react · robhogan · May 14, 2026
diff --git a/...abel-preset/src/__tests__/__fixtures__/output/hermes-stable-dev-preserve-block-scoping.js b/...abel-preset/src/__tests__/__fixtures__/output/hermes-stable-dev-preserve-block-scoping.js
@@ -0,0 +1,284 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @generated
+ * @noformat
+ * @noflow
+ * @nolint
+ *
+ * This is a snapshot of the transform output for testing purposes.
+ * To update, run: js1 test transform-snapshot-test.js -u
+ *
+ * Transform configuration:
+ *   - Hermes stable transform profile in development mode with unstable_preserveBlockScoping enabled
+ *   - Options: {"dev":true,"unstable_transformProfile":"hermes-stable","customTransformOptions":{"unstable_preserveBlockScoping":true}}
+ */
+
+var _interopRequireDefault = require("@babel/runtime/helpers/interopRequireDefault");
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.LegacyComponent = exports.Dog = exports.Counter = exports.Animal = void 0;
+exports.ModernComponent = ModernComponent;
+exports.MyClass = void 0;
+exports.asyncNumberGenerator = asyncNumberGenerator;
+Object.defineProperty(exports, "default", {
+  enumerable: true,
+  get: function () {
+    return _dataUtils.fetchData;
+  }
+});
+exports.getNestedValue = getNestedValue;
+exports.loadModule = loadModule;
+exports.matchEmoji = matchEmoji;
+exports.mergeConfigs = mergeConfigs;
+exports.parseDate = parseDate;
+exports.processUser = processUser;
+exports.safeJsonParse = safeJsonParse;
+exports.sumPairs = sumPairs;
+var _inherits2 = _interopRequireDefault(require("@babel/runtime/helpers/inherits"));
+var _setPrototypeOf2 = _interopRequireDefault(require("@babel/runtime/helpers/setPrototypeOf"));
+var _slicedToArray2 = _interopRequireDefault(require("@babel/runtime/helpers/slicedToArray"));
+var _asyncToGenerator2 = _interopRequireDefault(require("@babel/runtime/helpers/asyncToGenerator"));
+var _classPrivateFieldLooseBase2 = _interopRequireDefault(require("@babel/runtime/helpers/classPrivateFieldLooseBase"));
+var _classPrivateFieldLooseKey2 = _interopRequireDefault(require("@babel/runtime/helpers/classPrivateFieldLooseKey"));
+var _awaitAsyncGenerator2 = _interopRequireDefault(require("@babel/runtime/helpers/awaitAsyncGenerator"));
+var _wrapAsyncGenerator2 = _interopRequireDefault(require("@babel/runtime/helpers/wrapAsyncGenerator"));
+var _react = _interopRequireWildcard(require("react"));
+var React = _react;
+var _jsxRuntime = require("react/jsx-runtime");
+var _dataUtils = require("./data-utils");
+var _jsxFileName = "/absolute/path/to/input.js";
+function _interopRequireWildcard(e, t) { if ("function" == typeof WeakMap) var r = new WeakMap(), n = new WeakMap(); return (_interopRequireWildcard = function (e, t) { if (!t && e && e.__esModule) return e; var o, i, f = { __proto__: null, default: e }; if (null === e || "object" != typeof e && "function" != typeof e) return f; if (o = t ? n : r) { if (o.has(e)) return o.get(e); o.set(e, f); } for (const t in e) "default" !== t && {}.hasOwnProperty.call(e, t) && ((i = (o = Object.defineProperty) && Object.getOwnPropertyDescriptor(e, t)) && (i.get || i.set) ? o(f, t, i) : f[t] = e[t]); return f; })(e, t); }
+function _wrapRegExp() { _wrapRegExp = function (e, r) { return new BabelRegExp(e, void 0, r); }; var e = RegExp.prototype, r = new WeakMap(); function BabelRegExp(e, t, p) { var o = RegExp(e, t); return r.set(o, p || r.get(e)), (0, _setPrototypeOf2.default)(o, BabelRegExp.prototype); } function buildGroups(e, t) { var p = r.get(t); return Object.keys(p).reduce(function (r, t) { var o = p[t]; if ("number" == typeof o) r[t] = e[o];else { for (var i = 0; void 0 === e[o[i]] && i + 1 < o.length;) i++; r[t] = e[o[i]]; } return r; }, Object.create(null)); } return (0, _inherits2.default)(BabelRegExp, RegExp), BabelRegExp.prototype.exec = function (r) { var t = e.exec.call(this, r); if (t) { t.groups = buildGroups(t, this); var p = t.indices; p && (p.groups = buildGroups(p, this)); } return t; }, BabelRegExp.prototype[Symbol.replace] = function (t, p) { if ("string" == typeof p) { var o = r.get(this); return e[Symbol.replace].call(this, t, p.replace(/\$<([^>]+)(>|$)/g, function (e, r, t) { if ("" === t) return e; var p = o[r]; return Array.isArray(p) ? "$" + p.join("$") : "number" == typeof p ? "$" + p : ""; })); } if ("function" == typeof p) { var i = this; return e[Symbol.replace].call(this, t, function () { var e = arguments; return "object" != typeof e[e.length - 1] && (e = [].slice.call(e)).push(buildGroups(e, i)), p.apply(this, e); }); } return e[Symbol.replace].call(this, t, p); }, _wrapRegExp.apply(this, arguments); }
+function _createForOfIteratorHelperLoose(r, e) { var t = "undefined" != typeof Symbol && r[Symbol.iterator] || r["@@iterator"]; if (t) return (t = t.call(r)).next.bind(t); if (Array.isArray(r) || (t = _unsupportedIterableToArray(r)) || e && r && "number" == typeof r.length) { t && (r = t); var o = 0; return function () { return o >= r.length ? { done: !0 } : { done: !1, value: r[o++] }; }; } throw new TypeError("Invalid attempt to iterate non-iterable instance.\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method."); }
+function _unsupportedIterableToArray(r, a) { if (r) { if ("string" == typeof r) return _arrayLikeToArray(r, a); var t = {}.toString.call(r).slice(8, -1); return "Object" === t && r.constructor && (t = r.constructor.name), "Map" === t || "Set" === t ? Array.from(r) : "Arguments" === t || /^(?:Ui|I)nt(?:8|16|32)(?:Clamped)?Array$/.test(t) ? _arrayLikeToArray(r, a) : void 0; } }
+function _arrayLikeToArray(r, a) { (null == a || a > r.length) && (a = r.length); for (var e = 0, n = Array(a); e < a; e++) n[e] = r[e]; return n; }
+var _count = (0, _classPrivateFieldLooseKey2.default)("count");
+var _instances = (0, _classPrivateFieldLooseKey2.default)("instances");
+var _increment = (0, _classPrivateFieldLooseKey2.default)("increment");
+class Counter {
+  constructor() {
+    Object.defineProperty(this, _increment, {
+      value: _increment2
+    });
+    Object.defineProperty(this, _count, {
+      writable: true,
+      value: 0
+    });
+    (0, _classPrivateFieldLooseBase2.default)(Counter, _instances)[_instances]++;
+  }
+  get value() {
+    return (0, _classPrivateFieldLooseBase2.default)(this, _count)[_count];
+  }
+  increment() {
+    (0, _classPrivateFieldLooseBase2.default)(this, _increment)[_increment]();
+  }
+  static get instanceCount() {
+    return (0, _classPrivateFieldLooseBase2.default)(Counter, _instances)[_instances];
+  }
+}
+exports.Counter = Counter;
+function _increment2() {
+  (0, _classPrivateFieldLooseBase2.default)(this, _count)[_count]++;
+}
+Object.defineProperty(Counter, _instances, {
+  writable: true,
+  value: 0
+});
+function asyncNumberGenerator(_x) {
+  return _asyncNumberGenerator.apply(this, arguments);
+}
+function _asyncNumberGenerator() {
+  _asyncNumberGenerator = (0, _wrapAsyncGenerator2.default)(function* (max) {
+    for (let i = 0; i < max; i++) {
+      yield (0, _awaitAsyncGenerator2.default)(new Promise(resolve => setTimeout(resolve, 100)));
+      yield i;
+    }
+  });
+  return _asyncNumberGenerator.apply(this, arguments);
+}
+function fetchData(_x2) {
+  return _fetchData.apply(this, arguments);
+}
+function _fetchData() {
+  _fetchData = (0, _asyncToGenerator2.default)(function* (url) {
+    const response = yield fetch(url);
+    const data = yield response.json();
+    return {
+      data
+    };
+  });
+  return _fetchData.apply(this, arguments);
+}
+function getNestedValue(obj) {
+  var _obj$a$b$c, _obj$a;
+  return (_obj$a$b$c = obj == null || (_obj$a = obj.a) == null || (_obj$a = _obj$a.b) == null ? void 0 : _obj$a.c) != null ? _obj$a$b$c : 42;
+}
+class Animal {
+  #age;
+  constructor(name, age) {
+    this.name = name;
+    this.#age = age;
+  }
+  speak() {
+    return `${this.name} makes a sound`;
+  }
+  get age() {
+    return this.#age;
+  }
+}
+exports.Animal = Animal;
+class Dog extends Animal {
+  constructor(name, age, breed) {
+    super(name, age);
+    this.breed = breed;
+  }
+  speak() {
+    return `${this.name} barks!`;
+  }
+  fetchTreats() {
+    return (0, _asyncToGenerator2.default)(function* () {
+      yield new Promise(resolve => setTimeout(resolve, 100));
+      return ['bone', 'biscuit', 'toy'];
+    })();
+  }
+}
+exports.Dog = Dog;
+function processUser({
+  name,
+  age = 18,
+  ...rest
+}) {
+  const _rest$city = rest.city,
+    city = _rest$city === void 0 ? 'Unknown' : _rest$city;
+  return `${name} (${age}) from ${city}`;
+}
+function mergeConfigs(base, ...overrides) {
+  return {
+    ...base,
+    ...overrides.reduce((acc, o) => ({
+      ...acc,
+      ...o
+    }), {})
+  };
+}
+function sumPairs(pairs) {
+  let total = 0;
+  for (var _iterator = _createForOfIteratorHelperLoose(pairs), _step; !(_step = _iterator()).done;) {
+    const _ref = _step.value;
+    var _ref2 = (0, _slicedToArray2.default)(_ref, 2);
+    const a = _ref2[0];
+    const b = _ref2[1];
+    total += a + b;
+  }
+  return total;
+}
+function parseDate(dateString) {
+  const regex = _wrapRegExp(/(\d{4})-(\d{2})-(\d{2})/, {
+    year: 1,
+    month: 2,
+    day: 3
+  });
+  const match = dateString.match(regex);
+  if (match != null && match.groups) {
+    return {
+      year: match.groups.year,
+      month: match.groups.month,
+      day: match.groups.day
+    };
+  }
+  return null;
+}
+function safeJsonParse(input) {
+  try {
+    return JSON.parse(input);
+  } catch (_unused) {
+    return null;
+  }
+}
+function matchEmoji(text) {
+  const match = text.match(/(?:[#\*0-9\xA9\xAE\u203C\u2049\u2122\u2139\u2194-\u2199\u21A9\u21AA\u231A\u231B\u2328\u23CF\u23E9-\u23F3\u23F8-\u23FA\u24C2\u25AA\u25AB\u25B6\u25C0\u25FB-\u25FE\u2600-\u2604\u260E\u2611\u2614\u2615\u2618\u261D\u2620\u2622\u2623\u2626\u262A\u262E\u262F\u2638-\u263A\u2640\u2642\u2648-\u2653\u265F\u2660\u2663\u2665\u2666\u2668\u267B\u267E\u267F\u2692-\u2697\u2699\u269B\u269C\u26A0\u26A1\u26A7\u26AA\u26AB\u26B0\u26B1\u26BD\u26BE\u26C4\u26C5\u26C8\u26CE\u26CF\u26D1\u26D3\u26D4\u26E9\u26EA\u26F0-\u26F5\u26F7-\u26FA\u26FD\u2702\u2705\u2708-\u270D\u270F\u2712\u2714\u2716\u271D\u2721\u2728\u2733\u2734\u2744\u2747\u274C\u274E\u2753-\u2755\u2757\u2763\u2764\u2795-\u2797\u27A1\u27B0\u27BF\u2934\u2935\u2B05-\u2B07\u2B1B\u2B1C\u2B50\u2B55\u3030\u303D\u3297\u3299]|\uD83C[\uDC04\uDCCF\uDD70\uDD71\uDD7E\uDD7F\uDD8E\uDD91-\uDD9A\uDDE6-\uDDFF\uDE01\uDE02\uDE1A\uDE2F\uDE32-\uDE3A\uDE50\uDE51\uDF00-\uDF21\uDF24-\uDF93\uDF96\uDF97\uDF99-\uDF9B\uDF9E-\uDFF0\uDFF3-\uDFF5\uDFF7-\uDFFF]|\uD83D[\uDC00-\uDCFD\uDCFF-\uDD3D\uDD49-\uDD4E\uDD50-\uDD67\uDD6F\uDD70\uDD73-\uDD7A\uDD87\uDD8A-\uDD8D\uDD90\uDD95\uDD96\uDDA4\uDDA5\uDDA8\uDDB1\uDDB2\uDDBC\uDDC2-\uDDC4\uDDD1-\uDDD3\uDDDC-\uDDDE\uDDE1\uDDE3\uDDE8\uDDEF\uDDF3\uDDFA-\uDE4F\uDE80-\uDEC5\uDECB-\uDED2\uDED5-\uDED7\uDEDC-\uDEE5\uDEE9\uDEEB\uDEEC\uDEF0\uDEF3-\uDEFC\uDFE0-\uDFEB\uDFF0]|\uD83E[\uDD0C-\uDD3A\uDD3C-\uDD45\uDD47-\uDDFF\uDE70-\uDE7C\uDE80-\uDE89\uDE8F-\uDEC6\uDECE-\uDEDC\uDEDF-\uDEE9\uDEF0-\uDEF8])/);
+  return match == null ? void 0 : match[0];
+}
+const MyClass = class {
+  constructor(value) {
+    this.value = value;
+  }
+};
+exports.MyClass = MyClass;
+function loadModule() {
+  return _loadModule.apply(this, arguments);
+}
+function _loadModule() {
+  _loadModule = (0, _asyncToGenerator2.default)(function* () {
+    const module = yield import('./some-module');
+    return module.default;
+  });
+  return _loadModule.apply(this, arguments);
+}
+const LegacyComponent = exports.LegacyComponent = React.createClass({
+  displayName: 'LegacyComponent',
+  getInitialState() {
+    return {
+      count: 0
+    };
+  },
+  render() {
+    return (0, _jsxRuntime.jsx)("div", {
+      children: this.state.count
+    });
+  }
+});
+function ModernComponent({
+  initialCount = 0
+}) {
+  const _useState = (0, _react.useState)(initialCount),
+    _useState2 = (0, _slicedToArray2.default)(_useState, 2),
+    count = _useState2[0],
+    setCount = _useState2[1];
+  const _useState3 = (0, _react.useState)(Status.Active),
+    _useState4 = (0, _slicedToArray2.default)(_useState3, 2),
+    status = _useState4[0],
+    setStatus = _useState4[1];
+  (0, _react.useEffect)(() => {
+    const timer = setInterval(() => {
+      setCount(c => c + 1);
+    }, 1000);
+    return () => clearInterval(timer);
+  }, []);
+  function handleAsyncClick() {
+    return _handleAsyncClick.apply(this, arguments);
+  }
+  function _handleAsyncClick() {
+    _handleAsyncClick = (0, _asyncToGenerator2.default)(function* () {
+      const data = yield fetchData('/api/data');
+      console.log(data);
+    });
+    return _handleAsyncClick.apply(this, arguments);
+  }
+  const handleClick = function () {
+    var _ref3 = (0, _asyncToGenerator2.default)(function* () {
+      yield handleAsyncClick();
+      setStatus(Status.Pending);
+    });
+    return function handleClick() {
+      return _ref3.apply(this, arguments);
+    };
+  }();
+  return (0, _jsxRuntime.jsxs)("div", {
+    children: [(0, _jsxRuntime.jsx)("span", {
+      "data-testid": "count",
+      children: count
+    }), (0, _jsxRuntime.jsx)("span", {
+      "data-testid": "status",
+      children: String(status)
+    }), (0, _jsxRuntime.jsx)("button", {
+      onClick: handleClick,
+      children: "Increment"
+    }), (0, _jsxRuntime.jsx)(LegacyComponent, {})]
+  });
+}
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
@@ -129,6 +129,18 @@ const testConfigs = [
     description:
       'Hermes stable transform profile in development mode with unstable_preserveAsync enabled',
   },
+  {
+    name: 'hermes-stable-dev-preserve-block-scoping',
+    options: {
+      dev: true,
+      unstable_transformProfile: 'hermes-stable',
+      customTransformOptions: {
+        unstable_preserveBlockScoping: true,
+      },
+    },
+    description:
+      'Hermes stable transform profile in development mode with unstable_preserveBlockScoping enabled',
+  },
 ];
 
 function transformCode(
@@ -451,5 +463,44 @@ describe('react-native-babel-preset transform snapshots', () => {
       });
       expect(result).toContain('_asyncToGenerator');
     });
+
+    it('preserves block scoping with unstable_preserveBlockScoping', () => {
+      const code = `
+        let x = 1;
+        const y = 2;
+        {
+          let x = 3;
+          const z = 4;
+        }
+      `;
+      const result = transformCode(code, {
+        dev: false,
+        unstable_transformProfile: 'hermes-stable',
+        customTransformOptions: {
+          unstable_preserveBlockScoping: true,
+        },
+      });
+      expect(result).toContain('let x = 1');
+      expect(result).toContain('const y = 2');
+      expect(result).toContain('let x = 3');
+      expect(result).toContain('const z = 4');
+    });
+
+    it('transforms block scoping without unstable_preserveBlockScoping', () => {
+      const code = `
+        let x = 1;
+        const y = 2;
+        {
+          let x = 3;
+        }
+      `;
+      const result = transformCode(code, {
+        dev: false,
+        unstable_transformProfile: 'hermes-stable',
+      });
+      expect(result).toContain('var ');
+      expect(result).not.toContain('let ');
+      expect(result).not.toContain('const ');
+    });
   });
 });
diff --git a/packages/react-native-babel-preset/src/configs/main.js b/packages/react-native-babel-preset/src/configs/main.js
@@ -87,6 +87,11 @@ const getPreset = (src, options, babel) => {
     options?.customTransformOptions?.unstable_preserveAsync,
   );
 
+  // Preserve block scoping (let/const) if the experiment is enabled.
+  const preserveBlockScoping = TRUE_VALS.has(
+    options?.customTransformOptions?.unstable_preserveBlockScoping,
+  );
+
   const isNull = src == null;
   const hasClass = isNull || src.indexOf('class') !== -1;
 
@@ -227,7 +232,9 @@ const getPreset = (src, options, babel) => {
             },
           ],
           [require('babel-plugin-transform-flow-enums')],
-          [require('@babel/plugin-transform-block-scoping')],
+          ...(preserveBlockScoping
+            ? []
+            : [[require('@babel/plugin-transform-block-scoping')]]),
           ...(preserveClasses
             ? []
             : [[require('@babel/plugin-transform-class-properties'), {loose}]]),