diff --git a/crates/oxc_ecmascript/src/string_char_at.rs b/crates/oxc_ecmascript/src/string_char_at.rs
index fc018f1ee4129..c155fed42b516 100644
--- a/crates/oxc_ecmascript/src/string_char_at.rs
+++ b/crates/oxc_ecmascript/src/string_char_at.rs
@@ -9,26 +9,32 @@ pub trait StringCharAt {
 impl StringCharAt for &str {
     #[expect(clippy::cast_sign_loss)]
     fn char_at(&self, index: Option<f64>) -> Option<char> {
-        let index = index.map_or(0, |x| x.to_int_32() as isize);
+        let index = index.unwrap_or(0.0);
+        if index.fract() != 0.0 || index.is_nan() || index.is_infinite() {
+            return None;
+        }
+        let index = index.to_int_32() as isize;
         if index < 0 {
             None
         } else {
-            self.chars().nth(index as usize)
+            self.encode_utf16().nth(index as usize).and_then(|n| char::from_u32(u32::from(n)))
         }
     }
 }
 
 #[cfg(test)]
 mod test {
+    use super::StringCharAt;
 
     #[test]
     fn test_evaluate_string_char_at() {
-        use crate::string_char_at::StringCharAt;
-        assert_eq!("test".char_at(Some(0.0)), Some('t'));
-        assert_eq!("test".char_at(Some(1.0)), Some('e'));
-        assert_eq!("test".char_at(Some(2.0)), Some('s'));
-        assert_eq!("test".char_at(Some(-1.0)), None);
-        assert_eq!("test".char_at(Some(-1.1)), None);
-        assert_eq!("test".char_at(Some(-1_073_741_825.0)), None);
+        let s = "test";
+        assert_eq!(s.char_at(Some(0.0)), Some('t'));
+        assert_eq!(s.char_at(Some(1.0)), Some('e'));
+        assert_eq!(s.char_at(Some(2.0)), Some('s'));
+        assert_eq!(s.char_at(Some(0.5)), None);
+        assert_eq!(s.char_at(Some(-1.0)), None);
+        assert_eq!(s.char_at(Some(-1.1)), None);
+        assert_eq!(s.char_at(Some(-1_073_741_825.0)), None);
     }
 }
diff --git a/crates/oxc_ecmascript/src/string_char_code_at.rs b/crates/oxc_ecmascript/src/string_char_code_at.rs
index 59f5eb36d33d8..5f110e2daf625 100644
--- a/crates/oxc_ecmascript/src/string_char_code_at.rs
+++ b/crates/oxc_ecmascript/src/string_char_code_at.rs
@@ -14,19 +14,21 @@ impl StringCharCodeAt for &str {
 
 #[cfg(test)]
 mod test {
+    use super::StringCharCodeAt;
 
     #[test]
     fn test_evaluate_char_code_at() {
-        use crate::StringCharCodeAt;
-
-        assert_eq!("abcde".char_code_at(Some(0.0)), Some(97));
-        assert_eq!("abcde".char_code_at(Some(1.0)), Some(98));
-        assert_eq!("abcde".char_code_at(Some(2.0)), Some(99));
-        assert_eq!("abcde".char_code_at(Some(3.0)), Some(100));
-        assert_eq!("abcde".char_code_at(Some(4.0)), Some(101));
-        assert_eq!("abcde".char_code_at(Some(5.0)), None);
-        assert_eq!("abcde".char_code_at(Some(-1.0)), None);
-        assert_eq!("abcde".char_code_at(None), Some(97));
-        assert_eq!("abcde".char_code_at(Some(0.0)), Some(97));
+        let s = "abcde";
+        assert_eq!(s.char_code_at(Some(0.0)), Some(97));
+        assert_eq!(s.char_code_at(Some(1.0)), Some(98));
+        assert_eq!(s.char_code_at(Some(2.0)), Some(99));
+        assert_eq!(s.char_code_at(Some(3.0)), Some(100));
+        assert_eq!(s.char_code_at(Some(4.0)), Some(101));
+        assert_eq!(s.char_code_at(Some(5.0)), None);
+        assert_eq!(s.char_code_at(Some(-1.0)), None);
+        assert_eq!(s.char_code_at(None), Some(97));
+        assert_eq!(s.char_code_at(Some(0.0)), Some(97));
+        assert_eq!(s.char_code_at(Some(f64::NAN)), None);
+        assert_eq!(s.char_code_at(Some(f64::INFINITY)), None);
     }
 }
diff --git a/crates/oxc_minifier/src/ast_passes/peephole_replace_known_methods.rs b/crates/oxc_minifier/src/ast_passes/peephole_replace_known_methods.rs
index 435534cbf3ec9..93628ddbd70ab 100644
--- a/crates/oxc_minifier/src/ast_passes/peephole_replace_known_methods.rs
+++ b/crates/oxc_minifier/src/ast_passes/peephole_replace_known_methods.rs
@@ -185,6 +185,7 @@ impl<'a> PeepholeReplaceKnownMethods {
         Some(ctx.ast.expression_string_literal(ce.span, result, None))
     }
 
+    #[expect(clippy::cast_lossless)]
     fn try_fold_string_char_code_at(
         ce: &CallExpression<'a>,
         object: &Expression<'a>,
@@ -196,10 +197,14 @@ impl<'a> PeepholeReplaceKnownMethods {
             Some(Argument::SpreadElement(_)) => None,
             Some(e) => Ctx(ctx).get_side_free_number_value(e.to_expression()),
         }?;
-        // TODO: if `result` is `None`, return `NaN` instead of skipping the optimization
-        let result = s.value.as_str().char_code_at(Some(char_at_index))?;
-        #[expect(clippy::cast_lossless)]
-        Some(ctx.ast.expression_numeric_literal(ce.span, result as f64, None, NumberBase::Decimal))
+        let value = if (0.0..65536.0).contains(&char_at_index) {
+            s.value.as_str().char_code_at(Some(char_at_index))? as f64
+        } else if char_at_index.is_nan() || char_at_index.is_infinite() {
+            return None;
+        } else {
+            f64::NAN
+        };
+        Some(ctx.ast.expression_numeric_literal(ce.span, value, None, NumberBase::Decimal))
     }
 
     fn try_fold_string_replace(
@@ -606,18 +611,15 @@ mod test {
         fold("x = 'abcde'.charCodeAt(2)", "x = 99");
         fold("x = 'abcde'.charCodeAt(3)", "x = 100");
         fold("x = 'abcde'.charCodeAt(4)", "x = 101");
-        fold_same("x = 'abcde'.charCodeAt(5)"); // or x = (0/0)
-        fold_same("x = 'abcde'.charCodeAt(-1)"); // or x = (0/0)
+        fold_same("x = 'abcde'.charCodeAt(5)");
+        fold("x = 'abcde'.charCodeAt(-1)", "x = NaN");
         fold_same("x = 'abcde'.charCodeAt(y)");
-        // Seems that it does not handle this case
-        // fold("x = 'abcde'.charCodeAt()", "x = 97");
+        fold("x = 'abcde'.charCodeAt()", "x = 97");
         fold("x = 'abcde'.charCodeAt(0, ++z)", "x = 97");
         fold("x = 'abcde'.charCodeAt(null)", "x = 97");
         fold("x = 'abcde'.charCodeAt(true)", "x = 98");
         // fold("x = '\\ud834\udd1e'.charCodeAt(0)", "x = 55348");
         // fold("x = '\\ud834\udd1e'.charCodeAt(1)", "x = 56606");
-
-        // Template strings
         fold_same("x = `abcdef`.charCodeAt(0)");
         fold_same("x = `abcdef ${abc}`.charCodeAt(0)");
     }