From f4ee0f651c952eb85753633a990a598267981bcb Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Mon, 8 Mar 2021 20:40:21 +0000
Subject: [PATCH] fix corner case in `unsafe` (#4755)

---
 lib/compress.js           | 22 +++++++++++-----------
 test/compress/evaluate.js | 11 ++++++++---
 2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index f63c1869..a92ad97d 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -8581,17 +8581,17 @@ merge(Compressor.prototype, {
                 break;
               case "charAt":
                 if (self.args.length < 2) {
-                    var node = make_node(AST_Sub, self, {
-                        expression: exp.expression,
-                        property: self.args.length ? make_node(AST_Binary, self.args[0], {
-                            operator: "|",
-                            left: make_node(AST_Number, self, {
-                                value: 0
-                            }),
-                            right: self.args[0]
-                        }) : make_node(AST_Number, self, {
-                            value: 0
-                        })
+                    var node = make_node(AST_Binary, self, {
+                        operator: "||",
+                        left: make_node(AST_Sub, self, {
+                            expression: exp.expression,
+                            property: self.args.length ? make_node(AST_Binary, self.args[0], {
+                                operator: "|",
+                                left: make_node(AST_Number, self, { value: 0 }),
+                                right: self.args[0],
+                            }) : make_node(AST_Number, self, { value: 0 }),
+                        }).optimize(compressor),
+                        right: make_node(AST_String, self, { value: "" }),
                     });
                     node.is_string = return_true;
                     return node.optimize(compressor);
diff --git a/test/compress/evaluate.js b/test/compress/evaluate.js
index 597c7c87..c92e82bd 100644
--- a/test/compress/evaluate.js
+++ b/test/compress/evaluate.js
@@ -847,6 +847,8 @@ unsafe_charAt_noop: {
         unsafe: true,
     }
     input: {
+        s = "foo";
+        x = 42;
         console.log(
             s.charAt(0),
             "string".charAt(x),
@@ -854,12 +856,15 @@ unsafe_charAt_noop: {
         );
     }
     expect: {
+        s = "foo";
+        x = 42;
         console.log(
-            s[0],
-            "string"[0 | x],
-            (typeof x)[0]
+            s[0] || "",
+            "string"[0 | x] || "",
+            (typeof x)[0] || ""
         );
     }
+    expect_stdout: "f  n"
 }
 
 issue_1649: {
-- 
2.34.1