diff --git a/js/Interpreter.js b/js/Interpreter.js
index 83cdd11..f804f92 100644
--- a/js/Interpreter.js
+++ b/js/Interpreter.js
@@ -139,10 +139,11 @@ Interpreter.prototype.stepActiveThread = function() {
         // Advance the "program counter" to the next block before running the primitive.
         // Control flow primitives (e.g. if) may change activeThread.nextBlock.
         this.activeThread.nextBlock = b.nextBlock;
-        if(this.debugOps && this.debugFunc) {
-            var finalArgs = new Array(b.args.length);
-            for(var i=0; i<b.args.length; ++i)
-                finalArgs[i] = this.arg(b, i);
+        if (this.debugOps && this.debugFunc) {
+            var finalArgs = [];
+            for (var i = 0; i < b.args.length; ++i) {
+                finalArgs.push(this.arg(b, i));
+            }
 
             this.debugFunc(this.opCount2, b.op, finalArgs);
             ++this.opCount2;
@@ -150,13 +151,22 @@ Interpreter.prototype.stepActiveThread = function() {
         b.primFcn(b);
         if (this.yield) { this.activeThread.nextBlock = b; return; }
         b = this.activeThread.nextBlock; // refresh local variable b in case primitive did some control flow
-        while (b == null) {
+        while (!b) {
             // end of a substack; pop the owning control flow block from stack
             // Note: This is a loop to handle nested control flow blocks.
-            b = this.activeThread.stack.pop();
-            if ((b == null) || (b.isLoop)) {
-                this.activeThread.nextBlock = b;
-                return; // yield at the end of a loop or when stack is empty
+
+            // yield at the end of a loop or when stack is empty
+            if (this.activeThread.stack.length === 0) {
+                this.activeThread.nextBlock = null;
+                return;
+            } else {
+                b = this.activeThread.stack.pop();
+                if (b.isLoop) {
+                    this.activeThread.nextBlock = b; // preserve where it left off
+                    return;
+                } else {
+                    b = b.nextBlock; // skip and continue for non looping blocks
+                }
             }
         }
     }
@@ -206,6 +216,24 @@ Interpreter.prototype.arg = function(block, index) {
     return arg;
 };
 
+Interpreter.prototype.numarg = function(block, index) {
+    var arg = Number(this.arg(block, index));
+    if (arg !== arg) {
+        return 0;
+    }
+    return arg;
+};
+
+Interpreter.prototype.boolarg = function(block, index) {
+    var arg = this.arg(block, index);
+    if (typeof arg === 'boolean') {
+        return arg;
+    } else if (typeof arg === 'string') {
+        return !(arg === '' || arg === '0' || arg.toLowerCase() === 'false');
+    }
+    return Boolean(arg);
+};
+
 Interpreter.prototype.targetSprite = function() {
     return this.activeThread.target;
 };
@@ -242,14 +270,14 @@ Interpreter.prototype.initPrims = function() {
     this.primitiveTable['whenGreenFlag']       = this.primNoop;
     this.primitiveTable['whenKeyPressed']      = this.primNoop;
     this.primitiveTable['whenClicked']         = this.primNoop;
-    this.primitiveTable['if']                  = function(b) { if (interp.arg(b, 0)) interp.startSubstack(b); };
+    this.primitiveTable['if']                  = function(b) { if (interp.boolarg(b, 0)) interp.startSubstack(b); };
     this.primitiveTable['doForever']           = function(b) { interp.startSubstack(b, true); };
-    this.primitiveTable['doForeverIf']         = function(b) { if (interp.arg(b, 0)) interp.startSubstack(b, true); else interp.yield = true; };
-    this.primitiveTable['doIf']                = function(b) { if (interp.arg(b, 0)) interp.startSubstack(b); };
+    this.primitiveTable['doForeverIf']         = function(b) { if (interp.boolarg(b, 0)) interp.startSubstack(b, true); else interp.yield = true; };
+    this.primitiveTable['doIf']                = function(b) { if (interp.boolarg(b, 0)) interp.startSubstack(b); };
     this.primitiveTable['doRepeat']            = this.primRepeat;
-    this.primitiveTable['doIfElse']            = function(b) { if (interp.arg(b, 0)) interp.startSubstack(b); else interp.startSubstack(b, false, true); };
-    this.primitiveTable['doWaitUntil']         = function(b) { if (!interp.arg(b, 0)) interp.yield = true; };
-    this.primitiveTable['doUntil']             = function(b) { if (!interp.arg(b, 0)) interp.startSubstack(b, true); };
+    this.primitiveTable['doIfElse']            = function(b) { if (interp.boolarg(b, 0)) interp.startSubstack(b); else interp.startSubstack(b, false, true); };
+    this.primitiveTable['doWaitUntil']         = function(b) { if (!interp.boolarg(b, 0)) interp.yield = true; };
+    this.primitiveTable['doUntil']             = function(b) { if (!interp.boolarg(b, 0)) interp.startSubstack(b, true); };
     this.primitiveTable['doReturn']            = function(b) { interp.activeThread = new Thread(null); };
     this.primitiveTable['stopAll']             = function(b) { interp.activeThread = new Thread(null); interp.threads = []; }
     this.primitiveTable['whenIReceive']        = this.primNoop;
@@ -275,13 +303,13 @@ Interpreter.prototype.lookupPrim = function(op) {
 Interpreter.prototype.primNoop = function(b) { console.log(b.op); };
 
 Interpreter.prototype.primWait = function(b) {
-    if (interp.activeThread.firstTime) interp.startTimer(interp.arg(b, 0));
+    if (interp.activeThread.firstTime) interp.startTimer(interp.numarg(b, 0));
     else interp.checkTimer();
 };
 
 Interpreter.prototype.primRepeat = function(b) {
     if (b.tmp == -1) {
-        b.tmp = Math.max(interp.arg(b, 0), 0); // Initialize repeat count on this block
+        b.tmp = Math.max(interp.numarg(b, 0), 0); // Initialize repeat count on this block
     }
     if (b.tmp > 0) {
         b.tmp -= 1; // decrement count
@@ -299,7 +327,7 @@ Interpreter.prototype.broadcast = function(b, waitFlag) {
         var receivers = [];
         var msg = String(interp.arg(b, 0)).toLowerCase();
         var findReceivers = function(stack, target) {
-            if ((stack.op == "whenIReceive") && (stack.args[0].toLowerCase() == msg)) {
+            if ((stack.op == 'whenIReceive') && (stack.args[0].toLowerCase() == msg)) {
                 receivers.push([stack, target]);
             }
         }
@@ -334,10 +362,8 @@ Interpreter.prototype.isRunning = function(b) {
 
 Interpreter.prototype.startSubstack = function(b, isLoop, secondSubstack) {
     // Start the substack of a control structure command such as if or forever.
-    if (isLoop) {
-        b.isLoop = true;
-        this.activeThread.stack.push(b); // remember the block that started the substack
-    }
+    b.isLoop = !!isLoop;
+    this.activeThread.stack.push(b); // remember the block that started the substack
     if (!secondSubstack) {
         this.activeThread.nextBlock = b.substack;
     } else {
diff --git a/js/primitives/LooksPrims.js b/js/primitives/LooksPrims.js
index 174a4c7..5eb0835 100644
--- a/js/primitives/LooksPrims.js
+++ b/js/primitives/LooksPrims.js
@@ -18,35 +18,35 @@
 var LooksPrims = function() {};
 
 LooksPrims.prototype.addPrimsTo = function(primTable) {
-    primTable["show"]               = this.primShow;
-    primTable["hide"]               = this.primHide;
+    primTable['show']               = this.primShow;
+    primTable['hide']               = this.primHide;
 
-    primTable["nextCostume"]        = this.primNextCostume;
-    primTable["lookLike:"]          = this.primShowCostume;
-    primTable["costumeIndex"]       = this.primCostumeNum;
+    primTable['nextCostume']        = this.primNextCostume;
+    primTable['lookLike:']          = this.primShowCostume;
+    primTable['costumeIndex']       = this.primCostumeNum;
 
-    primTable["nextScene"]     = this.primNextCostume;
-    primTable["showBackground:"]    = this.primShowCostume;
-    primTable["backgroundIndex"]    = this.primCostumeNum;
+    primTable['nextScene']     = this.primNextCostume;
+    primTable['showBackground:']    = this.primShowCostume;
+    primTable['backgroundIndex']    = this.primCostumeNum;
 
-    primTable["startScene"]         = this.primStartScene;
-    primTable["backgroundIndex"]    = this.primCostumeNum;
+    primTable['startScene']         = this.primStartScene;
+    primTable['backgroundIndex']    = this.primCostumeNum;
 
-    primTable["changeSizeBy:"]      = this.primChangeSize;
-    primTable["setSizeTo:"]         = this.primSetSize;
-    primTable["scale"]              = this.primSize;
+    primTable['changeSizeBy:']      = this.primChangeSize;
+    primTable['setSizeTo:']         = this.primSetSize;
+    primTable['scale']              = this.primSize;
 
-    primTable["comeToFront"]        = this.primGoFront;
-    primTable["goBackByLayers:"]    = this.primGoBack;
+    primTable['comeToFront']        = this.primGoFront;
+    primTable['goBackByLayers:']    = this.primGoBack;
 
-    primTable["changeGraphicEffect:by:"] = this.primChangeEffect;
-    primTable["setGraphicEffect:to:"]    = this.primSetEffect;
-    primTable["filterReset"]             = this.primClearEffects;
+    primTable['changeGraphicEffect:by:'] = this.primChangeEffect;
+    primTable['setGraphicEffect:to:']    = this.primSetEffect;
+    primTable['filterReset']             = this.primClearEffects;
 
-    primTable["say:"] = function(b) { showBubble(b, 'say'); };
-    primTable["say:duration:elapsed:from:"] = function(b) { showBubbleAndWait(b, 'say'); };
-    primTable["think:"] = function(b) { showBubble(b, 'think'); };
-    primTable["think:duration:elapsed:from:"] = function(b) { showBubbleAndWait(b, 'think'); };
+    primTable['say:'] = function(b) { showBubble(b, 'say'); };
+    primTable['say:duration:elapsed:from:'] = function(b) { showBubbleAndWait(b, 'say'); };
+    primTable['think:'] = function(b) { showBubble(b, 'think'); };
+    primTable['think:duration:elapsed:from:'] = function(b) { showBubbleAndWait(b, 'think'); };
 };
 
 LooksPrims.prototype.primShow = function(b) {
@@ -71,7 +71,7 @@ LooksPrims.prototype.primShowCostume = function(b) {
     if (typeof(arg) == 'number') {
         s.showCostume(arg - 1);
     } else {
-        if ((arg == 'CAMERA') || (arg == "CAMERA - MIRROR")) {
+        if ((arg == 'CAMERA') || (arg == 'CAMERA - MIRROR')) {
             s.showCostumeNamed(arg);
             return;
         }
@@ -79,8 +79,8 @@ LooksPrims.prototype.primShowCostume = function(b) {
         if (i >= 0) {
             s.showCostume(i);
         } else {
-            var n = Number(arg);
-            if (!isNaN(n)) {
+            var n = parseInt(arg, 10);
+            if (n === n) { // if n is not NaN
                 s.showCostume(n - 1);
             } else {
                 return;  // arg did not match a costume name nor is a valid number
@@ -96,7 +96,7 @@ LooksPrims.prototype.primStartScene = function(b) {
     if (typeof(arg) == 'number') {
         s.showCostume(arg - 1);
     } else {
-        if ((arg == 'CAMERA') || (arg == "CAMERA - MIRROR")) {
+        if ((arg == 'CAMERA') || (arg == 'CAMERA - MIRROR')) {
             s.showCostumeNamed(arg);
             return;
         }
@@ -104,8 +104,8 @@ LooksPrims.prototype.primStartScene = function(b) {
         if (i >= 0) {
             s.showCostume(i);
         } else {
-            var n = Number(arg);
-            if (!isNaN(n)) {
+            var n = parseInt(arg, 10);
+            if (n === n) { // fast !isNaN check
                 s.showCostume(n - 1);
             } else {
                 return;  // arg did not match a costume name nor is a valid number
@@ -123,14 +123,14 @@ LooksPrims.prototype.primCostumeNum = function(b) {
 LooksPrims.prototype.primChangeSize = function(b) {
     var s = interp.targetSprite();
     if (s == null) return;
-    s.setSize(s.getSize() + interp.arg(b, 0));
+    s.setSize(s.getSize() + interp.numarg(b, 0));
     if (s.visible) interp.redraw();
 };
 
 LooksPrims.prototype.primSetSize = function(b) {
     var s = interp.targetSprite();
     if (s == null) return;
-    s.setSize(interp.arg(b, 0));
+    s.setSize(interp.numarg(b, 0));
     if (s.visible) interp.redraw();
 };
 
@@ -148,8 +148,8 @@ LooksPrims.prototype.primGoFront = function(b) {
 
 LooksPrims.prototype.primGoBack = function(b) {
     var s = interp.targetSprite();
-    runtime.reassignZ(s, interp.arg(b, 0));
-    if (s.visible) interp.redraw();
+    runtime.reassignZ(s, interp.numarg(b, 0));
+    if(s.visible) interp.redraw();
 };
 
 LooksPrims.prototype.primChangeEffect = function(b) {};
@@ -168,7 +168,7 @@ var showBubbleAndWait = function(b, type) {
     if (s == null) return;
     if (interp.activeThread.firstTime) {
         var text = interp.arg(b, 0);
-        var secs = interp.arg(b, 1);
+        var secs = interp.numarg(b, 1);
         s.showBubble(text, type);
         if (s.visible) interp.redraw();
         interp.startTimer(secs);
diff --git a/js/primitives/MotionAndPenPrims.js b/js/primitives/MotionAndPenPrims.js
index 00345e9..f6fa8fa 100644
--- a/js/primitives/MotionAndPenPrims.js
+++ b/js/primitives/MotionAndPenPrims.js
@@ -22,42 +22,42 @@ MotionAndPenPrims.prototype.addPrimsTo = function(primTable) {
     primTable['turnLeft:']          = this.primTurnLeft;
     primTable['turnRight:']         = this.primTurnRight;
     primTable['heading:']           = this.primSetDirection;
-    primTable["pointTowards:"]      = this.primPointTowards;
-    primTable["gotoX:y:"]           = this.primGoTo;
-    primTable["gotoSpriteOrMouse:"]  = this.primGoToSpriteOrMouse;
-    primTable["glideSecs:toX:y:elapsed:from:"] = this.primGlide;
+    primTable['pointTowards:']      = this.primPointTowards;
+    primTable['gotoX:y:']           = this.primGoTo;
+    primTable['gotoSpriteOrMouse:']  = this.primGoToSpriteOrMouse;
+    primTable['glideSecs:toX:y:elapsed:from:'] = this.primGlide;
 
-    primTable["changeXposBy:"]      = this.primChangeX;
-    primTable["xpos:"]              = this.primSetX;
-    primTable["changeYposBy:"]      = this.primChangeY;
-    primTable["ypos:"]              = this.primSetY;
+    primTable['changeXposBy:']      = this.primChangeX;
+    primTable['xpos:']              = this.primSetX;
+    primTable['changeYposBy:']      = this.primChangeY;
+    primTable['ypos:']              = this.primSetY;
 
-    primTable["bounceOffEdge"]      = this.primBounceOffEdge;
-    primTable["setRotationStyle"]   = this.primSetRotationStyle;
+    primTable['bounceOffEdge']      = this.primBounceOffEdge;
+    primTable['setRotationStyle']   = this.primSetRotationStyle;
 
-    primTable["xpos"]               = this.primXPosition;
-    primTable["ypos"]               = this.primYPosition;
-    primTable["heading"]            = this.primDirection;
+    primTable['xpos']               = this.primXPosition;
+    primTable['ypos']               = this.primYPosition;
+    primTable['heading']            = this.primDirection;
 
-    primTable["clearPenTrails"]     = this.primClear;
-    primTable["putPenDown"]         = this.primPenDown;
-    primTable["putPenUp"]           = this.primPenUp;
-    primTable["penColor:"]          = this.primSetPenColor;
-    primTable["setPenHueTo:"]       = this.primSetPenHue;
-    primTable["changePenHueBy:"]    = this.primChangePenHue;
-    primTable["setPenShadeTo:"]     = this.primSetPenShade;
-    primTable["changePenShadeBy:"]  = this.primChangePenShade;
-    primTable["penSize:"]           = this.primSetPenSize;
-    primTable["changePenSizeBy:"]   = this.primChangePenSize;
+    primTable['clearPenTrails']     = this.primClear;
+    primTable['putPenDown']         = this.primPenDown;
+    primTable['putPenUp']           = this.primPenUp;
+    primTable['penColor:']          = this.primSetPenColor;
+    primTable['setPenHueTo:']       = this.primSetPenHue;
+    primTable['changePenHueBy:']    = this.primChangePenHue;
+    primTable['setPenShadeTo:']     = this.primSetPenShade;
+    primTable['changePenShadeBy:']  = this.primChangePenShade;
+    primTable['penSize:']           = this.primSetPenSize;
+    primTable['changePenSizeBy:']   = this.primChangePenSize;
 
-    primTable["stampCostume"]       = this.primStamp;
-    primTable["stampTransparent"]   = this.primStampTransparent;
+    primTable['stampCostume']       = this.primStamp;
+    primTable['stampTransparent']   = this.primStampTransparent;
 };
 
 MotionAndPenPrims.prototype.primMove = function(b) {
     var s = interp.targetSprite();
     var radians = (90 - s.direction) * Math.PI / 180;
-    var d = interp.arg(b, 0);
+    var d = interp.numarg(b, 0);
 
     moveSpriteTo(s, s.scratchX + d * Math.cos(radians), s.scratchY + d * Math.sin(radians));
     if (s.visible) interp.redraw();
@@ -65,21 +65,21 @@ MotionAndPenPrims.prototype.primMove = function(b) {
 
 MotionAndPenPrims.prototype.primTurnLeft = function(b) {
     var s = interp.targetSprite();
-    var d = s.direction - interp.arg(b, 0);
+    var d = s.direction - interp.numarg(b, 0);
     s.setDirection(d);
     if (s.visible) interp.redraw();
 };
 
 MotionAndPenPrims.prototype.primTurnRight = function(b) {
     var s = interp.targetSprite();
-    var d = s.direction + interp.arg(b, 0);
+    var d = s.direction + interp.numarg(b, 0);
     s.setDirection(d);
     if (s.visible) interp.redraw();
 };
 
 MotionAndPenPrims.prototype.primSetDirection = function(b) {
     var s = interp.targetSprite();
-    s.setDirection(interp.arg(b, 0));
+    s.setDirection(interp.numarg(b, 0));
     if (s.visible) interp.redraw();
 };
 
@@ -96,7 +96,7 @@ MotionAndPenPrims.prototype.primPointTowards = function(b) {
 
 MotionAndPenPrims.prototype.primGoTo = function(b) {
     var s = interp.targetSprite();
-    if (s != null) moveSpriteTo(s, interp.arg(b, 0), interp.arg(b, 1));
+    if (s != null) moveSpriteTo(s, interp.numarg(b, 0), interp.numarg(b, 1));
 };
 
 MotionAndPenPrims.prototype.primGoToSpriteOrMouse = function(b) {
@@ -110,9 +110,9 @@ MotionAndPenPrims.prototype.primGlide = function(b) {
     var s = interp.targetSprite();
     if (s == null) return;
     if (interp.activeThread.firstTime) {
-        var secs = interp.arg(b, 0);
-        var destX = interp.arg(b, 1);
-        var destY = interp.arg(b, 2);
+        var secs = interp.numarg(b, 0);
+        var destX = interp.numarg(b, 1);
+        var destY = interp.numarg(b, 2);
         if (secs <= 0) {
             moveSpriteTo(s, destX, destY);
             return;
@@ -138,22 +138,22 @@ MotionAndPenPrims.prototype.primGlide = function(b) {
 
 MotionAndPenPrims.prototype.primChangeX = function(b) {
     var s = interp.targetSprite();
-    if (s != null) moveSpriteTo(s, s.scratchX + interp.arg(b, 0), s.scratchY);
+    if (s != null) moveSpriteTo(s, s.scratchX + interp.numarg(b, 0), s.scratchY);
 };
 
 MotionAndPenPrims.prototype.primSetX = function(b) {
     var s = interp.targetSprite();
-    if (s != null) moveSpriteTo(s, interp.arg(b, 0), s.scratchY);
+    if (s != null) moveSpriteTo(s, interp.numarg(b, 0), s.scratchY);
 };
 
 MotionAndPenPrims.prototype.primChangeY = function(b) {
     var s = interp.targetSprite();
-    if (s != null) moveSpriteTo(s, s.scratchX, s.scratchY + interp.arg(b, 0));
+    if (s != null) moveSpriteTo(s, s.scratchX, s.scratchY + interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primSetY = function(b) {
     var s = interp.targetSprite();
-    if (s != null) moveSpriteTo(s, s.scratchX, interp.arg(b, 0));
+    if (s != null) moveSpriteTo(s, s.scratchX, interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primBounceOffEdge = function(b) {
@@ -209,38 +209,38 @@ MotionAndPenPrims.prototype.primPenUp = function(b) {
 
 MotionAndPenPrims.prototype.primSetPenColor = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.setPenColor(interp.arg(b, 0));
+    if (s != null) s.setPenColor(interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primSetPenHue = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.setPenHue(interp.arg(b, 0));
+    if (s != null) s.setPenHue(interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primChangePenHue = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.setPenHue(s.penHue + interp.arg(b, 0));
+    if (s != null) s.setPenHue(s.penHue + interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primSetPenShade = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.setPenShade(interp.arg(b, 0));
+    if (s != null) s.setPenShade(interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primChangePenShade = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.setPenShade(s.penShade + interp.arg(b, 0));
+    if (s != null) s.setPenShade(s.penShade + interp.numarg(b, 0));
 };
 
 MotionAndPenPrims.prototype.primSetPenSize = function(b) {
     var s = interp.targetSprite();
-    var w = Math.max(0, Math.min(interp.arg(b, 0), 100));
+    var w = Math.max(0, Math.min(interp.numarg(b, 0), 100));
     if (s != null) s.penWidth = w;
 };
 
 MotionAndPenPrims.prototype.primChangePenSize = function(b) {
     var s = interp.targetSprite();
-    var w = Math.max(0, Math.min(s.penWidth + interp.arg(b, 0), 100));
+    var w = Math.max(0, Math.min(s.penWidth + interp.numarg(b, 0), 100));
     if (s != null) s.penWidth = w;
 };
 
@@ -251,7 +251,7 @@ MotionAndPenPrims.prototype.primStamp = function(b) {
 
 MotionAndPenPrims.prototype.primStampTransparent = function(b) {
     var s = interp.targetSprite();
-    var transparency = Math.max(0, Math.min(interp.arg(b, 0), 100));
+    var transparency = Math.max(0, Math.min(interp.numarg(b, 0), 100));
     var alpha = 100 - transparency;
     s.stamp(runtime.stage.lineCache, alpha);
 };
@@ -263,7 +263,7 @@ var stroke = function(s, oldX, oldY, newX, newY) {
 };
 
 var mouseOrSpritePosition = function(arg) {
-    if (arg == "_mouse_") {
+    if (arg == '_mouse_') {
         var w = runtime.stage;
         return new Point(runtime.mousePos[0], runtime.mousePos[1]);
     } else {
diff --git a/js/primitives/Primitives.js b/js/primitives/Primitives.js
index 035cff7..6000f00 100644
--- a/js/primitives/Primitives.js
+++ b/js/primitives/Primitives.js
@@ -26,29 +26,29 @@ var Primitives = function() {}
 
 Primitives.prototype.addPrimsTo = function(primTable) {
     // Math primitives
-    primTable["+"]        = function(b) { return interp.arg(b, 0) + interp.arg(b, 1); };
-    primTable["-"]        = function(b) { return interp.arg(b, 0) - interp.arg(b, 1); };
-    primTable["*"]        = function(b) { return interp.arg(b, 0) * interp.arg(b, 1); };
-    primTable["/"]        = function(b) { return interp.arg(b, 0) / interp.arg(b, 1); };
-    primTable["%"]        = function(b) { return interp.arg(b, 0) % interp.arg(b, 1); };
-    primTable["randomFrom:to:"] = this.primRandom;
-    primTable["<"]        = function(b) { return (interp.arg(b, 0) < interp.arg(b, 1)); };
-    primTable["="]        = function(b) { return (interp.arg(b, 0) == interp.arg(b, 1)); };
-    primTable[">"]        = function(b) { return (interp.arg(b, 0) > interp.arg(b, 1)); };
-    primTable["&"]        = function(b) { return interp.arg(b, 0) && interp.arg(b, 1); };
-    primTable["|"]        = function(b) { return interp.arg(b, 0) || interp.arg(b, 1); };
-    primTable["not"]      = function(b) { return !interp.arg(b, 0) };
-    primTable["abs"]      = function(b) { return Math.abs(interp.arg(b, 0)) };
-    primTable["sqrt"]     = function(b) { return Math.sqrt(interp.arg(b, 0)) };
+    primTable['+']        = function(b) { return interp.numarg(b, 0) + interp.numarg(b, 1); };
+    primTable['-']        = function(b) { return interp.numarg(b, 0) - interp.numarg(b, 1); };
+    primTable['*']        = function(b) { return interp.numarg(b, 0) * interp.numarg(b, 1); };
+    primTable['/']        = function(b) { return interp.numarg(b, 0) / interp.numarg(b, 1); };
+    primTable['%']        = function(b) { return interp.numarg(b, 0) % interp.numarg(b, 1); };
+    primTable['randomFrom:to:'] = this.primRandom;
+    primTable['<']        = function(b) { return (interp.numarg(b, 0) < interp.numarg(b, 1)); };
+    primTable['=']        = function(b) { return (interp.arg(b, 0) == interp.arg(b, 1)); };
+    primTable['>']        = function(b) { return (interp.numarg(b, 0) > interp.numarg(b, 1)); };
+    primTable['&']        = function(b) { return interp.boolarg(b, 0) && interp.boolarg(b, 1); };
+    primTable['|']        = function(b) { return interp.boolarg(b, 0) || interp.boolarg(b, 1); };
+    primTable['not']      = function(b) { return !interp.boolarg(b, 0); };
+    primTable['abs']      = function(b) { return Math.abs(interp.numarg(b, 0)); };
+    primTable['sqrt']     = function(b) { return Math.sqrt(interp.numarg(b, 0)); };
 
-    primTable["\\\\"]               = this.primModulo;
-    primTable["rounded"]            = function(b) { return Math.round(interp.arg(b, 0)); };
-    primTable["computeFunction:of:"] = this.primMathFunction;
+    primTable['\\\\']               = this.primModulo;
+    primTable['rounded']            = function(b) { return Math.round(interp.numarg(b, 0)); };
+    primTable['computeFunction:of:'] = this.primMathFunction;
 
     // String primitives
-    primTable["concatenate:with:"]  = function(b) { return "" + interp.arg(b, 0) + interp.arg(b, 1); };
-    primTable["letter:of:"]         = this.primLetterOf;
-    primTable["stringLength:"]      = function(b) { return interp.arg(b, 0).length; };
+    primTable['concatenate:with:']  = function(b) { return '' + interp.arg(b, 0) + interp.arg(b, 1); };
+    primTable['letter:of:']         = this.primLetterOf;
+    primTable['stringLength:']      = function(b) { return interp.arg(b, 0).length; };
 
     new VarListPrims().addPrimsTo(primTable);
     new MotionAndPenPrims().addPrimsTo(primTable);
@@ -58,8 +58,8 @@ Primitives.prototype.addPrimsTo = function(primTable) {
 }
 
 Primitives.prototype.primRandom = function(b) {
-    var n1 = interp.arg(b, 0);
-    var n2 = interp.arg(b, 1);
+    var n1 = interp.numarg(b, 0);
+    var n2 = interp.numarg(b, 1);
     var low = n1 <= n2 ? n1 : n2;
     var hi = n1 <= n2 ? n2 : n1;
     if (low == hi) return low;
@@ -72,34 +72,34 @@ Primitives.prototype.primRandom = function(b) {
 
 Primitives.prototype.primLetterOf = function(b) {
     var s = interp.arg(b, 1);
-    var i = interp.arg(b, 0) - 1;
-    if (i < 0 || i >= s.length) return "";
+    var i = interp.numarg(b, 0) - 1;
+    if (i < 0 || i >= s.length) return '';
     return s.charAt(i);
 }
 
 Primitives.prototype.primModulo = function(b) {
-    var modulus = interp.arg(b, 1);
-    var n = interp.arg(b, 0) % modulus;
+    var modulus = interp.numarg(b, 1);
+    var n = interp.numarg(b, 0) % modulus;
     if (n < 0) n += modulus;
     return n;
 }
 
 Primitives.prototype.primMathFunction = function(b) {
-    var op = interp.arg(b, 0);
-    var n = interp.arg(b, 1);
+    var op = interp.numarg(b, 0);
+    var n = interp.numarg(b, 1);
     switch(op) {
-        case "abs": return Math.abs(n);
-        case "sqrt": return Math.sqrt(n);
-        case "sin": return Math.sin(n * Math.PI / 180);
-        case "cos": return Math.cos(n * Math.PI / 180);
-        case "tan": return Math.tan(n * Math.PI / 180);
-        case "asin": return Math.asin(n) * 180 / Math.PI;
-        case "acos": return Math.acos(n) * 180 / Math.PI;
-        case "atan": return Math.atan(n) * 180 / Math.PI;
-        case "ln": return Math.log(n);
-        case "log": return Math.log(n) / Math.LN10;
-        case "e ^": return Math.exp(n);
-        case "10 ^": return Math.exp(n * Math.LN10);
+        case 'abs': return Math.abs(n);
+        case 'sqrt': return Math.sqrt(n);
+        case 'sin': return Math.sin(n * Math.PI / 180);
+        case 'cos': return Math.cos(n * Math.PI / 180);
+        case 'tan': return Math.tan(n * Math.PI / 180);
+        case 'asin': return Math.asin(n) * 180 / Math.PI;
+        case 'acos': return Math.acos(n) * 180 / Math.PI;
+        case 'atan': return Math.atan(n) * 180 / Math.PI;
+        case 'ln': return Math.log(n);
+        case 'log': return Math.log(n) / Math.LN10;
+        case 'e ^': return Math.exp(n);
+        case '10 ^': return Math.exp(n * Math.LN10);
     }
     return 0;
 }
diff --git a/js/primitives/SoundPrims.js b/js/primitives/SoundPrims.js
index a33f067..d1f4ce1 100644
--- a/js/primitives/SoundPrims.js
+++ b/js/primitives/SoundPrims.js
@@ -155,8 +155,8 @@ SoundPrims.prototype.primPlayNote = function(b) {
     var s = interp.targetSprite();
     if (s == null) return;
     if (interp.activeThread.firstTime) {
-        var key = interp.arg(b, 0);
-        var secs = beatsToSeconds(interp.arg(b, 1));
+        var key = interp.numarg(b, 0);
+        var secs = beatsToSeconds(interp.numarg(b, 1));
         playNote(s.instrument, key, secs, s);
         interp.startTimer(secs);
     } else {
@@ -168,8 +168,8 @@ SoundPrims.prototype.primPlayDrum = function(b) {
     var s = interp.targetSprite();
     if (s == null) return;
     if (interp.activeThread.firstTime) {
-        var drum = Math.round(interp.arg(b, 0));
-        var secs = beatsToSeconds(interp.arg(b, 1));
+        var drum = Math.round(interp.numarg(b, 0));
+        var secs = beatsToSeconds(interp.numarg(b, 1));
         playDrum(drum, secs, s);
         interp.startTimer(secs);
     } else {
@@ -181,7 +181,7 @@ SoundPrims.prototype.primPlayRest = function(b) {
     var s = interp.targetSprite();
     if (s == null) return;
     if (interp.activeThread.firstTime) {
-        var secs = beatsToSeconds(interp.arg(b, 0));
+        var secs = beatsToSeconds(interp.numarg(b, 0));
         interp.startTimer(secs);
     } else {
         interp.checkTimer();
@@ -199,12 +199,12 @@ SoundPrims.prototype.primStopAllSounds = function(b) {
 
 SoundPrims.prototype.primChangeVolume = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.volume += interp.arg(b, 0);
+    if (s != null) s.volume += interp.numarg(b, 0);
 };
 
 SoundPrims.prototype.primSetVolume = function(b) {
     var s = interp.targetSprite();
-    if (s != null) s.volume = interp.arg(b, 0);
+    if (s != null) s.volume = interp.numarg(b, 0);
 };
 
 SoundPrims.prototype.primVolume = function(b) {
diff --git a/js/primitives/VarListPrims.js b/js/primitives/VarListPrims.js
index a8f734a..38b1f5b 100644
--- a/js/primitives/VarListPrims.js
+++ b/js/primitives/VarListPrims.js
@@ -67,11 +67,9 @@ VarListPrims.prototype.primChangeVar = function(b) {
     if (s == null) return;
     var targetVar = interp.arg(b, 0);
     if (targetVar in s.variables) {
-        s.variables[targetVar] = parseFloat(s.variables[targetVar]);
-        s.variables[targetVar] += interp.arg(b, 1);
+        s.variables[targetVar] = parseFloat(s.variables[targetVar]) + interp.numarg(b, 1);
     } else if (targetVar in runtime.stage.variables) {
-        runtime.stage.variables[targetVar] = parseFloat(runtime.stage.variables[targetVar]);
-        runtime.stage.variables[targetVar] += interp.arg(b, 1);
+        runtime.stage.variables[targetVar] = parseFloat(runtime.stage.variables[targetVar]) + interp.numarg(b, 1);
     }
 };
 
@@ -127,7 +125,7 @@ VarListPrims.prototype.primListDeleteLine = function(b) {
     var line = interp.arg(b, 0);
     if (line == 'all' || list.length == 0) list.length = 0;
     else if (line == 'last') list.splice(list.length - 1, 1);
-    else if (parseInt(line) - 1 in list) list.splice(parseInt(line) - 1, 1);
+    else if (parseInt(line, 10) - 1 in list) list.splice(parseInt(line, 10) - 1, 1);
 };
 
 VarListPrims.prototype.primListInsertAt = function(b) {
@@ -138,7 +136,7 @@ VarListPrims.prototype.primListInsertAt = function(b) {
     var position = interp.arg(b, 1);
     if (position == 'last') position = list.length;
     else if (position == 'random') position = Math.round(Math.random() * list.length);
-    else position = parseInt(position) - 1;
+    else position = parseInt(position, 10) - 1;
     if (position > list.length) return;
 
     list.splice(position, 0, newItem);
@@ -152,7 +150,7 @@ VarListPrims.prototype.primListSetLine = function(b) {
 
     if (position == 'last') position = list.length - 1;
     else if (position == 'random') position = Math.floor(Math.random() * list.length);
-    else position = parseInt(position) - 1;
+    else position = parseInt(position, 10) - 1;
 
     if (position > list.length - 1) return;