1 files changed, 24 insertions, 9 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 9420d8d2..3d1b17f3 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -5194,16 +5194,17 @@ merge(Compressor.prototype, {
         return if_break_in_loop(self, compressor);
     });
 
-    function mark_locally_defined(condition, consequent, alternative, operator) {
+    function mark_locally_defined(condition, consequent, alternative) {
         if (!(condition instanceof AST_Binary)) return;
         if (!(condition.left instanceof AST_String)) {
-            if (!operator) operator = condition.operator;
-            if (condition.operator != operator) return;
-            switch (operator) {
+            switch (condition.operator) {
               case "&&":
+                mark_locally_defined(condition.left, consequent);
+                mark_locally_defined(condition.right, consequent);
+                break;
               case "||":
-                mark_locally_defined(condition.left, consequent, alternative, operator);
-                mark_locally_defined(condition.right, consequent, alternative, operator);
+                mark_locally_defined(negate(condition.left), alternative);
+                mark_locally_defined(negate(condition.right), alternative);
                 break;
             }
             return;
@@ -5235,6 +5236,20 @@ merge(Compressor.prototype, {
             if (node instanceof AST_SymbolRef && node.definition() === def) node.defined = true;
         });
         body.walk(tw);
+
+        function negate(node) {
+            if (!(node instanceof AST_Binary)) return;
+            switch (node.operator) {
+              case "==":
+                node = node.clone();
+                node.operator = "!=";
+                return node;
+              case "!=":
+                node = node.clone();
+                node.operator = "==";
+                return node;
+            }
+        }
     }
 
     OPT(AST_If, function(self, compressor) {
@@ -6623,7 +6638,7 @@ merge(Compressor.prototype, {
                 (self.left.is_number(compressor) && self.right.is_number(compressor)) ||
                 (self.left.is_boolean(compressor) && self.right.is_boolean(compressor)) ||
                 self.left.equivalent_to(self.right)) {
-                self.operator = self.operator.substr(0, 2);
+                self.operator = self.operator.slice(0, 2);
             }
             // XXX: intentionally falling down to the next case
           case "==":
@@ -7102,10 +7117,10 @@ merge(Compressor.prototype, {
         }
         if (compressor.option("typeofs")) switch (self.operator) {
           case "&&":
-            mark_locally_defined(self.left, self.right, null, "&&");
+            mark_locally_defined(self.left, self.right, null);
             break;
           case "||":
-            mark_locally_defined(self.left, null, self.right, "||");
+            mark_locally_defined(self.left, null, self.right);
             break;
         }
         if (compressor.option("unsafe")) {