10,7 @@ function Clock (runtime) {
      * @type{!Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 Clock.prototype.projectTimer = function () {
     if (this._paused) {
diff --git a/src/io/keyboard.js b/src/io/keyboard.js
index ea423fd40..290d24348 100644
--- a/src/io/keyboard.js
+++ b/src/io/keyboard.js
@@ -1,6 +1,6 @@
 var Cast = require('../util/cast');
 
-function Keyboard (runtime) {
+var Keyboard = function (runtime) {
     /**
      * List of currently pressed keys.
      * @type{Array.<number>}
@@ -12,7 +12,7 @@ function Keyboard (runtime) {
      * @type{!Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 /**
  * Convert a Scratch key name to a DOM keyCode.
@@ -20,7 +20,7 @@ function Keyboard (runtime) {
  * @return {number} Key code corresponding to a DOM event.
  */
 Keyboard.prototype._scratchKeyToKeyCode = function (keyName) {
-    if (typeof keyName == 'number') {
+    if (typeof keyName === 'number') {
         // Key codes placed in with number blocks.
         return keyName;
     }
@@ -75,7 +75,7 @@ Keyboard.prototype.postData = function (data) {
 };
 
 Keyboard.prototype.getKeyIsDown = function (key) {
-    if (key == 'any') {
+    if (key === 'any') {
         return this._keysPressed.length > 0;
     }
     var keyCode = this._scratchKeyToKeyCode(key);
diff --git a/src/io/mouse.js b/src/io/mouse.js
index 58df9df1f..c54c97511 100644
--- a/src/io/mouse.js
+++ b/src/io/mouse.js
@@ -1,6 +1,6 @@
 var MathUtil = require('../util/math-util');
 
-function Mouse (runtime) {
+var Mouse = function (runtime) {
     this._x = 0;
     this._y = 0;
     this._isDown = false;
@@ -10,14 +10,14 @@ function Mouse (runtime) {
      * @type{!Runtime}
      */
     this.runtime = runtime;
-}
+};
 
 Mouse.prototype.postData = function (data) {
     if (data.x) {
-        this._x = data.x - data.canvasWidth / 2;
+        this._x = data.x - (data.canvasWidth / 2);
     }
     if (data.y) {
-        this._y = data.y - data.canvasHeight / 2;
+        this._y = data.y - (data.canvasHeight / 2);
     }
     if (typeof data.isDown !== 'undefined') {
         this._isDown = data.isDown;
@@ -33,7 +33,7 @@ Mouse.prototype._activateClickHats = function (x, y) {
         for (var i = 0; i < this.runtime.targets.length; i++) {
             var target = this.runtime.targets[i];
             if (target.hasOwnProperty('drawableID') &&
-                target.drawableID == drawableID) {
+                target.drawableID === drawableID) {
                 this.runtime.startHats('event_whenthisspriteclicked',
                     null, target);
                 return;
diff --git a/src/sprites/clone.js b/src/sprites/clone.js
index 79ead30e4..c5e8e0ca6 100644
--- a/src/sprites/clone.js
+++ b/src/sprites/clone.js
@@ -8,7 +8,7 @@ var Target = require('../engine/target');
  * @param {Runtime} runtime Reference to the runtime.
  * @constructor
  */
-function Clone (sprite, runtime) {
+var Clone = function (sprite, runtime) {
     Target.call(this, sprite.blocks);
     this.runtime = runtime;
     /**
@@ -43,7 +43,7 @@ function Clone (sprite, runtime) {
         'brightness': 0,
         'ghost': 0
     };
-}
+};
 util.inherits(Clone, Target);
 
 /**
@@ -166,7 +166,7 @@ Clone.prototype._getRenderedDirectionAndScale = function () {
     // Default: no changes to `this.direction` or `this.scale`.
     var finalDirection = this.direction;
     var finalScale = [this.size, this.size];
-    if (this.rotationStyle == Clone.ROTATION_STYLE_NONE) {
+    if (this.rotationStyle === Clone.ROTATION_STYLE_NONE) {
         // Force rendered direction to be 90.
         finalDirection = 90;
     } else if (this.rotationStyle === Clone.ROTATION_STYLE_LEFT_RIGHT) {
@@ -319,11 +319,11 @@ Clone.prototype.setCostume = function (index) {
  * @param {!string} rotationStyle New rotation style.
  */
 Clone.prototype.setRotationStyle = function (rotationStyle) {
-    if (rotationStyle == Clone.ROTATION_STYLE_NONE) {
+    if (rotationStyle === Clone.ROTATION_STYLE_NONE) {
         this.rotationStyle = Clone.ROTATION_STYLE_NONE;
-    } else if (rotationStyle == Clone.ROTATION_STYLE_ALL_AROUND) {
+    } else if (rotationStyle === Clone.ROTATION_STYLE_ALL_AROUND) {
         this.rotationStyle = Clone.ROTATION_STYLE_ALL_AROUND;
-    } else if (rotationStyle == Clone.ROTATION_STYLE_LEFT_RIGHT) {
+    } else if (rotationStyle === Clone.ROTATION_STYLE_LEFT_RIGHT) {
         this.rotationStyle = Clone.ROTATION_STYLE_LEFT_RIGHT;
     }
     if (this.renderer) {
@@ -345,7 +345,7 @@ Clone.prototype.setRotationStyle = function (rotationStyle) {
  */
 Clone.prototype.getCostumeIndexByName = function (costumeName) {
     for (var i = 0; i < this.sprite.costumes.length; i++) {
-        if (this.sprite.costumes[i].name == costumeName) {
+        if (this.sprite.costumes[i].name === costumeName) {
             return i;
         }
     }
@@ -408,8 +408,8 @@ Clone.prototype.isTouchingPoint = function (x, y) {
         // Limits test to this Drawable, so this will return true
         // even if the clone is obscured by another Drawable.
         var pickResult = this.runtime.renderer.pick(
-            x + this.runtime.constructor.STAGE_WIDTH / 2,
-            -y + this.runtime.constructor.STAGE_HEIGHT / 2,
+            x + (this.runtime.constructor.STAGE_WIDTH / 2),
+            -y + (this.runtime.constructor.STAGE_HEIGHT / 2),
             null, null,
             [this.drawableID]
         );
@@ -506,11 +506,11 @@ Clone.prototype.goBackLayers = function (nLayers) {
  * Keep a desired position within a fence.
  * @param {number} newX New desired X position.
  * @param {number} newY New desired Y position.
- * @param {Object=} opt_fence Optional fence with left, right, top bottom.
+ * @param {Object=} optFence Optional fence with left, right, top bottom.
  * @return {Array.<number>} Fenced X and Y coordinates.
  */
-Clone.prototype.keepInFence = function (newX, newY, opt_fence) {
-    var fence = opt_fence;
+Clone.prototype.keepInFence = function (newX, newY, optFence) {
+    var fence = optFence;
     if (!fence) {
         fence = {
             left: -this.runtime.constructor.STAGE_WIDTH / 2,
diff --git a/src/sprites/sprite.js b/src/sprites/sprite.js
index 2afb25e5e..351d40a3f 100644
--- a/src/sprites/sprite.js
+++ b/src/sprites/sprite.js
@@ -8,7 +8,7 @@ var Blocks = require('../engine/blocks');
  * @param {Runtime} runtime Reference to the runtime.
  * @constructor
  */
-function Sprite (blocks, runtime) {
+var Sprite = function (blocks, runtime) {
     this.runtime = runtime;
     if (!blocks) {
         // Shared set of blocks for all clones.
@@ -38,7 +38,7 @@ function Sprite (blocks, runtime) {
      * @type {Array.<!Clone>}
      */
     this.clones = [];
-}
+};
 
 /**
  * Create a clone of this sprite.
@@ -46,7 +46,7 @@ function Sprite (blocks, runtime) {
  */
 Sprite.prototype.createClone = function () {
     var newClone = new Clone(this, this.runtime);
-    newClone.isOriginal = this.clones.length == 0;
+    newClone.isOriginal = this.clones.length === 0;
     this.clones.push(newClone);
     if (newClone.isOriginal) {
         newClone.initDrawable();
diff --git a/src/util/cast.js b/src/util/cast.js
index dda55bf8e..f14e97df5 100644
--- a/src/util/cast.js
+++ b/src/util/cast.js
@@ -1,6 +1,6 @@
 var Color = require('../util/color');
 
-function Cast () {}
+var Cast = function () {};
 
 /**
  * @fileoverview
@@ -44,9 +44,9 @@ Cast.toBoolean = function (value) {
     }
     if (typeof value === 'string') {
         // These specific strings are treated as false in Scratch.
-        if ((value == '') ||
-            (value == '0') ||
-            (value.toLowerCase() == 'false')) {
+        if ((value === '') ||
+            (value === '0') ||
+            (value.toLowerCase() === 'false')) {
             return false;
         }
         // All other strings treated as true.
@@ -72,7 +72,7 @@ Cast.toString = function (value) {
  */
 Cast.toRgbColorList = function (value) {
     var color;
-    if (typeof value == 'string' && value.substring(0, 1) == '#') {
+    if (typeof value === 'string' && value.substring(0, 1) === '#') {
         color = Color.hexToRgb(value);
     } else {
         color = Color.decimalToRgb(Cast.toNumber(value));
@@ -114,7 +114,7 @@ Cast.isInt = function (val) {
             return true;
         }
         // True if it's "round" (e.g., 2.0 and 2).
-        return val == parseInt(val);
+        return val === parseInt(val, 10);
     } else if (typeof val === 'boolean') {
         // `True` and `false` always represent integer after Scratch cast.
         return true;
@@ -138,15 +138,15 @@ Cast.LIST_ALL = 'ALL';
  */
 Cast.toListIndex = function (index, length) {
     if (typeof index !== 'number') {
-        if (index == 'all') {
+        if (index === 'all') {
             return Cast.LIST_ALL;
         }
-        if (index == 'last') {
+        if (index === 'last') {
             if (length > 0) {
                 return length;
             }
             return Cast.LIST_INVALID;
-        } else if (index == 'random' || index == 'any') {
+        } else if (index === 'random' || index === 'any') {
             if (length > 0) {
                 return 1 + Math.floor(Math.random() * length);
             }
diff --git a/src/util/color.js b/src/util/color.js
index 6660ccf29..3e5054652 100644
--- a/src/util/color.js
+++ b/src/util/color.js
@@ -1,4 +1,4 @@
-function Color () {}
+var Color = function () {};
 
 /**
  * Convert a Scratch decimal color to a hex string, #RRGGBB.
diff --git a/src/util/math-util.js b/src/util/math-util.js
index 14ebb450e..c27e730bb 100644
--- a/src/util/math-util.js
+++ b/src/util/math-util.js
@@ -1,4 +1,4 @@
-function MathUtil () {}
+var MathUtil = function () {};
 
 /**
  * Convert a value from degrees to radians.
@@ -42,7 +42,7 @@ MathUtil.clamp = function (n, min, max) {
  */
 MathUtil.wrapClamp = function (n, min, max) {
     var range = (max - min) + 1;
-    return n - Math.floor((n - min) / range) * range;
+    return n - (Math.floor((n - min) / range) * range);
 };
 
 module.exports = MathUtil;
diff --git a/src/util/timer.js b/src/util/timer.js
index 48dd223d5..17c66a567 100644
--- a/src/util/timer.js
+++ b/src/util/timer.js
@@ -15,7 +15,7 @@
 /**
  * @constructor
  */
-function Timer () {}
+var Timer = function () {};
 
 /**
  * Used to store the start time of a timer action.
diff --git a/test/.eslintrc.js b/test/.eslintrc.js
new file mode 100644
index 000000000..666194097
--- /dev/null
+++ b/test/.eslintrc.js
@@ -0,0 +1,5 @@
+module.exports = {
+    rules: {
+        'no-undefined': [0]
+    }
+};
diff --git a/test/unit/engine_adapter.js b/test/unit/engine_adapter.js
index 58a820743..7c6cd2da1 100644
--- a/test/unit/engine_adapter.js
+++ b/test/unit/engine_adapter.js
@@ -61,7 +61,7 @@ test('create with branch', function (t) {
     // Find actual branch block
     var branchBlock = null;
     for (var i = 0; i < result.length; i++) {
-        if (result[i].id == branchBlockId) {
+        if (result[i].id === branchBlockId) {
             branchBlock = result[i];
         }
     }
@@ -93,10 +93,10 @@ test('create with two branches', function (t) {
     var firstBranchBlock = null;
     var secondBranchBlock = null;
     for (var i = 0; i < result.length; i++) {
-        if (result[i].id == firstBranchBlockId) {
+        if (result[i].id === firstBranchBlockId) {
             firstBranchBlock = result[i];
         }
-        if (result[i].id == secondBranchBlockId) {
+        if (result[i].id === secondBranchBlockId) {
             secondBranchBlock = result[i];
         }
     }