From 91d9289c49815cad5b8f7c840e4d8cf3a3a322e3 Mon Sep 17 00:00:00 2001
From: Rob Hogan <robhogan@meta.com>
Date: Tue, 3 Mar 2026 07:02:20 -0800
Subject: [PATCH] Babel preset: Add unstable_preserveClassPrivate to experiment
 with disabling private class transforms for SH (#55880)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55880

Disable `babel/plugin-transform-private-methods` and `babel/plugin-transform-private-property-in-object` when `customTransformOptions.unstable_preserveClassPrivate` is truthy. This allows us to experiment with native private class field and method support in Static Hermes.

Changelog: [Internal]

Reviewed By: vzaidman, javache

Differential Revision: D93010263
---
 .../src/__tests__/transform-snapshot-test.js  | 36 +++++++++++++++++++
 .../src/configs/main.js                       | 23 +++++++++---
 2 files changed, 54 insertions(+), 5 deletions(-)

diff --git a/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js b/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js
index 430d0a78e25a..8451562fe50d 100644
--- a/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js
+++ b/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js
@@ -352,5 +352,41 @@ describe('react-native-babel-preset transform snapshots', () => {
       expect(result).toContain('import');
       expect(result).toContain('export');
     });
+
+    it('preserves private class fields with unstable_preserveClassPrivate', () => {
+      const code = `
+        class Counter {
+          #count = 0;
+          #privateMethod() { return this.#count; }
+          increment() { this.#count++; }
+        }
+      `;
+      const result = transformCode(code, {
+        dev: false,
+        unstable_transformProfile: 'hermes-stable',
+        customTransformOptions: {
+          unstable_preserveClassPrivate: true,
+        },
+      });
+      expect(result).toContain('#count');
+      expect(result).toContain('#privateMethod');
+    });
+
+    it('transforms private class fields without unstable_preserveClassPrivate', () => {
+      const code = `
+        class Counter {
+          #count = 0;
+          #privateMethod() { return this.#count; }
+          increment() { this.#count++; }
+        }
+      `;
+      const result = transformCode(code, {
+        dev: false,
+        unstable_transformProfile: 'hermes-stable',
+      });
+      expect(result).not.toContain('#count');
+      expect(result).not.toContain('#privateMethod');
+      expect(result).toContain('_classPrivateFieldLooseKey');
+    });
   });
 });
diff --git a/packages/react-native-babel-preset/src/configs/main.js b/packages/react-native-babel-preset/src/configs/main.js
index ad3dfe462803..5229c59ca9f7 100644
--- a/packages/react-native-babel-preset/src/configs/main.js
+++ b/packages/react-native-babel-preset/src/configs/main.js
@@ -21,6 +21,10 @@ const EXCLUDED_FIRST_PARTY_PATHS = [
   /[/\\]private[/\\]react-native-fantom[/\\]/,
 ];
 
+// customTransformOptions may be strings from URL params, or booleans passed
+// programatically. For strings, handle them as Metro does when parsing URLs.
+const TRUE_VALS = new Set([true, 'true', '1']);
+
 function isTypeScriptSource(fileName) {
   return !!fileName && fileName.endsWith('.ts');
 }
@@ -73,6 +77,11 @@ const getPreset = (src, options, babel) => {
   // Preserve class syntax and related if we're using Hermes V1.
   const preserveClasses = isHermesV1;
 
+  // Preserve private class fields and methods if the experiment is enabled.
+  const preserveClassPrivate = TRUE_VALS.has(
+    options?.customTransformOptions?.unstable_preserveClassPrivate,
+  );
+
   const isNull = src == null;
   const hasClass = isNull || src.indexOf('class') !== -1;
 
@@ -217,11 +226,15 @@ const getPreset = (src, options, babel) => {
           ...(preserveClasses
             ? []
             : [[require('@babel/plugin-transform-class-properties'), {loose}]]),
-          [require('@babel/plugin-transform-private-methods'), {loose}],
-          [
-            require('@babel/plugin-transform-private-property-in-object'),
-            {loose},
-          ],
+          ...(preserveClassPrivate
+            ? []
+            : [
+                [require('@babel/plugin-transform-private-methods'), {loose}],
+                [
+                  require('@babel/plugin-transform-private-property-in-object'),
+                  {loose},
+                ],
+              ]),
           [require('@babel/plugin-syntax-dynamic-import')],
           [require('@babel/plugin-syntax-export-default-from')],
           ...passthroughSyntaxPlugins,