facebook · kassens · Aug 25, 2025 · Aug 20, 2025
diff --git a/.eslintrc.js b/.eslintrc.js
@@ -583,8 +583,11 @@ module.exports = {
     mixin$Animatable: 'readonly',
     MouseEventHandler: 'readonly',
     NavigateEvent: 'readonly',
+    PerformanceMeasureOptions: 'readonly',
     PropagationPhases: 'readonly',
     PropertyDescriptor: 'readonly',
+    PropertyDescriptorMap: 'readonly',
+    Proxy$traps: 'readonly',
     React$Component: 'readonly',
     React$Config: 'readonly',
     React$Context: 'readonly',

diff --git a/package.json b/package.json
@@ -74,15 +74,15 @@
     "eslint-plugin-react-internal": "link:./scripts/eslint-rules",
     "fbjs-scripts": "^3.0.1",
     "filesize": "^6.0.1",
-    "flow-bin": "^0.274",
-    "flow-remove-types": "^2.274",
+    "flow-bin": "^0.279.0",
+    "flow-remove-types": "^2.279.0",
     "flow-typed": "^4.1.1",
     "glob": "^7.1.6",
     "glob-stream": "^6.1.0",
     "google-closure-compiler": "^20230206.0.0",
     "gzip-size": "^5.1.1",
-    "hermes-eslint": "^0.25.1",
-    "hermes-parser": "^0.25.1",
+    "hermes-eslint": "^0.32.0",
+    "hermes-parser": "^0.32.0",
     "jest": "^29.4.2",
     "jest-cli": "^29.4.2",
     "jest-diff": "^29.4.2",

diff --git a/packages/react-client/src/ReactFlightPerformanceTrack.js b/packages/react-client/src/ReactFlightPerformanceTrack.js
@@ -153,7 +153,7 @@ export function logComponentAborted(
     const entryName =
       isPrimaryEnv || env === undefined ? name : name + ' [' + env + ']';
     if (__DEV__) {
-      const properties = [
+      const properties: Array<[string, string]> = [
         [
           'Aborted',
           'The stream was aborted before this Component finished rendering.',
@@ -215,7 +215,7 @@ export function logComponentErrored(
             String(error.message)
           : // eslint-disable-next-line react-internal/safe-string-coercion
             String(error);
-      const properties = [['Error', message]];
+      const properties: Array<[string, string]> = [['Error', message]];
       if (componentInfo.key != null) {
         addValueToProperties('key', componentInfo.key, properties, 0, '');
       }

diff --git a/packages/react-debug-tools/src/ReactDebugHooks.js b/packages/react-debug-tools/src/ReactDebugHooks.js
@@ -789,7 +789,7 @@ const Dispatcher: DispatcherType = {
 
 // create a proxy to throw a custom error
 // in case future versions of React adds more hooks
-const DispatcherProxyHandler = {
+const DispatcherProxyHandler: Proxy$traps<DispatcherType> = {
   get(target: DispatcherType, prop: string) {
     if (target.hasOwnProperty(prop)) {
       // $FlowFixMe[invalid-computed-prop]

diff --git a/packages/react-devtools-shared/src/backend/fiber/renderer.js b/packages/react-devtools-shared/src/backend/fiber/renderer.js
@@ -2217,7 +2217,7 @@ export function attach(
     }
     if (typeof instance.getClientRects === 'function') {
       // DOM
-      const result = [];
+      const result: Array<Rect> = [];
       const doc = instance.ownerDocument;
       const win = doc && doc.defaultView;
       const scrollX = win ? win.scrollX : 0;

diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementHooksTree.js b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementHooksTree.js
@@ -223,7 +223,10 @@ function HookView({
   const hookDisplayName = hookName ? (
     <>
       {name}
-      {!!hookName && <span className={styles.HookName}>({hookName})</span>}
+      {
+        // $FlowFixMe[constant-condition]
+        !!hookName && <span className={styles.HookName}>({hookName})</span>
+      }
     </>
   ) : (
     name

diff --git a/packages/react-devtools-shared/src/devtools/views/ModalDialog.js b/packages/react-devtools-shared/src/devtools/views/ModalDialog.js
@@ -75,7 +75,7 @@ function dialogReducer(state: State, action: Action) {
             content: action.content,
             id: action.id,
             title: action.title || null,
-          },
+          } as Dialog,
         ],
       };
     default:

diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js b/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
@@ -64,7 +64,7 @@ function getTooltipPosition(
     mouseY: number,
     width: number,
   },
-) {
+): {left: string, top: string} {
   const {height, mouseX, mouseY, width} = mousePosition;
   let top: number | string = 0;
   let left: number | string = 0;
@@ -108,7 +108,13 @@ function getMousePosition(
       }
     }
 
-    const {height, left, top, width} = targetContainer.getBoundingClientRect();
+    const {height, left, top, width} =
+      targetContainer.getBoundingClientRect() as {
+        height: number,
+        left: number,
+        top: number,
+        width: number,
+      };
 
     const mouseX = mouseEvent.clientX - left;
     const mouseY = mouseEvent.clientY - top;

diff --git a/packages/react-devtools-shared/src/hooks/SourceMapConsumer.js b/packages/react-devtools-shared/src/hooks/SourceMapConsumer.js
@@ -145,13 +145,21 @@ type Section = {
 };
 
 function IndexedSourceMapConsumer(sourceMapJSON: IndexSourceMap) {
-  let lastOffset = {
+  let lastOffset: {
+    line: number,
+    column: number,
+    ...
+  } = {
     line: -1,
     column: 0,
   };
 
   const sections: Array<Section> = sourceMapJSON.sections.map(section => {
-    const offset = section.offset;
+    const offset: {
+      line: number,
+      column: number,
+      ...
+    } = section.offset;
     const offsetLine = offset.line;
     const offsetColumn = offset.column;
 

diff --git a/packages/react-devtools-shell/src/app/InspectableElements/Contexts.js b/packages/react-devtools-shell/src/app/InspectableElements/Contexts.js
@@ -28,7 +28,7 @@ const contextData = {
   bool: true,
   func: someNamedFunction,
   number: 123,
-  object: {outer: {inner: {}}},
+  object: {outer: {inner: {} as {...}}},
   string: 'abc',
   symbol: Symbol.for('symbol'),
   null: null,

diff --git a/packages/react-devtools-timeline/src/import-worker/preprocessData.js b/packages/react-devtools-timeline/src/import-worker/preprocessData.js
@@ -28,6 +28,7 @@ import type {
   TimelineData,
   SchedulingEvent,
   SuspenseEvent,
+  Snapshot,
 } from '../types';
 import {
   REACT_TOTAL_NUM_LANES,
@@ -350,7 +351,7 @@ function processScreenshot(
 ) {
   const encodedSnapshot = event.args.snapshot; // Base 64 encoded
 
-  const snapshot = {
+  const snapshot: Snapshot = {
     height: 0,
     image: null,
     imageSource: `data:image/png;base64,${encodedSnapshot}`,

diff --git a/...t-devtools-timeline/src/view-base/vertical-scroll-overflow/withVerticalScrollbarLayout.js b/...t-devtools-timeline/src/view-base/vertical-scroll-overflow/withVerticalScrollbarLayout.js
@@ -7,7 +7,7 @@
  * @flow
  */
 
-import type {Layouter} from '../layouter';
+import type {LayoutInfo, Layouter} from '../layouter';
 
 /**
  * Assumes {@param layout} will only contain 2 views.
@@ -25,7 +25,7 @@ export const withVerticalScrollbarLayout: Layouter = (
     ? scrollbarLayoutInfo.view.desiredSize().width
     : 0;
 
-  const laidOutContentLayoutInfo = {
+  const laidOutContentLayoutInfo: LayoutInfo = {
     ...contentLayoutInfo,
     frame: {
       origin: contentLayoutInfo.view.frame.origin,
@@ -35,7 +35,7 @@ export const withVerticalScrollbarLayout: Layouter = (
       },
     },
   };
-  const laidOutScrollbarLayoutInfo = {
+  const laidOutScrollbarLayoutInfo: LayoutInfo = {
     ...scrollbarLayoutInfo,
     frame: {
       origin: {

diff --git a/packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js b/packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js
@@ -2274,6 +2274,7 @@ function mergeTranslate(translateA: ?string, translateB: ?string): string {
     return translateB || '';
   }
   if (!translateB || translateB === 'none') {
+    // $FlowFixMe[constant-condition]
     return translateA || '';
   }
   const partsA = translateA.split(' ');

diff --git a/packages/react-reconciler/src/ReactCapturedValue.js b/packages/react-reconciler/src/ReactCapturedValue.js
@@ -30,7 +30,7 @@ export function createCapturedValueAtFiber<T>(
     if (existing !== undefined) {
       return existing;
     }
-    const captured = {
+    const captured: CapturedValue<T> = {
       value,
       source,
       stack: getStackByFiberInDevAndProd(source),

diff --git a/packages/react-reconciler/src/ReactFiberPerformanceTrack.js b/packages/react-reconciler/src/ReactFiberPerformanceTrack.js
@@ -179,7 +179,7 @@ const reusableComponentDevToolDetails = {
   track: COMPONENTS_TRACK,
 };
 
-const reusableComponentOptions = {
+const reusableComponentOptions: PerformanceMeasureOptions = {
   start: -0,
   end: -0,
   detail: {
@@ -351,7 +351,7 @@ export function logComponentErrored(
         // error boundary itself.
         debugTask = fiber._debugTask;
       }
-      const options = {
+      const options: PerformanceMeasureOptions = {
         start: startTime,
         end: endTime,
         detail: {
@@ -992,7 +992,7 @@ export function logRecoveredRenderPhase(
               String(error);
         properties.push(['Recoverable Error', message]);
       }
-      const options = {
+      const options: PerformanceMeasureOptions = {
         start: startTime,
         end: endTime,
         detail: {
@@ -1199,7 +1199,7 @@ export function logCommitErrored(
               String(error);
         properties.push(['Error', message]);
       }
-      const options = {
+      const options: PerformanceMeasureOptions = {
         start: startTime,
         end: endTime,
         detail: {

diff --git a/packages/react-server-dom-esm/src/ReactFlightESMReferences.js b/packages/react-server-dom-esm/src/ReactFlightESMReferences.js
@@ -66,7 +66,7 @@ function bind(this: ServerReference<any>): any {
     const $$bound = {value: this.$$bound ? this.$$bound.concat(args) : args};
     return Object.defineProperties(
       (newFn: any),
-      __DEV__
+      (__DEV__
         ? {
             $$typeof,
             $$id,
@@ -82,7 +82,7 @@ function bind(this: ServerReference<any>): any {
             $$id,
             $$bound,
             bind: {value: bind, configurable: true},
-          },
+          }) as PropertyDescriptorMap,
     );
   }
   return newFn;
@@ -101,7 +101,7 @@ export function registerServerReference<T: Function>(
   const $$bound = {value: null, configurable: true};
   return Object.defineProperties(
     (reference: any),
-    __DEV__
+    (__DEV__
       ? {
           $$typeof,
           $$id,
@@ -117,6 +117,6 @@ export function registerServerReference<T: Function>(
           $$id,
           $$bound,
           bind: {value: bind, configurable: true},
-        },
+        }) as PropertyDescriptorMap,
   );
 }
diff --git a/packages/react-server-dom-parcel/src/ReactFlightParcelReferences.js b/packages/react-server-dom-parcel/src/ReactFlightParcelReferences.js
@@ -73,7 +73,7 @@ function bind(this: ServerReference<any>): any {
     const $$bound = {value: this.$$bound ? this.$$bound.concat(args) : args};
     return Object.defineProperties(
       (newFn: any),
-      __DEV__
+      (__DEV__
         ? {
             $$typeof,
             $$id,
@@ -89,7 +89,7 @@ function bind(this: ServerReference<any>): any {
             $$id,
             $$bound,
             bind: {value: bind, configurable: true},
-          },
+          }) as PropertyDescriptorMap,
     );
   }
   return newFn;
@@ -108,7 +108,7 @@ export function registerServerReference<T>(
   const $$bound = {value: null, configurable: true};
   return Object.defineProperties(
     (reference: any),
-    __DEV__
+    (__DEV__
       ? {
           $$typeof,
           $$id,
@@ -124,6 +124,6 @@ export function registerServerReference<T>(
           $$id,
           $$bound,
           bind: {value: bind, configurable: true},
-        },
+        }) as PropertyDescriptorMap,
   );
 }
diff --git a/packages/react-server-dom-parcel/src/server/ReactFlightDOMServerBrowser.js b/packages/react-server-dom-parcel/src/server/ReactFlightDOMServerBrowser.js
@@ -240,7 +240,7 @@ export function prerender(
   });
 }
 
-let serverManifest = {};
+let serverManifest: ServerManifest = {};
 export function registerServerActions(manifest: ServerManifest) {
   // This function is called by the bundler to register the manifest.
   serverManifest = manifest;

diff --git a/packages/react-server-dom-parcel/src/server/ReactFlightDOMServerEdge.js b/packages/react-server-dom-parcel/src/server/ReactFlightDOMServerEdge.js
@@ -245,7 +245,7 @@ export function prerender(
   });
 }
 
-let serverManifest = {};
+let serverManifest: ServerManifest = {};
 export function registerServerActions(manifest: ServerManifest) {
   // This function is called by the bundler to register the manifest.
   serverManifest = manifest;

diff --git a/packages/react-server-dom-parcel/src/server/ReactFlightDOMServerNode.js b/packages/react-server-dom-parcel/src/server/ReactFlightDOMServerNode.js
@@ -554,7 +554,7 @@ export function prerender(
   });
 }
 
-let serverManifest = {};
+let serverManifest: ServerManifest = {};
 export function registerServerActions(manifest: ServerManifest) {
   // This function is called by the bundler to register the manifest.
   serverManifest = manifest;

diff --git a/packages/react-server-dom-turbopack/src/ReactFlightTurbopackReferences.js b/packages/react-server-dom-turbopack/src/ReactFlightTurbopackReferences.js
@@ -80,7 +80,7 @@ function bind(this: ServerReference<any>): any {
     const $$bound = {value: this.$$bound ? this.$$bound.concat(args) : args};
     return Object.defineProperties(
       (newFn: any),
-      __DEV__
+      (__DEV__
         ? {
             $$typeof,
             $$id,
@@ -96,7 +96,7 @@ function bind(this: ServerReference<any>): any {
             $$id,
             $$bound,
             bind: {value: bind, configurable: true},
-          },
+          }) as PropertyDescriptorMap,
     );
   }
   return newFn;
@@ -115,7 +115,7 @@ export function registerServerReference<T: Function>(
   const $$bound = {value: null, configurable: true};
   return Object.defineProperties(
     (reference: any),
-    __DEV__
+    (__DEV__
       ? {
           $$typeof,
           $$id,
@@ -131,13 +131,13 @@ export function registerServerReference<T: Function>(
           $$id,
           $$bound,
           bind: {value: bind, configurable: true},
-        },
+        }) as PropertyDescriptorMap,
   );
 }
 
 const PROMISE_PROTOTYPE = Promise.prototype;
 
-const deepProxyHandlers = {
+const deepProxyHandlers: Proxy$traps<mixed> = {
   get: function (
     target: Function,
     name: string | symbol,