From fa76365dff650e2a610daebf110472908f87cff4 Mon Sep 17 00:00:00 2001
From: sapphi-red <49056869+sapphi-red@users.noreply.github.com>
Date: Sat, 20 Sep 2025 13:02:22 +0000
Subject: [PATCH] feat(minifier): only apply `arguments` copy loop
 transformation in strict mode (#13951)

Made the arguments copy loop transformation to only apply in strict mode to reduce the assumptions held.
---
 crates/oxc_minifier/docs/ASSUMPTIONS.md         | 10 +++-------
 .../src/peephole/substitute_alternate_syntax.rs | 17 ++++++++++++++++-
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/crates/oxc_minifier/docs/ASSUMPTIONS.md b/crates/oxc_minifier/docs/ASSUMPTIONS.md
index 054c8bcbede72..88bfdc3298321 100644
--- a/crates/oxc_minifier/docs/ASSUMPTIONS.md
+++ b/crates/oxc_minifier/docs/ASSUMPTIONS.md
@@ -98,17 +98,13 @@ eval('var x = 1');
 new Function('return x');
 ```
 
-### `arguments` is always [the arguments object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments)
+### No access to a variable named `arguments` outside functions
 
-`arguments` variables are only accessed inside functions and not assigned with a different value. We intend to change this assumption to optional in the future.
+`arguments` variables are only accessed inside functions. We intend to change this assumption to optional in the future.
 
 ```javascript
 // The minifier assumes this never happens:
-console.log(arguments); // This is not the arguments object
-function f(a) {
-  arguments = 2; // This makes the arguments variable point not to point to the arguments object
-  return a;
-}
+console.log(arguments);
 ```
 
 ## Optional Assumptions
diff --git a/crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs b/crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs
index 5d1726418aa1c..0c9c976ffb48c 100644
--- a/crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs
+++ b/crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs
@@ -530,6 +530,11 @@ impl<'a> PeepholeOptimizations {
             }
         }
 
+        // In non-strict mode, a different value may be reassigned to the `arguments` variable
+        if !ctx.current_scope_flags().is_strict_mode() {
+            return;
+        }
+
         // FIXME: this function treats `arguments` not inside a function scope as if they are inside it
         //        we should check in a different way than `ctx.is_global_reference`
 
@@ -1517,12 +1522,16 @@ where
 /// Port from <https://github.com/google/closure-compiler/blob/v20240609/test/com/google/javascript/jscomp/PeepholeSubstituteAlternateSyntaxTest.java>
 #[cfg(test)]
 mod test {
+    use oxc_span::SourceType;
     use oxc_syntax::es_target::ESTarget;
 
     use crate::{
         CompressOptions, CompressOptionsUnused,
         options::CompressOptionsKeepNames,
-        tester::{default_options, test, test_options, test_same, test_same_options},
+        tester::{
+            default_options, test, test_options, test_same, test_same_options,
+            test_same_options_source_type,
+        },
     };
 
     #[test]
@@ -2346,6 +2355,12 @@ mod test {
         test_same(
             "for (var e = arguments.length, r = Array(e > 1 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a];",
         );
+
+        test_same_options_source_type(
+            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
+            SourceType::cjs(),
+            &default_options(),
+        );
     }
 
     #[test]