enhance side_effects & strings (#5704)

Author: alexlamsl

lib/compress.js

@@ -8823,7 +8823,8 @@ Compressor.prototype.compress = function(node) {
                 var negated = node.clone();
                 negated.operator = op == "&&" ? "||" : "&&";
                 negated.left = left.negate(compressor, first_in_statement);
-                if (negated.operator == negated.right.operator) swap_chain(negated);
+                var negated_rhs = negated.right.tail_node();
+                if (negated_rhs instanceof AST_Binary && negated.operator == negated_rhs.operator) swap_chain(negated);
                 var best = first_in_statement ? best_of_statement : best_of_expression;
                 return op == "&&" ? best(node, negated) : best(negated, node);
             }
@@ -11607,7 +11608,14 @@ Compressor.prototype.compress = function(node) {
     }
 
     function swap_chain(self, compressor) {
-        var rhs = self.right;
+        var rhs = self.right.tail_node();
+        if (rhs !== self.right) {
+            var exprs = self.right.expressions.slice(0, -1);
+            exprs.push(rhs.left);
+            rhs = rhs.clone();
+            rhs.left = make_sequence(self.right, exprs);
+            self.right = rhs;
+        }
         self.left = make_node(AST_Binary, self, {
             operator: self.operator,
             left: self.left,
@@ -11808,16 +11816,7 @@ Compressor.prototype.compress = function(node) {
             // x && (y && z) ---> x && y && z
             // w || (x, y || z) ---> w || (x, y) || z
             var rhs = self.right.tail_node();
-            if (rhs instanceof AST_Binary && self.operator == rhs.operator) {
-                if (rhs !== self.right) {
-                    var exprs = self.right.expressions.slice(0, -1);
-                    exprs.push(rhs.left);
-                    rhs = rhs.clone();
-                    rhs.left = make_sequence(self.right, exprs);
-                    self.right = rhs;
-                }
-                swap_chain(self, compressor);
-            }
+            if (rhs instanceof AST_Binary && self.operator == rhs.operator) swap_chain(self, compressor);
         }
         if (compressor.option("strings") && self.operator == "+") {
             // "foo" + 42 + "" ---> "foo" + 42
@@ -11843,12 +11842,13 @@ Compressor.prototype.compress = function(node) {
                 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.left.is_constant() || !self.right.right.has_side_effects(compressor)))) {
+            // w + (x, "y" + z) ---> w + (x, "y") + z
+            var rhs = self.right.tail_node();
+            if (rhs instanceof AST_Binary
+                && self.operator == rhs.operator
+                && (self.left.is_string(compressor) && rhs.is_string(compressor)
+                    || rhs.left.is_string(compressor)
+                        && (self.left.is_constant() || !rhs.right.has_side_effects(compressor)))) {
                 swap_chain(self, compressor);
             }
         }

test/compress/concat-strings.js

@@ -273,6 +273,23 @@ concat_9: {
     expect_stdout: true
 }
 
+concat_sequence: {
+    options = {
+        collapse_vars: true,
+        strings: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        console.log(12 + (a = null, "34" + a));
+    }
+    expect: {
+        console.log(12 + "34" + null);
+    }
+    expect_stdout: "1234null"
+}
+
 issue_3689: {
     options = {
         strings: true,