From ebe3496bdc9d8b205af86c74bba270e32617ac5f Mon Sep 17 00:00:00 2001
From: Dunqing <dengqing0821@gmail.com>
Date: Wed, 16 Apr 2025 21:26:53 +0800
Subject: [PATCH] perf(transformer/class-properties): return early if no
 private fields are found (#10418)

If a class or its parent classes don't have a private field, we don't
need to transform the expression. Because we only transform expressions
that include a private field.

Co-authored-by: Dunqing <29533304+Dunqing@users.noreply.github.com>
---
 .../oxc_transformer/src/es2022/class_properties/class.rs  | 5 ++++-
 crates/oxc_transformer/src/es2022/class_properties/mod.rs | 8 ++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/crates/oxc_transformer/src/es2022/class_properties/class.rs b/crates/oxc_transformer/src/es2022/class_properties/class.rs
index 373dae9c9c..4054d8c681 100644
--- a/crates/oxc_transformer/src/es2022/class_properties/class.rs
+++ b/crates/oxc_transformer/src/es2022/class_properties/class.rs
@@ -164,6 +164,8 @@ impl<'a> ClassProperties<'a, '_> {
             }
         }
 
+        self.private_field_count += private_props.len();
+
         // Exit if nothing to transform
         if instance_prop_count == 0
             && !has_static_prop
@@ -385,7 +387,6 @@ impl<'a> ClassProperties<'a, '_> {
         } else {
             debug_assert!(class_details.bindings.temp.is_none());
         }
-
         // Pop off stack. We're done!
         self.classes_stack.pop();
     }
@@ -444,6 +445,7 @@ impl<'a> ClassProperties<'a, '_> {
         }
 
         if let Some(private_props) = &class_details.private_props {
+            self.private_field_count -= private_props.len();
             if self.private_fields_as_properties {
                 let mut private_props = private_props
                     .iter()
@@ -588,6 +590,7 @@ impl<'a> ClassProperties<'a, '_> {
         // Babel has these always go first, regardless of order of class elements.
         // Also insert `var _prop;` temp var declarations for private static props.
         if let Some(private_props) = &class_details.private_props {
+            self.private_field_count -= private_props.len();
             // Insert `var _prop;` declarations here rather than when binding was created to maintain
             // same order of `var` declarations as Babel.
             // `c = class C { #x = 1; static y = 2; }` -> `var _C, _x;`
diff --git a/crates/oxc_transformer/src/es2022/class_properties/mod.rs b/crates/oxc_transformer/src/es2022/class_properties/mod.rs
index 141fb7d7e4..fbf026a812 100644
--- a/crates/oxc_transformer/src/es2022/class_properties/mod.rs
+++ b/crates/oxc_transformer/src/es2022/class_properties/mod.rs
@@ -277,6 +277,8 @@ pub struct ClassProperties<'a, 'ctx> {
     /// Keys are symbols' IDs.
     /// Values are initially the original name of binding, later on the name of new UID name.
     clashing_constructor_symbols: FxHashMap<SymbolId, Atom<'a>>,
+    /// Count of private fields in class and also included parent classes.
+    private_field_count: usize,
 
     // ----- State used only during exit phase -----
     //
@@ -312,6 +314,7 @@ impl<'a, 'ctx> ClassProperties<'a, 'ctx> {
             instance_inits_constructor_scope_id: None,
             // `Vec`s and `FxHashMap`s which are reused for every class being transformed
             clashing_constructor_symbols: FxHashMap::default(),
+            private_field_count: 0,
             insert_before: vec![],
             insert_after_exprs: vec![],
             insert_after_stmts: vec![],
@@ -339,6 +342,11 @@ impl<'a> Traverse<'a> for ClassProperties<'a, '_> {
     // `#[inline]` for fast exit for expressions which are not any of the transformed types
     #[inline]
     fn enter_expression(&mut self, expr: &mut Expression<'a>, ctx: &mut TraverseCtx<'a>) {
+        // Return early if no private fields are found.
+        if self.private_field_count == 0 {
+            return;
+        }
+
         match expr {
             // `object.#prop`
             Expression::PrivateFieldExpression(_) => {