From 2a49760032dea89032d6bc82dabab5e9e53bb310 Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Wed, 3 Mar 2021 10:47:17 +0000
Subject: enhance `side_effects` (#4727)

---
 lib/compress.js | 61 +++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 38 insertions(+), 23 deletions(-)

(limited to 'lib')

diff --git a/lib/compress.js b/lib/compress.js
index 1b22d861..99e67b91 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -4797,9 +4797,15 @@ merge(Compressor.prototype, {
                 || this.right.has_side_effects(compressor);
         });
         def(AST_Binary, function(compressor) {
-            return this.left.has_side_effects(compressor)
-                || this.right.has_side_effects(compressor)
-                || this.operator == "in" && !is_object(this.right);
+            var lhs = this.left;
+            if (lhs.has_side_effects(compressor)) return true;
+            var rhs = this.right;
+            var op = this.operator;
+            if (!rhs.has_side_effects(compressor)) return op == "in" && !is_object(rhs);
+            if (op == "&&" && rhs instanceof AST_PropAccess && lhs.equivalent_to(rhs.expression)) {
+                return rhs instanceof AST_Sub && rhs.property.has_side_effects(compressor);
+            }
+            return true;
         });
         def(AST_Block, function(compressor) {
             return any(this.body, compressor);
@@ -7208,33 +7214,42 @@ merge(Compressor.prototype, {
             return node;
         });
         def(AST_Binary, function(compressor, first_in_statement) {
-            if (this.operator == "in" && !is_object(this.right)) {
-                var left = this.left.drop_side_effect_free(compressor, first_in_statement);
-                if (left === this.left) return this;
+            var left = this.left;
+            var right = this.right;
+            var op = this.operator;
+            if (op == "in" && !is_object(right)) {
+                var lhs = left.drop_side_effect_free(compressor, first_in_statement);
+                if (lhs === left) return this;
                 var node = this.clone();
-                node.left = left || make_node(AST_Number, this.left, {
-                    value: 0
-                });
+                node.left = lhs || make_node(AST_Number, left, { value: 0 });
                 return node;
             }
-            var right = this.right.drop_side_effect_free(compressor, first_in_statement);
-            if (!right) return this.left.drop_side_effect_free(compressor, first_in_statement);
-            if (lazy_op[this.operator] && !(right instanceof AST_Function)) {
+            var rhs = right.drop_side_effect_free(compressor, first_in_statement);
+            if (!rhs) return left.drop_side_effect_free(compressor, first_in_statement);
+            if (lazy_op[op] && !(rhs instanceof AST_Function)) {
                 var node = this;
-                if (right !== node.right) {
-                    node = this.clone();
-                    node.right = right.drop_side_effect_free(compressor);
-                }
-                if (this.operator == "??") return node;
+                 if (op == "&&"
+                    && rhs instanceof AST_PropAccess
+                    && left.equivalent_to(rhs.expression)
+                    && !left.has_side_effects(compressor)) {
+                    var prop = rhs instanceof AST_Sub && rhs.property.drop_side_effect_free(compressor);
+                    if (!prop) return left.drop_side_effect_free(compressor, first_in_statement);
+                    node = node.clone();
+                    node.right = prop;
+                } else if (rhs !== right) {
+                    node = node.clone();
+                    node.right = rhs.drop_side_effect_free(compressor);
+                }
+                if (op == "??") return node;
                 return (first_in_statement ? best_of_statement : best_of_expression)(node, make_node(AST_Binary, this, {
-                    operator: node.operator == "&&" ? "||" : "&&",
-                    left: node.left.negate(compressor, first_in_statement),
-                    right: node.right
+                    operator: op == "&&" ? "||" : "&&",
+                    left: left.negate(compressor, first_in_statement),
+                    right: node.right,
                 }));
             } else {
-                var left = this.left.drop_side_effect_free(compressor, first_in_statement);
-                if (!left) return right;
-                return make_sequence(this, [ left, right.drop_side_effect_free(compressor) ]);
+                var lhs = left.drop_side_effect_free(compressor, first_in_statement);
+                if (!lhs) return rhs;
+                return make_sequence(this, [ lhs, rhs.drop_side_effect_free(compressor) ]);
             }
         });
         def(AST_Call, function(compressor, first_in_statement) {
-- 
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