minor improvement to string optimisation (#1514)

- "" + "a"     => "a"
- "" + a + "b" => a + "b"
- "a" + ""     => "a" (improving on #45)

1 files changed, 19 insertions, 10 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 24550973..1d4e719a 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1054,12 +1054,6 @@ merge(Compressor.prototype, {
         def(AST_Conditional, function(compressor){
             return this.consequent.is_string(compressor) && this.alternative.is_string(compressor);
         });
-        def(AST_Call, function(compressor){
-            return compressor.option("unsafe")
-                && this.expression instanceof AST_SymbolRef
-                && this.expression.name == "String"
-                && this.expression.undeclared();
-        });
     })(function(node, func){
         node.DEFMETHOD("is_string", func);
     });
@@ -2985,10 +2979,25 @@ merge(Compressor.prototype, {
                 }
             }
         }
-        if (self.operator == "+" && self.right instanceof AST_String
-            && self.right.getValue() === "" && self.left instanceof AST_Binary
-            && self.left.operator == "+" && self.left.is_string(compressor)) {
-            return self.left;
+        if (self.operator == "+") {
+            if (self.right instanceof AST_String
+                && self.right.getValue() == ""
+                && self.left.is_string(compressor)) {
+                return self.left;
+            }
+            if (self.left instanceof AST_String
+                && self.left.getValue() == ""
+                && self.right.is_string(compressor)) {
+                return self.right;
+            }
+            if (self.left instanceof AST_Binary
+                && self.left.operator == "+"
+                && self.left.left instanceof AST_String
+                && self.left.left.getValue() == ""
+                && self.right.is_string(compressor)) {
+                self.left = self.left.right;
+                return self.transform(compressor);
+            }
         }
         if (compressor.option("evaluate")) {
             switch (self.operator) {