From 2879a4d42b18e7e9d6bc12119a799c6a8c95e108 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Donny/=EA=B0=95=EB=8F=99=EC=9C=A4?= <kdy1997.dev@gmail.com>
Date: Sun, 2 Jun 2024 12:35:43 +0900
Subject: [PATCH] fix(es/minifier): Do not index a string with a surrogate pair
 (#9013)

**Related issue:**

 - Closes #9008
---
 crates/swc_ecma_minifier/tests/exec.rs        |  5 +++++
 .../src/simplify/expr/mod.rs                  | 20 ++++++++++---------
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/crates/swc_ecma_minifier/tests/exec.rs b/crates/swc_ecma_minifier/tests/exec.rs
index 8ac36c3f02e1..30ea00ed50b2 100644
--- a/crates/swc_ecma_minifier/tests/exec.rs
+++ b/crates/swc_ecma_minifier/tests/exec.rs
@@ -11294,6 +11294,11 @@ fn issue_8964() {
     );
 }
 
+#[test]
+fn issue_9008() {
+    run_default_exec_test("console.log('💖'[0]);")
+}
+
 #[test]
 fn issue_8982_1() {
     run_default_exec_test(
diff --git a/crates/swc_ecma_transforms_optimization/src/simplify/expr/mod.rs b/crates/swc_ecma_transforms_optimization/src/simplify/expr/mod.rs
index 4823a7c59f53..87ee73f94dd3 100644
--- a/crates/swc_ecma_transforms_optimization/src/simplify/expr/mod.rs
+++ b/crates/swc_ecma_transforms_optimization/src/simplify/expr/mod.rs
@@ -159,13 +159,11 @@ impl SimplifyExpr {
 
                 // 'foo'[1]
                 KnownOp::Index(idx) if (idx as usize) < value.len() => {
-                    self.changed = true;
-
                     if idx < 0 {
+                        self.changed = true;
                         *expr = *undefined(*span)
-                    } else {
-                        let value = nth_char(value, idx as _);
-
+                    } else if let Some(value) = nth_char(value, idx as _) {
+                        self.changed = true;
                         *expr = Expr::Lit(Lit::Str(Str {
                             raw: None,
                             value: value.into(),
@@ -1661,9 +1659,13 @@ where
     ctx.preserve_effects(span, Expr::Lit(Lit::Bool(Bool { value, span })), orig)
 }
 
-fn nth_char(s: &str, mut idx: usize) -> Cow<str> {
+fn nth_char(s: &str, mut idx: usize) -> Option<Cow<str>> {
+    if s.chars().any(|c| c.len_utf16() > 1) {
+        return None;
+    }
+
     if !s.contains("\\ud") && !s.contains("\\uD") {
-        return Cow::Owned(s.chars().nth(idx).unwrap().to_string());
+        return Some(Cow::Owned(s.chars().nth(idx).unwrap().to_string()));
     }
 
     let mut iter = s.chars().peekable();
@@ -1674,7 +1676,7 @@ fn nth_char(s: &str, mut idx: usize) -> Cow<str> {
                 let mut buf = String::new();
                 buf.push('\\');
                 buf.extend(iter.take(5));
-                return Cow::Owned(buf);
+                return Some(Cow::Owned(buf));
             } else {
                 for _ in 0..5 {
                     iter.next();
@@ -1683,7 +1685,7 @@ fn nth_char(s: &str, mut idx: usize) -> Cow<str> {
         }
 
         if idx == 0 {
-            return Cow::Owned(c.to_string());
+            return Some(Cow::Owned(c.to_string()));
         }
 
         idx -= 1;