1 files changed, 15 insertions, 9 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 4a5f6395..f2269a2f 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -765,7 +765,7 @@ merge(Compressor.prototype, {
                             CHANGED = true;
                             stat = stat.clone();
                             stat.alternative = ret[0] || make_node(AST_Return, stat, {
-                                value: make_node(AST_Undefined, stat)
+                                value: null
                             });
                             ret[0] = stat.transform(compressor);
                             continue loop;
@@ -798,7 +798,7 @@ merge(Compressor.prototype, {
                             && !stat.alternative) {
                             CHANGED = true;
                             ret.push(make_node(AST_Return, ret[0], {
-                                value: make_node(AST_Undefined, ret[0])
+                                value: null
                             }).transform(compressor));
                             ret.unshift(stat);
                             continue loop;
@@ -1055,6 +1055,10 @@ merge(Compressor.prototype, {
         }));
     };
 
+    function is_undefined(node) {
+        return node instanceof AST_Undefined || node.is_undefined;
+    }
+
     /* -----[ boolean/negation helpers ]----- */
 
     // methods to determine whether an expression has a boolean result type
@@ -2402,8 +2406,8 @@ merge(Compressor.prototype, {
             return make_node(self.body.CTOR, self, {
                 value: make_node(AST_Conditional, self, {
                     condition   : self.condition,
-                    consequent  : self.body.value || make_node(AST_Undefined, self.body).optimize(compressor),
-                    alternative : self.alternative.value || make_node(AST_Undefined, self.alternative).optimize(compressor)
+                    consequent  : self.body.value || make_node(AST_Undefined, self.body),
+                    alternative : self.alternative.value || make_node(AST_Undefined, self.alternative)
                 })
             }).transform(compressor);
         }
@@ -2834,7 +2838,7 @@ merge(Compressor.prototype, {
                 return self.car;
             }
         }
-        if (self.cdr instanceof AST_Undefined) {
+        if (is_undefined(self.cdr)) {
             return make_node(AST_UnaryPrefix, self, {
                 operator   : "void",
                 expression : self.car
@@ -2873,7 +2877,7 @@ merge(Compressor.prototype, {
                 self.expression = e;
                 return self;
             } else {
-                return make_node(AST_Undefined, self);
+                return make_node(AST_Undefined, self).transform(compressor);
             }
         }
         if (compressor.option("booleans") && compressor.in_boolean_context()) {
@@ -3354,7 +3358,7 @@ merge(Compressor.prototype, {
             && (!self.scope.uses_with || !compressor.find_parent(AST_With))) {
             switch (self.name) {
               case "undefined":
-                return make_node(AST_Undefined, self);
+                return make_node(AST_Undefined, self).transform(compressor);
               case "NaN":
                 return make_node(AST_NaN, self).transform(compressor);
               case "Infinity":
@@ -3397,11 +3401,13 @@ merge(Compressor.prototype, {
             var scope = compressor.find_parent(AST_Scope);
             var undef = scope.find_variable("undefined");
             if (undef) {
-                return make_node(AST_SymbolRef, self, {
+                var ref = make_node(AST_SymbolRef, self, {
                     name   : "undefined",
                     scope  : scope,
                     thedef : undef
                 });
+                ref.is_undefined = true;
+                return ref;
             }
         }
         return self;
@@ -3688,7 +3694,7 @@ merge(Compressor.prototype, {
     OPT(AST_RegExp, literals_in_boolean_context);
 
     OPT(AST_Return, function(self, compressor){
-        if (self.value instanceof AST_Undefined) {
+        if (self.value && is_undefined(self.value)) {
             self.value = null;
         }
         return self;