fix corner case in string concatenations (#3692)

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

fixes #3689

README.md

@@ -736,6 +736,8 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
   annotation `/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For
   example: `/*@__PURE__*/foo();`
 
+- `strings` (default: `true`) -- compact string concatenations.
+
 - `switches` (default: `true`) -- de-duplicate and remove unreachable `switch` branches
 
 - `toplevel` (default: `false`) -- drop unreferenced functions (`"funcs"`) and/or

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) {

test/compress/arrays.js

@@ -16,6 +16,7 @@ holes_and_undefined: {
 constant_join: {
     options = {
         evaluate: true,
+        strings: true,
         unsafe: true,
     }
     input: {
@@ -65,6 +66,7 @@ constant_join: {
 constant_join_2: {
     options = {
         evaluate: true,
+        strings: true,
         unsafe: true,
     }
     input: {
@@ -94,9 +96,11 @@ constant_join_2: {
 constant_join_3: {
     options = {
         evaluate: true,
+        strings: true,
         unsafe: true,
     }
     input: {
+        var foo, bar, baz;
         var a = [ null ].join();
         var b = [ , ].join();
         var c = [ , 1, , 3 ].join();
@@ -111,6 +115,7 @@ constant_join_3: {
         var l = [ foo, bar + "baz" ].join("");
     }
     expect: {
+        var foo, bar, baz;
         var a = "";
         var b = "";
         var c = ",1,,3";

test/compress/concat-strings.js

@@ -26,7 +26,9 @@ concat_1: {
 }
 
 concat_2: {
-    options = {}
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             1 + (2 + 3),
@@ -55,7 +57,9 @@ concat_2: {
 }
 
 concat_3: {
-    options = {}
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             1 + 2 + (3 + 4 + 5),
@@ -84,7 +88,9 @@ concat_3: {
 }
 
 concat_4: {
-    options = {}
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             1 + "2" + (3 + 4 + 5),
@@ -113,7 +119,9 @@ concat_4: {
 }
 
 concat_5: {
-    options = {}
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             "1" + 2 + (3 + 4 + 5),
@@ -142,7 +150,9 @@ concat_5: {
 }
 
 concat_6: {
-    options = {}
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             "1" + "2" + (3 + 4 + 5),
@@ -171,6 +181,9 @@ concat_6: {
 }
 
 concat_7: {
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             "" + 1,
@@ -197,6 +210,9 @@ concat_7: {
 }
 
 concat_8: {
+    options = {
+        strings: true,
+    }
     input: {
         console.log(
             1 + "",
@@ -221,3 +237,20 @@ concat_8: {
     }
     expect_stdout: true
 }
+
+issue_3689: {
+    options = {
+        strings: true,
+    }
+    input: {
+        console.log(function(a) {
+            return a + ("" + (a[0] = 0));
+        }([]));
+    }
+    expect: {
+        console.log(function(a) {
+            return a + ("" + (a[0] = 0));
+        }([]));
+    }
+    expect_stdout: "00"
+}

test/compress/dead-code.js

@@ -830,6 +830,7 @@ issue_3552: {
 unreachable_assign: {
     options = {
         dead_code: true,
+        strings: true,
     }
     input: {
         console.log(A = "P" + (A = "A" + (B = "S" + (A = B = "S"))), A, B);