fix corner case in string concatenations (#3692)

- migrate de-facto compression to `conditionals` & `strings`

fixes #3689

1 files changed, 33 insertions, 21 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 4e967aef..e14d9d06 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -83,6 +83,7 @@ function Compressor(options, false_by_default) {
         reduce_vars     : !false_by_default,
         sequences       : !false_by_default,
         side_effects    : !false_by_default,
+        strings         : !false_by_default,
         switches        : !false_by_default,
         top_retain      : null,
         toplevel        : !!(options && options["top_retain"]),
@@ -6188,6 +6189,18 @@ merge(Compressor.prototype, {
                 self.right = tmp;
             }
         }
+        function swap_chain() {
+            var rhs = self.right;
+            self.left = make_node(AST_Binary, self, {
+                operator: self.operator,
+                left: self.left,
+                right: rhs.left,
+                start: self.left.start,
+                end: rhs.left.end
+            });
+            self.right = rhs.right;
+            self.left = self.left.transform(compressor);
+        }
         if (commutativeOperators[self.operator]
             && self.right.is_constant()
             && !self.left.is_constant()
@@ -6338,17 +6351,28 @@ merge(Compressor.prototype, {
               case ">=": reverse("<="); break;
             }
         }
-        if (self.operator == "+") {
+        // x && (y && z) => x && y && z
+        // x || (y || z) => x || y || z
+        if (compressor.option("conditionals")
+            && lazy_op[self.operator]
+            && self.right instanceof AST_Binary
+            && self.operator == self.right.operator) {
+            swap_chain();
+        }
+        if (compressor.option("strings") && self.operator == "+") {
+            // "foo" + 42 + "" => "foo" + 42
             if (self.right instanceof AST_String
                 && self.right.value == ""
                 && self.left.is_string(compressor)) {
                 return self.left.optimize(compressor);
             }
+            // "" + ("foo" + 42) => "foo" + 42
             if (self.left instanceof AST_String
                 && self.left.value == ""
                 && self.right.is_string(compressor)) {
                 return self.right.optimize(compressor);
             }
+            // "" + 42 + "foo" => 42 + "foo"
             if (self.left instanceof AST_Binary
                 && self.left.operator == "+"
                 && self.left.left instanceof AST_String
@@ -6357,6 +6381,14 @@ merge(Compressor.prototype, {
                 self.left = self.left.right;
                 return self.optimize(compressor);
             }
+            // "x" + (y + "z") => "x" + y + "z"
+            // x + ("y" + z) => x + "y" + z
+            if (self.right instanceof AST_Binary
+                && self.operator == self.right.operator
+                && (self.left.is_string(compressor) && self.right.is_string(compressor)
+                    || self.right.left.is_string(compressor) && !self.right.right.has_side_effects(compressor))) {
+                swap_chain();
+            }
         }
         if (compressor.option("evaluate")) {
             var associative = true;
@@ -6727,26 +6759,6 @@ merge(Compressor.prototype, {
                 return node.optimize(compressor);
             }
         }
-        // x && (y && z) => x && y && z
-        // x || (y || z) => x || y || z
-        // x + ("y" + z) => x + "y" + z
-        // "x" + (y + "z") => "x" + y + "z"
-        if (self.right instanceof AST_Binary
-            && self.right.operator == self.operator
-            && (lazy_op[self.operator]
-                || (self.operator == "+"
-                    && (self.right.left.is_string(compressor)
-                        || (self.left.is_string(compressor)
-                            && self.right.right.is_string(compressor))))))
-        {
-            self.left = make_node(AST_Binary, self.left, {
-                operator : self.operator,
-                left     : self.left,
-                right    : self.right.left
-            });
-            self.right = self.right.right;
-            return self.transform(compressor);
-        }
         return try_evaluate(compressor, self);
 
         function align(ref, op) {