diff --git a/packages/react-debug-tools/src/ReactDebugHooks.js b/packages/react-debug-tools/src/ReactDebugHooks.js
index 2cb6e7fbdf386..a09da97f90d72 100644
--- a/packages/react-debug-tools/src/ReactDebugHooks.js
+++ b/packages/react-debug-tools/src/ReactDebugHooks.js
@@ -783,7 +783,7 @@ export function inspectHooksOfFiber(
   fiber: Fiber,
   currentDispatcher: ?CurrentDispatcherRef,
   includeHooksSource?: boolean = false,
-) {
+): HooksTree {
   // DevTools will pass the current renderer's injected dispatcher.
   // Other apps might compile debug hooks as part of their app though.
   if (currentDispatcher == null) {
diff --git a/packages/react-dom/src/client/ReactDOMHostConfig.js b/packages/react-dom/src/client/ReactDOMHostConfig.js
index c9c6e11fdfe00..64195ac7f5957 100644
--- a/packages/react-dom/src/client/ReactDOMHostConfig.js
+++ b/packages/react-dom/src/client/ReactDOMHostConfig.js
@@ -680,7 +680,7 @@ export function clearContainer(container: Container): void {
 
 export const supportsHydration = true;
 
-export function isHydratableResource(type: string, props: Props) {
+export function isHydratableResource(type: string, props: Props): boolean {
   return (
     type === 'link' &&
     typeof (props: any).precedence === 'string' &&
@@ -726,11 +726,13 @@ export function canHydrateSuspenseInstance(
   return ((instance: any): SuspenseInstance);
 }
 
-export function isSuspenseInstancePending(instance: SuspenseInstance) {
+export function isSuspenseInstancePending(instance: SuspenseInstance): boolean {
   return instance.data === SUSPENSE_PENDING_START_DATA;
 }
 
-export function isSuspenseInstanceFallback(instance: SuspenseInstance) {
+export function isSuspenseInstanceFallback(
+  instance: SuspenseInstance,
+): boolean {
   return instance.data === SUSPENSE_FALLBACK_START_DATA;
 }
 
diff --git a/packages/react-dom/src/client/inputValueTracking.js b/packages/react-dom/src/client/inputValueTracking.js
index 6bec33f9b0432..2c279331e8adc 100644
--- a/packages/react-dom/src/client/inputValueTracking.js
+++ b/packages/react-dom/src/client/inputValueTracking.js
@@ -123,7 +123,7 @@ export function track(node: ElementWithValueTracker) {
   node._valueTracker = trackValueOnNode(node);
 }
 
-export function updateValueIfChanged(node: ElementWithValueTracker) {
+export function updateValueIfChanged(node: ElementWithValueTracker): boolean {
   if (!node) {
     return false;
   }
diff --git a/packages/react-dom/src/events/checkPassiveEvents.js b/packages/react-dom/src/events/checkPassiveEvents.js
index 12680a885ebde..396060867414b 100644
--- a/packages/react-dom/src/events/checkPassiveEvents.js
+++ b/packages/react-dom/src/events/checkPassiveEvents.js
@@ -9,7 +9,7 @@
 
 import {canUseDOM} from 'shared/ExecutionEnvironment';
 
-export let passiveBrowserEventsSupported = false;
+export let passiveBrowserEventsSupported: boolean = false;
 
 // Check if browser support events with passive listeners
 // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#Safely_detecting_option_support
diff --git a/packages/react-dom/src/shared/DOMProperty.js b/packages/react-dom/src/shared/DOMProperty.js
index 09f1dd2b66a31..0ae244d8c98a3 100644
--- a/packages/react-dom/src/shared/DOMProperty.js
+++ b/packages/react-dom/src/shared/DOMProperty.js
@@ -63,10 +63,10 @@ export type PropertyInfo = {
 export const ATTRIBUTE_NAME_START_CHAR =
   ':A-Z_a-z\\u00C0-\\u00D6\\u00D8-\\u00F6\\u00F8-\\u02FF\\u0370-\\u037D\\u037F-\\u1FFF\\u200C-\\u200D\\u2070-\\u218F\\u2C00-\\u2FEF\\u3001-\\uD7FF\\uF900-\\uFDCF\\uFDF0-\\uFFFD';
 /* eslint-enable max-len */
-export const ATTRIBUTE_NAME_CHAR =
+export const ATTRIBUTE_NAME_CHAR: string =
   ATTRIBUTE_NAME_START_CHAR + '\\-.0-9\\u00B7\\u0300-\\u036F\\u203F-\\u2040';
 
-export const VALID_ATTRIBUTE_NAME_REGEX = new RegExp(
+export const VALID_ATTRIBUTE_NAME_REGEX: RegExp = new RegExp(
   '^[' + ATTRIBUTE_NAME_START_CHAR + '][' + ATTRIBUTE_NAME_CHAR + ']*$',
 );
 
diff --git a/packages/react-dom/src/shared/isCustomComponent.js b/packages/react-dom/src/shared/isCustomComponent.js
index aa6b2903c0e71..e94bc239d9c48 100644
--- a/packages/react-dom/src/shared/isCustomComponent.js
+++ b/packages/react-dom/src/shared/isCustomComponent.js
@@ -7,7 +7,7 @@
  * @flow
  */
 
-function isCustomComponent(tagName: string, props: Object) {
+function isCustomComponent(tagName: string, props: Object): boolean {
   if (tagName.indexOf('-') === -1) {
     return typeof props.is === 'string';
   }
diff --git a/packages/react-is/src/ReactIs.js b/packages/react-is/src/ReactIs.js
index 8c5d26a08c2ae..26bf7e3c6b36d 100644
--- a/packages/react-is/src/ReactIs.js
+++ b/packages/react-is/src/ReactIs.js
@@ -82,7 +82,7 @@ let hasWarnedAboutDeprecatedIsAsyncMode = false;
 let hasWarnedAboutDeprecatedIsConcurrentMode = false;
 
 // AsyncMode should be deprecated
-export function isAsyncMode(object: any) {
+export function isAsyncMode(object: any): boolean {
   if (__DEV__) {
     if (!hasWarnedAboutDeprecatedIsAsyncMode) {
       hasWarnedAboutDeprecatedIsAsyncMode = true;
@@ -95,7 +95,7 @@ export function isAsyncMode(object: any) {
   }
   return false;
 }
-export function isConcurrentMode(object: any) {
+export function isConcurrentMode(object: any): boolean {
   if (__DEV__) {
     if (!hasWarnedAboutDeprecatedIsConcurrentMode) {
       hasWarnedAboutDeprecatedIsConcurrentMode = true;
@@ -108,43 +108,43 @@ export function isConcurrentMode(object: any) {
   }
   return false;
 }
-export function isContextConsumer(object: any) {
+export function isContextConsumer(object: any): boolean {
   return typeOf(object) === REACT_CONTEXT_TYPE;
 }
-export function isContextProvider(object: any) {
+export function isContextProvider(object: any): boolean {
   return typeOf(object) === REACT_PROVIDER_TYPE;
 }
-export function isElement(object: any) {
+export function isElement(object: any): boolean {
   return (
     typeof object === 'object' &&
     object !== null &&
     object.$$typeof === REACT_ELEMENT_TYPE
   );
 }
-export function isForwardRef(object: any) {
+export function isForwardRef(object: any): boolean {
   return typeOf(object) === REACT_FORWARD_REF_TYPE;
 }
-export function isFragment(object: any) {
+export function isFragment(object: any): boolean {
   return typeOf(object) === REACT_FRAGMENT_TYPE;
 }
-export function isLazy(object: any) {
+export function isLazy(object: any): boolean {
   return typeOf(object) === REACT_LAZY_TYPE;
 }
-export function isMemo(object: any) {
+export function isMemo(object: any): boolean {
   return typeOf(object) === REACT_MEMO_TYPE;
 }
-export function isPortal(object: any) {
+export function isPortal(object: any): boolean {
   return typeOf(object) === REACT_PORTAL_TYPE;
 }
-export function isProfiler(object: any) {
+export function isProfiler(object: any): boolean {
   return typeOf(object) === REACT_PROFILER_TYPE;
 }
-export function isStrictMode(object: any) {
+export function isStrictMode(object: any): boolean {
   return typeOf(object) === REACT_STRICT_MODE_TYPE;
 }
-export function isSuspense(object: any) {
+export function isSuspense(object: any): boolean {
   return typeOf(object) === REACT_SUSPENSE_TYPE;
 }
-export function isSuspenseList(object: any) {
+export function isSuspenseList(object: any): boolean {
   return typeOf(object) === REACT_SUSPENSE_LIST_TYPE;
 }
diff --git a/packages/react-reconciler/src/ReactCurrentFiber.js b/packages/react-reconciler/src/ReactCurrentFiber.js
index 08853b384bc65..5c9fa7a052362 100644
--- a/packages/react-reconciler/src/ReactCurrentFiber.js
+++ b/packages/react-reconciler/src/ReactCurrentFiber.js
@@ -73,7 +73,7 @@ export function setIsRendering(rendering: boolean) {
   }
 }
 
-export function getIsRendering() {
+export function getIsRendering(): void | boolean {
   if (__DEV__) {
     return isRendering;
   }
diff --git a/packages/react-reconciler/src/ReactEventPriorities.js b/packages/react-reconciler/src/ReactEventPriorities.js
index 46223aa70df55..faf30c3338ad9 100644
--- a/packages/react-reconciler/src/ReactEventPriorities.js
+++ b/packages/react-reconciler/src/ReactEventPriorities.js
@@ -58,7 +58,7 @@ export function getCurrentUpdatePriority(): EventPriority {
     : (getCurrentUpdatePriority_old(): any);
 }
 
-export function setCurrentUpdatePriority(priority: EventPriority) {
+export function setCurrentUpdatePriority(priority: EventPriority): void {
   return enableNewReconciler
     ? setCurrentUpdatePriority_new((priority: any))
     : setCurrentUpdatePriority_old((priority: any));
diff --git a/packages/react-reconciler/src/ReactFiber.new.js b/packages/react-reconciler/src/ReactFiber.new.js
index d6a6c8f7adc19..484f954a6f4df 100644
--- a/packages/react-reconciler/src/ReactFiber.new.js
+++ b/packages/react-reconciler/src/ReactFiber.new.js
@@ -225,7 +225,7 @@ function shouldConstruct(Component: Function) {
   return !!(prototype && prototype.isReactComponent);
 }
 
-export function isSimpleFunctionComponent(type: any) {
+export function isSimpleFunctionComponent(type: any): boolean {
   return (
     typeof type === 'function' &&
     !shouldConstruct(type) &&
@@ -355,7 +355,10 @@ export function createWorkInProgress(current: Fiber, pendingProps: any): Fiber {
 }
 
 // Used to reuse a Fiber for a second pass.
-export function resetWorkInProgress(workInProgress: Fiber, renderLanes: Lanes) {
+export function resetWorkInProgress(
+  workInProgress: Fiber,
+  renderLanes: Lanes,
+): Fiber {
   // This resets the Fiber to what createFiber or createWorkInProgress would
   // have set the values to before during the first pass. Ideally this wouldn't
   // be necessary but unfortunately many code paths reads from the workInProgress
diff --git a/packages/react-reconciler/src/ReactFiber.old.js b/packages/react-reconciler/src/ReactFiber.old.js
index d8f1ef424bd13..60d52c2d0b827 100644
--- a/packages/react-reconciler/src/ReactFiber.old.js
+++ b/packages/react-reconciler/src/ReactFiber.old.js
@@ -225,7 +225,7 @@ function shouldConstruct(Component: Function) {
   return !!(prototype && prototype.isReactComponent);
 }
 
-export function isSimpleFunctionComponent(type: any) {
+export function isSimpleFunctionComponent(type: any): boolean {
   return (
     typeof type === 'function' &&
     !shouldConstruct(type) &&
@@ -355,7 +355,10 @@ export function createWorkInProgress(current: Fiber, pendingProps: any): Fiber {
 }
 
 // Used to reuse a Fiber for a second pass.
-export function resetWorkInProgress(workInProgress: Fiber, renderLanes: Lanes) {
+export function resetWorkInProgress(
+  workInProgress: Fiber,
+  renderLanes: Lanes,
+): Fiber {
   // This resets the Fiber to what createFiber or createWorkInProgress would
   // have set the values to before during the first pass. Ideally this wouldn't
   // be necessary but unfortunately many code paths reads from the workInProgress
diff --git a/packages/react-reconciler/src/ReactFiberAct.new.js b/packages/react-reconciler/src/ReactFiberAct.new.js
index bad7fd3dc52f7..53a4922ad5417 100644
--- a/packages/react-reconciler/src/ReactFiberAct.new.js
+++ b/packages/react-reconciler/src/ReactFiberAct.new.js
@@ -15,7 +15,7 @@ import {warnsIfNotActing} from './ReactFiberHostConfig';
 
 const {ReactCurrentActQueue} = ReactSharedInternals;
 
-export function isLegacyActEnvironment(fiber: Fiber) {
+export function isLegacyActEnvironment(fiber: Fiber): boolean {
   if (__DEV__) {
     // Legacy mode. We preserve the behavior of React 17's act. It assumes an
     // act environment whenever `jest` is defined, but you can still turn off
diff --git a/packages/react-reconciler/src/ReactFiberAct.old.js b/packages/react-reconciler/src/ReactFiberAct.old.js
index 6358ab66a3485..32d3a0f270719 100644
--- a/packages/react-reconciler/src/ReactFiberAct.old.js
+++ b/packages/react-reconciler/src/ReactFiberAct.old.js
@@ -15,7 +15,7 @@ import {warnsIfNotActing} from './ReactFiberHostConfig';
 
 const {ReactCurrentActQueue} = ReactSharedInternals;
 
-export function isLegacyActEnvironment(fiber: Fiber) {
+export function isLegacyActEnvironment(fiber: Fiber): boolean {
   if (__DEV__) {
     // Legacy mode. We preserve the behavior of React 17's act. It assumes an
     // act environment whenever `jest` is defined, but you can still turn off
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.new.js b/packages/react-reconciler/src/ReactFiberBeginWork.new.js
index fa417e08114c9..913a054be2dd2 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.new.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.new.js
@@ -281,7 +281,7 @@ let didWarnAboutModulePatternComponent;
 let didWarnAboutContextTypeOnFunctionComponent;
 let didWarnAboutGetDerivedStateOnFunctionComponent;
 let didWarnAboutFunctionRefs;
-export let didWarnAboutReassigningProps;
+export let didWarnAboutReassigningProps: boolean;
 let didWarnAboutRevealOrder;
 let didWarnAboutTailOptions;
 let didWarnAboutDefaultPropsOnFunctionComponent;
@@ -3479,7 +3479,7 @@ export function markWorkInProgressReceivedUpdate() {
   didReceiveUpdate = true;
 }
 
-export function checkIfWorkInProgressReceivedUpdate() {
+export function checkIfWorkInProgressReceivedUpdate(): boolean {
   return didReceiveUpdate;
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.old.js b/packages/react-reconciler/src/ReactFiberBeginWork.old.js
index 449f306e7ef89..382e9075716af 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.old.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.old.js
@@ -281,7 +281,7 @@ let didWarnAboutModulePatternComponent;
 let didWarnAboutContextTypeOnFunctionComponent;
 let didWarnAboutGetDerivedStateOnFunctionComponent;
 let didWarnAboutFunctionRefs;
-export let didWarnAboutReassigningProps;
+export let didWarnAboutReassigningProps: boolean;
 let didWarnAboutRevealOrder;
 let didWarnAboutTailOptions;
 let didWarnAboutDefaultPropsOnFunctionComponent;
@@ -3479,7 +3479,7 @@ export function markWorkInProgressReceivedUpdate() {
   didReceiveUpdate = true;
 }
 
-export function checkIfWorkInProgressReceivedUpdate() {
+export function checkIfWorkInProgressReceivedUpdate(): boolean {
   return didReceiveUpdate;
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js
index 5bdc8fc6a82a0..5bf0a7a376cec 100644
--- a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js
+++ b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js
@@ -164,7 +164,7 @@ let hasForceUpdate = false;
 
 let didWarnUpdateInsideUpdate;
 let currentlyProcessingQueue;
-export let resetCurrentlyProcessingQueue;
+export let resetCurrentlyProcessingQueue: () => void;
 if (__DEV__) {
   didWarnUpdateInsideUpdate = false;
   currentlyProcessingQueue = null;
diff --git a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js
index bdff2ea93e704..c9918ae104c01 100644
--- a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js
+++ b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js
@@ -164,7 +164,7 @@ let hasForceUpdate = false;
 
 let didWarnUpdateInsideUpdate;
 let currentlyProcessingQueue;
-export let resetCurrentlyProcessingQueue;
+export let resetCurrentlyProcessingQueue: () => void;
 if (__DEV__) {
   didWarnUpdateInsideUpdate = false;
   currentlyProcessingQueue = null;
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.new.js b/packages/react-reconciler/src/ReactFiberCommitWork.new.js
index c70e4ae3470e7..1710bcfc861e7 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.new.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.new.js
@@ -320,7 +320,7 @@ let shouldFireAfterActiveInstanceBlur: boolean = false;
 export function commitBeforeMutationEffects(
   root: FiberRoot,
   firstChild: Fiber,
-) {
+): boolean {
   focusedInstanceHandle = prepareForCommit(root.containerInfo);
 
   nextEffect = firstChild;
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.old.js b/packages/react-reconciler/src/ReactFiberCommitWork.old.js
index 4b5076a61c51f..a7a8929d2b68c 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.old.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.old.js
@@ -320,7 +320,7 @@ let shouldFireAfterActiveInstanceBlur: boolean = false;
 export function commitBeforeMutationEffects(
   root: FiberRoot,
   firstChild: Fiber,
-) {
+): boolean {
   focusedInstanceHandle = prepareForCommit(root.containerInfo);
 
   nextEffect = firstChild;
diff --git a/packages/react-reconciler/src/ReactFiberHiddenContext.new.js b/packages/react-reconciler/src/ReactFiberHiddenContext.new.js
index 3ce3490a3990e..d44178a3efc8c 100644
--- a/packages/react-reconciler/src/ReactFiberHiddenContext.new.js
+++ b/packages/react-reconciler/src/ReactFiberHiddenContext.new.js
@@ -66,6 +66,6 @@ export function popHiddenContext(fiber: Fiber): void {
   pop(prevRenderLanesStackCursor, fiber);
 }
 
-export function isCurrentTreeHidden() {
+export function isCurrentTreeHidden(): boolean {
   return currentTreeHiddenStackCursor.current !== null;
 }
diff --git a/packages/react-reconciler/src/ReactFiberHiddenContext.old.js b/packages/react-reconciler/src/ReactFiberHiddenContext.old.js
index 565dc4ae20e9f..1886dfef8bb53 100644
--- a/packages/react-reconciler/src/ReactFiberHiddenContext.old.js
+++ b/packages/react-reconciler/src/ReactFiberHiddenContext.old.js
@@ -66,6 +66,6 @@ export function popHiddenContext(fiber: Fiber): void {
   pop(prevRenderLanesStackCursor, fiber);
 }
 
-export function isCurrentTreeHidden() {
+export function isCurrentTreeHidden(): boolean {
   return currentTreeHiddenStackCursor.current !== null;
 }
diff --git a/packages/react-reconciler/src/ReactFiberHooks.new.js b/packages/react-reconciler/src/ReactFiberHooks.new.js
index 602f855c76d85..b602c3691471e 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.new.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.new.js
@@ -571,7 +571,7 @@ export function renderWithHooks<Props, SecondArg>(
   return children;
 }
 
-export function checkDidRenderIdHook() {
+export function checkDidRenderIdHook(): boolean {
   // This should be called immediately after every renderWithHooks call.
   // Conceptually, it's part of the return value of renderWithHooks; it's only a
   // separate function to avoid using an array tuple.
diff --git a/packages/react-reconciler/src/ReactFiberHooks.old.js b/packages/react-reconciler/src/ReactFiberHooks.old.js
index f2b024bee943d..9f0cb1914581f 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.old.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.old.js
@@ -571,7 +571,7 @@ export function renderWithHooks<Props, SecondArg>(
   return children;
 }
 
-export function checkDidRenderIdHook() {
+export function checkDidRenderIdHook(): boolean {
   // This should be called immediately after every renderWithHooks call.
   // Conceptually, it's part of the return value of renderWithHooks; it's only a
   // separate function to avoid using an array tuple.
diff --git a/packages/react-reconciler/src/ReactFiberHotReloading.js b/packages/react-reconciler/src/ReactFiberHotReloading.js
index 0d3fdca45ceff..385ee8bfcd8e2 100644
--- a/packages/react-reconciler/src/ReactFiberHotReloading.js
+++ b/packages/react-reconciler/src/ReactFiberHotReloading.js
@@ -7,6 +7,8 @@
  * @flow
  */
 
+import type {Fiber} from './ReactInternalTypes';
+import type {ReactElement} from '../../shared/ReactElementType';
 import type {Instance} from './ReactFiberHostConfig';
 import type {FiberRoot} from './ReactInternalTypes';
 import type {ReactNodeList} from 'shared/ReactTypes';
@@ -58,9 +60,9 @@ export type FindHostInstancesForRefresh = (
   families: Array<Family>,
 ) => Set<Instance>;
 
-export const setRefreshHandler = enableNewReconciler
-  ? setRefreshHandler_new
-  : setRefreshHandler_old;
+export const setRefreshHandler: (
+  handler: RefreshHandler | null,
+) => void = enableNewReconciler ? setRefreshHandler_new : setRefreshHandler_old;
 export const resolveFunctionForHotReloading = enableNewReconciler
   ? resolveFunctionForHotReloading_new
   : resolveFunctionForHotReloading_old;
@@ -70,18 +72,23 @@ export const resolveClassForHotReloading = enableNewReconciler
 export const resolveForwardRefForHotReloading = enableNewReconciler
   ? resolveForwardRefForHotReloading_new
   : resolveForwardRefForHotReloading_old;
-export const isCompatibleFamilyForHotReloading = enableNewReconciler
+export const isCompatibleFamilyForHotReloading: (
+  fiber: Fiber,
+  element: ReactElement,
+) => boolean = enableNewReconciler
   ? isCompatibleFamilyForHotReloading_new
   : isCompatibleFamilyForHotReloading_old;
-export const markFailedErrorBoundaryForHotReloading = enableNewReconciler
+export const markFailedErrorBoundaryForHotReloading: (
+  fiber: Fiber,
+) => void = enableNewReconciler
   ? markFailedErrorBoundaryForHotReloading_new
   : markFailedErrorBoundaryForHotReloading_old;
-export const scheduleRefresh = enableNewReconciler
+export const scheduleRefresh: ScheduleRefresh = enableNewReconciler
   ? scheduleRefresh_new
   : scheduleRefresh_old;
-export const scheduleRoot = enableNewReconciler
+export const scheduleRoot: ScheduleRoot = enableNewReconciler
   ? scheduleRoot_new
   : scheduleRoot_old;
-export const findHostInstancesForRefresh = enableNewReconciler
+export const findHostInstancesForRefresh: FindHostInstancesForRefresh = enableNewReconciler
   ? findHostInstancesForRefresh_new
   : findHostInstancesForRefresh_old;
diff --git a/packages/react-reconciler/src/ReactFiberHydrationContext.new.js b/packages/react-reconciler/src/ReactFiberHydrationContext.new.js
index 23010daad4970..66c28274ae0cf 100644
--- a/packages/react-reconciler/src/ReactFiberHydrationContext.new.js
+++ b/packages/react-reconciler/src/ReactFiberHydrationContext.new.js
@@ -109,7 +109,7 @@ export function markDidThrowWhileHydratingDEV() {
   }
 }
 
-export function didSuspendOrErrorWhileHydratingDEV() {
+export function didSuspendOrErrorWhileHydratingDEV(): boolean {
   if (__DEV__) {
     return didSuspendOrErrorDEV;
   }
@@ -685,7 +685,7 @@ function popHydrationState(fiber: Fiber): boolean {
   return true;
 }
 
-function hasUnhydratedTailNodes() {
+function hasUnhydratedTailNodes(): boolean {
   return isHydrating && nextHydratableInstance !== null;
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberHydrationContext.old.js b/packages/react-reconciler/src/ReactFiberHydrationContext.old.js
index 28371e823e5c8..10e59b1d2bf0a 100644
--- a/packages/react-reconciler/src/ReactFiberHydrationContext.old.js
+++ b/packages/react-reconciler/src/ReactFiberHydrationContext.old.js
@@ -109,7 +109,7 @@ export function markDidThrowWhileHydratingDEV() {
   }
 }
 
-export function didSuspendOrErrorWhileHydratingDEV() {
+export function didSuspendOrErrorWhileHydratingDEV(): boolean {
   if (__DEV__) {
     return didSuspendOrErrorDEV;
   }
@@ -685,7 +685,7 @@ function popHydrationState(fiber: Fiber): boolean {
   return true;
 }
 
-function hasUnhydratedTailNodes() {
+function hasUnhydratedTailNodes(): boolean {
   return isHydrating && nextHydratableInstance !== null;
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberLane.new.js b/packages/react-reconciler/src/ReactFiberLane.new.js
index 2dcbe70ee7960..36af3b95becea 100644
--- a/packages/react-reconciler/src/ReactFiberLane.new.js
+++ b/packages/react-reconciler/src/ReactFiberLane.new.js
@@ -435,7 +435,7 @@ export function markStarvedLanesAsExpired(
 
 // This returns the highest priority pending lanes regardless of whether they
 // are suspended.
-export function getHighestPriorityPendingLanes(root: FiberRoot) {
+export function getHighestPriorityPendingLanes(root: FiberRoot): Lanes {
   return getHighestPriorityLanes(root.pendingLanes);
 }
 
@@ -458,25 +458,25 @@ export function getLanesToRetrySynchronouslyOnError(
   return NoLanes;
 }
 
-export function includesSyncLane(lanes: Lanes) {
+export function includesSyncLane(lanes: Lanes): boolean {
   return (lanes & SyncLane) !== NoLanes;
 }
 
-export function includesNonIdleWork(lanes: Lanes) {
+export function includesNonIdleWork(lanes: Lanes): boolean {
   return (lanes & NonIdleLanes) !== NoLanes;
 }
-export function includesOnlyRetries(lanes: Lanes) {
+export function includesOnlyRetries(lanes: Lanes): boolean {
   return (lanes & RetryLanes) === lanes;
 }
-export function includesOnlyNonUrgentLanes(lanes: Lanes) {
+export function includesOnlyNonUrgentLanes(lanes: Lanes): boolean {
   const UrgentLanes = SyncLane | InputContinuousLane | DefaultLane;
   return (lanes & UrgentLanes) === NoLanes;
 }
-export function includesOnlyTransitions(lanes: Lanes) {
+export function includesOnlyTransitions(lanes: Lanes): boolean {
   return (lanes & TransitionLanes) === lanes;
 }
 
-export function includesBlockingLane(root: FiberRoot, lanes: Lanes) {
+export function includesBlockingLane(root: FiberRoot, lanes: Lanes): boolean {
   if (
     allowConcurrentByDefault &&
     (root.current.mode & ConcurrentUpdatesByDefaultMode) !== NoMode
@@ -492,13 +492,13 @@ export function includesBlockingLane(root: FiberRoot, lanes: Lanes) {
   return (lanes & SyncDefaultLanes) !== NoLanes;
 }
 
-export function includesExpiredLane(root: FiberRoot, lanes: Lanes) {
+export function includesExpiredLane(root: FiberRoot, lanes: Lanes): boolean {
   // This is a separate check from includesBlockingLane because a lane can
   // expire after a render has already started.
   return (lanes & root.expiredLanes) !== NoLanes;
 }
 
-export function isTransitionLane(lane: Lane) {
+export function isTransitionLane(lane: Lane): boolean {
   return (lane & TransitionLanes) !== NoLanes;
 }
 
@@ -543,11 +543,11 @@ function laneToIndex(lane: Lane) {
   return pickArbitraryLaneIndex(lane);
 }
 
-export function includesSomeLane(a: Lanes | Lane, b: Lanes | Lane) {
+export function includesSomeLane(a: Lanes | Lane, b: Lanes | Lane): boolean {
   return (a & b) !== NoLanes;
 }
 
-export function isSubsetOfLanes(set: Lanes, subset: Lanes | Lane) {
+export function isSubsetOfLanes(set: Lanes, subset: Lanes | Lane): boolean {
   return (set & subset) === subset;
 }
 
@@ -569,7 +569,7 @@ export function laneToLanes(lane: Lane): Lanes {
   return lane;
 }
 
-export function higherPriorityLane(a: Lane, b: Lane) {
+export function higherPriorityLane(a: Lane, b: Lane): Lane {
   // This works because the bit ranges decrease in priority as you go left.
   return a !== NoLane && a < b ? a : b;
 }
diff --git a/packages/react-reconciler/src/ReactFiberLane.old.js b/packages/react-reconciler/src/ReactFiberLane.old.js
index 6042a1a31c429..38fe4700dff47 100644
--- a/packages/react-reconciler/src/ReactFiberLane.old.js
+++ b/packages/react-reconciler/src/ReactFiberLane.old.js
@@ -435,7 +435,7 @@ export function markStarvedLanesAsExpired(
 
 // This returns the highest priority pending lanes regardless of whether they
 // are suspended.
-export function getHighestPriorityPendingLanes(root: FiberRoot) {
+export function getHighestPriorityPendingLanes(root: FiberRoot): Lanes {
   return getHighestPriorityLanes(root.pendingLanes);
 }
 
@@ -458,25 +458,25 @@ export function getLanesToRetrySynchronouslyOnError(
   return NoLanes;
 }
 
-export function includesSyncLane(lanes: Lanes) {
+export function includesSyncLane(lanes: Lanes): boolean {
   return (lanes & SyncLane) !== NoLanes;
 }
 
-export function includesNonIdleWork(lanes: Lanes) {
+export function includesNonIdleWork(lanes: Lanes): boolean {
   return (lanes & NonIdleLanes) !== NoLanes;
 }
-export function includesOnlyRetries(lanes: Lanes) {
+export function includesOnlyRetries(lanes: Lanes): boolean {
   return (lanes & RetryLanes) === lanes;
 }
-export function includesOnlyNonUrgentLanes(lanes: Lanes) {
+export function includesOnlyNonUrgentLanes(lanes: Lanes): boolean {
   const UrgentLanes = SyncLane | InputContinuousLane | DefaultLane;
   return (lanes & UrgentLanes) === NoLanes;
 }
-export function includesOnlyTransitions(lanes: Lanes) {
+export function includesOnlyTransitions(lanes: Lanes): boolean {
   return (lanes & TransitionLanes) === lanes;
 }
 
-export function includesBlockingLane(root: FiberRoot, lanes: Lanes) {
+export function includesBlockingLane(root: FiberRoot, lanes: Lanes): boolean {
   if (
     allowConcurrentByDefault &&
     (root.current.mode & ConcurrentUpdatesByDefaultMode) !== NoMode
@@ -492,13 +492,13 @@ export function includesBlockingLane(root: FiberRoot, lanes: Lanes) {
   return (lanes & SyncDefaultLanes) !== NoLanes;
 }
 
-export function includesExpiredLane(root: FiberRoot, lanes: Lanes) {
+export function includesExpiredLane(root: FiberRoot, lanes: Lanes): boolean {
   // This is a separate check from includesBlockingLane because a lane can
   // expire after a render has already started.
   return (lanes & root.expiredLanes) !== NoLanes;
 }
 
-export function isTransitionLane(lane: Lane) {
+export function isTransitionLane(lane: Lane): boolean {
   return (lane & TransitionLanes) !== NoLanes;
 }
 
@@ -543,11 +543,11 @@ function laneToIndex(lane: Lane) {
   return pickArbitraryLaneIndex(lane);
 }
 
-export function includesSomeLane(a: Lanes | Lane, b: Lanes | Lane) {
+export function includesSomeLane(a: Lanes | Lane, b: Lanes | Lane): boolean {
   return (a & b) !== NoLanes;
 }
 
-export function isSubsetOfLanes(set: Lanes, subset: Lanes | Lane) {
+export function isSubsetOfLanes(set: Lanes, subset: Lanes | Lane): boolean {
   return (set & subset) === subset;
 }
 
@@ -569,7 +569,7 @@ export function laneToLanes(lane: Lane): Lanes {
   return lane;
 }
 
-export function higherPriorityLane(a: Lane, b: Lane) {
+export function higherPriorityLane(a: Lane, b: Lane): Lane {
   // This works because the bit ranges decrease in priority as you go left.
   return a !== NoLane && a < b ? a : b;
 }
diff --git a/packages/react-reconciler/src/ReactFiberNewContext.new.js b/packages/react-reconciler/src/ReactFiberNewContext.new.js
index eae7b232c5eb5..a311b101d3a4e 100644
--- a/packages/react-reconciler/src/ReactFiberNewContext.new.js
+++ b/packages/react-reconciler/src/ReactFiberNewContext.new.js
@@ -598,7 +598,9 @@ function propagateParentContextChanges(
   workInProgress.flags |= DidPropagateContext;
 }
 
-export function checkIfContextChanged(currentDependencies: Dependencies) {
+export function checkIfContextChanged(
+  currentDependencies: Dependencies,
+): boolean {
   if (!enableLazyContextPropagation) {
     return false;
   }
diff --git a/packages/react-reconciler/src/ReactFiberNewContext.old.js b/packages/react-reconciler/src/ReactFiberNewContext.old.js
index cbc2d0069b356..0833d58806e9b 100644
--- a/packages/react-reconciler/src/ReactFiberNewContext.old.js
+++ b/packages/react-reconciler/src/ReactFiberNewContext.old.js
@@ -598,7 +598,9 @@ function propagateParentContextChanges(
   workInProgress.flags |= DidPropagateContext;
 }
 
-export function checkIfContextChanged(currentDependencies: Dependencies) {
+export function checkIfContextChanged(
+  currentDependencies: Dependencies,
+): boolean {
   if (!enableLazyContextPropagation) {
     return false;
   }
diff --git a/packages/react-reconciler/src/ReactFiberShellHydration.js b/packages/react-reconciler/src/ReactFiberShellHydration.js
index caadb978f69d0..296c763e01b05 100644
--- a/packages/react-reconciler/src/ReactFiberShellHydration.js
+++ b/packages/react-reconciler/src/ReactFiberShellHydration.js
@@ -13,7 +13,7 @@ import type {RootState} from './ReactFiberRoot.new';
 // This is imported by the event replaying implementation in React DOM. It's
 // in a separate file to break a circular dependency between the renderer and
 // the reconciler.
-export function isRootDehydrated(root: FiberRoot) {
+export function isRootDehydrated(root: FiberRoot): boolean {
   const currentState: RootState = root.current.memoizedState;
   return currentState.isDehydrated;
 }
diff --git a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js
index 0f72c3fbc863a..73c04034b7e9d 100644
--- a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js
+++ b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js
@@ -48,7 +48,7 @@ export function flushSyncCallbacksOnlyInLegacyMode() {
   }
 }
 
-export function flushSyncCallbacks() {
+export function flushSyncCallbacks(): null {
   if (!isFlushingSyncQueue && syncQueue !== null) {
     // Prevent re-entrance.
     isFlushingSyncQueue = true;
diff --git a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js
index 56422a38b137a..12d8371b72251 100644
--- a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js
+++ b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js
@@ -48,7 +48,7 @@ export function flushSyncCallbacksOnlyInLegacyMode() {
   }
 }
 
-export function flushSyncCallbacks() {
+export function flushSyncCallbacks(): null {
   if (!isFlushingSyncQueue && syncQueue !== null) {
     // Prevent re-entrance.
     isFlushingSyncQueue = true;
diff --git a/packages/react-reconciler/src/ReactFiberWakeable.new.js b/packages/react-reconciler/src/ReactFiberWakeable.new.js
index e494aa34fc1f2..11f470cfab888 100644
--- a/packages/react-reconciler/src/ReactFiberWakeable.new.js
+++ b/packages/react-reconciler/src/ReactFiberWakeable.new.js
@@ -24,11 +24,11 @@ let lastUsedThenable: Thenable<any> | null = null;
 
 const MAX_AD_HOC_SUSPEND_COUNT = 50;
 
-export function isTrackingSuspendedThenable() {
+export function isTrackingSuspendedThenable(): boolean {
   return suspendedThenable !== null;
 }
 
-export function suspendedThenableDidResolve() {
+export function suspendedThenableDidResolve(): boolean {
   if (suspendedThenable !== null) {
     const status = suspendedThenable.status;
     return status === 'fulfilled' || status === 'rejected';
diff --git a/packages/react-reconciler/src/ReactFiberWakeable.old.js b/packages/react-reconciler/src/ReactFiberWakeable.old.js
index e494aa34fc1f2..11f470cfab888 100644
--- a/packages/react-reconciler/src/ReactFiberWakeable.old.js
+++ b/packages/react-reconciler/src/ReactFiberWakeable.old.js
@@ -24,11 +24,11 @@ let lastUsedThenable: Thenable<any> | null = null;
 
 const MAX_AD_HOC_SUSPEND_COUNT = 50;
 
-export function isTrackingSuspendedThenable() {
+export function isTrackingSuspendedThenable(): boolean {
   return suspendedThenable !== null;
 }
 
-export function suspendedThenableDidResolve() {
+export function suspendedThenableDidResolve(): boolean {
   if (suspendedThenable !== null) {
     const status = suspendedThenable.status;
     return status === 'fulfilled' || status === 'rejected';
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index ca6bbcae278ab..ccd1821db2c90 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -357,7 +357,7 @@ let workInProgressRootRenderTargetTime: number = Infinity;
 const RENDER_TIMEOUT_MS = 500;
 
 let workInProgressTransitions: Array<Transition> | null = null;
-export function getWorkInProgressTransitions() {
+export function getWorkInProgressTransitions(): null | Array<Transition> {
   return workInProgressTransitions;
 }
 
@@ -814,7 +814,7 @@ export function scheduleInitialHydrationOnRoot(
   ensureRootIsScheduled(root, eventTime);
 }
 
-export function isUnsafeClassRenderPhaseUpdate(fiber: Fiber) {
+export function isUnsafeClassRenderPhaseUpdate(fiber: Fiber): boolean {
   // Check if this is a render phase update. Only called by class components,
   // which special (deprecated) behavior for UNSAFE_componentWillReceive props.
   return (
@@ -1557,7 +1557,7 @@ declare function flushSync<R>(fn: () => R): R;
 // eslint-disable-next-line no-redeclare
 declare function flushSync(): void;
 // eslint-disable-next-line no-redeclare
-export function flushSync(fn) {
+export function flushSync(fn): void {
   // In legacy mode, we flush pending passive effects at the beginning of the
   // next event, not at the end of the previous one.
   if (
@@ -1596,7 +1596,7 @@ export function flushSync(fn) {
   }
 }
 
-export function isAlreadyRendering() {
+export function isAlreadyRendering(): boolean {
   // Used by the renderer to print a warning if certain APIs are called from
   // the wrong context.
   return (
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 4715d3fdf4df9..dc592a912e3a1 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -357,7 +357,7 @@ let workInProgressRootRenderTargetTime: number = Infinity;
 const RENDER_TIMEOUT_MS = 500;
 
 let workInProgressTransitions: Array<Transition> | null = null;
-export function getWorkInProgressTransitions() {
+export function getWorkInProgressTransitions(): null | Array<Transition> {
   return workInProgressTransitions;
 }
 
@@ -814,7 +814,7 @@ export function scheduleInitialHydrationOnRoot(
   ensureRootIsScheduled(root, eventTime);
 }
 
-export function isUnsafeClassRenderPhaseUpdate(fiber: Fiber) {
+export function isUnsafeClassRenderPhaseUpdate(fiber: Fiber): boolean {
   // Check if this is a render phase update. Only called by class components,
   // which special (deprecated) behavior for UNSAFE_componentWillReceive props.
   return (
@@ -1557,7 +1557,7 @@ declare function flushSync<R>(fn: () => R): R;
 // eslint-disable-next-line no-redeclare
 declare function flushSync(): void;
 // eslint-disable-next-line no-redeclare
-export function flushSync(fn) {
+export function flushSync(fn): void {
   // In legacy mode, we flush pending passive effects at the beginning of the
   // next event, not at the end of the previous one.
   if (
@@ -1596,7 +1596,7 @@ export function flushSync(fn) {
   }
 }
 
-export function isAlreadyRendering() {
+export function isAlreadyRendering(): boolean {
   // Used by the renderer to print a warning if certain APIs are called from
   // the wrong context.
   return (
diff --git a/packages/react-reconciler/src/clz32.js b/packages/react-reconciler/src/clz32.js
index 80a9cfb911482..fe827c4ca0870 100644
--- a/packages/react-reconciler/src/clz32.js
+++ b/packages/react-reconciler/src/clz32.js
@@ -9,7 +9,9 @@
 
 // TODO: This is pretty well supported by browsers. Maybe we can drop it.
 
-export const clz32 = Math.clz32 ? Math.clz32 : clz32Fallback;
+export const clz32: (x: number) => number = Math.clz32
+  ? Math.clz32
+  : clz32Fallback;
 
 // Count leading zeros.
 // Based on:
diff --git a/packages/react-refresh/src/ReactFreshRuntime.js b/packages/react-refresh/src/ReactFreshRuntime.js
index 99d4d03199d70..56af77a8e8584 100644
--- a/packages/react-refresh/src/ReactFreshRuntime.js
+++ b/packages/react-refresh/src/ReactFreshRuntime.js
@@ -599,13 +599,13 @@ export function injectIntoGlobalHook(globalObject: any): void {
   }
 }
 
-export function hasUnrecoverableErrors() {
+export function hasUnrecoverableErrors(): boolean {
   // TODO: delete this after removing dependency in RN.
   return false;
 }
 
 // Exposed for testing.
-export function _getMountedRootCount() {
+export function _getMountedRootCount(): number {
   if (__DEV__) {
     return mountedRoots.size;
   } else {
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
index 737a8502801bb..3182d1ab16cdf 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
@@ -94,7 +94,7 @@ export async function getSource(
   url: string,
   context: GetSourceContext,
   defaultGetSource: GetSourceFunction,
-) {
+): Promise<{source: Source}> {
   // We stash this in case we end up needing to resolve export * statements later.
   stashedGetSource = defaultGetSource;
   return defaultGetSource(url, context, defaultGetSource);
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
index 98d3058029fc5..bbc26b6c84f83 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
@@ -29,7 +29,7 @@ class ClientReferenceDependency extends ModuleDependency {
     super(request);
   }
 
-  get type() {
+  get type(): string {
     return 'client-reference';
   }
 }
diff --git a/packages/react-server/src/ReactFizzContext.js b/packages/react-server/src/ReactFizzContext.js
index 3834a6f1076ff..005fb776fed05 100644
--- a/packages/react-server/src/ReactFizzContext.js
+++ b/packages/react-server/src/ReactFizzContext.js
@@ -17,7 +17,8 @@ if (__DEV__) {
   warnedAboutMissingGetChildContext = {};
 }
 
-export const emptyContextObject = {};
+// $FlowFixMe[incompatible-exact]
+export const emptyContextObject: {} = {};
 if (__DEV__) {
   Object.freeze(emptyContextObject);
 }
diff --git a/packages/react-server/src/ReactFizzHooks.js b/packages/react-server/src/ReactFizzHooks.js
index de67ccebcaf6b..f6c8847a81b2d 100644
--- a/packages/react-server/src/ReactFizzHooks.js
+++ b/packages/react-server/src/ReactFizzHooks.js
@@ -239,13 +239,13 @@ export function finishHooks(
   return children;
 }
 
-export function getThenableStateAfterSuspending() {
+export function getThenableStateAfterSuspending(): null | ThenableState {
   const state = thenableState;
   thenableState = null;
   return state;
 }
 
-export function checkDidRenderIdHook() {
+export function checkDidRenderIdHook(): boolean {
   // This should be called immediately after every finishHooks call.
   // Conceptually, it's part of the return value of finishHooks; it's only a
   // separate function to avoid using an array tuple.
diff --git a/packages/react-server/src/ReactFlightHooks.js b/packages/react-server/src/ReactFlightHooks.js
index dfb234b15ad27..c2d319074e7f8 100644
--- a/packages/react-server/src/ReactFlightHooks.js
+++ b/packages/react-server/src/ReactFlightHooks.js
@@ -39,7 +39,7 @@ export function prepareToUseHooksForComponent(
   thenableState = prevThenableState;
 }
 
-export function getThenableStateAfterSuspending() {
+export function getThenableStateAfterSuspending(): null | ThenableState {
   const state = thenableState;
   thenableState = null;
   return state;
diff --git a/packages/react/src/BadMapPolyfill.js b/packages/react/src/BadMapPolyfill.js
index 9cad26144490e..89615303ce4e5 100644
--- a/packages/react/src/BadMapPolyfill.js
+++ b/packages/react/src/BadMapPolyfill.js
@@ -6,7 +6,7 @@
  * @flow
  */
 
-export let hasBadMapPolyfill;
+export let hasBadMapPolyfill: boolean;
 
 if (__DEV__) {
   hasBadMapPolyfill = false;
diff --git a/packages/scheduler/src/forks/SchedulerPostTask.js b/packages/scheduler/src/forks/SchedulerPostTask.js
index 8169bf1acc945..36074d5a97c39 100644
--- a/packages/scheduler/src/forks/SchedulerPostTask.js
+++ b/packages/scheduler/src/forks/SchedulerPostTask.js
@@ -59,7 +59,7 @@ let currentPriorityLevel_DEPRECATED = NormalPriority;
 
 // `isInputPending` is not available. Since we have no way of knowing if
 // there's pending input, always yield at the end of the frame.
-export function unstable_shouldYield() {
+export function unstable_shouldYield(): boolean {
   return getCurrentTime() >= deadline;
 }
 
diff --git a/packages/shared/CheckStringCoercion.js b/packages/shared/CheckStringCoercion.js
index e8fbf64a99b88..a6c72c4bbe77a 100644
--- a/packages/shared/CheckStringCoercion.js
+++ b/packages/shared/CheckStringCoercion.js
@@ -73,7 +73,7 @@ function testStringCoercion(value: mixed) {
 export function checkAttributeStringCoercion(
   value: mixed,
   attributeName: string,
-) {
+): void | string {
   if (__DEV__) {
     if (willCoercionThrow(value)) {
       console.error(
@@ -87,7 +87,7 @@ export function checkAttributeStringCoercion(
   }
 }
 
-export function checkKeyStringCoercion(value: mixed) {
+export function checkKeyStringCoercion(value: mixed): void | string {
   if (__DEV__) {
     if (willCoercionThrow(value)) {
       console.error(
@@ -100,7 +100,10 @@ export function checkKeyStringCoercion(value: mixed) {
   }
 }
 
-export function checkPropStringCoercion(value: mixed, propName: string) {
+export function checkPropStringCoercion(
+  value: mixed,
+  propName: string,
+): void | string {
   if (__DEV__) {
     if (willCoercionThrow(value)) {
       console.error(
@@ -114,7 +117,10 @@ export function checkPropStringCoercion(value: mixed, propName: string) {
   }
 }
 
-export function checkCSSPropertyStringCoercion(value: mixed, propName: string) {
+export function checkCSSPropertyStringCoercion(
+  value: mixed,
+  propName: string,
+): void | string {
   if (__DEV__) {
     if (willCoercionThrow(value)) {
       console.error(
@@ -128,7 +134,7 @@ export function checkCSSPropertyStringCoercion(value: mixed, propName: string) {
   }
 }
 
-export function checkHtmlStringCoercion(value: mixed) {
+export function checkHtmlStringCoercion(value: mixed): void | string {
   if (__DEV__) {
     if (willCoercionThrow(value)) {
       console.error(
@@ -141,7 +147,7 @@ export function checkHtmlStringCoercion(value: mixed) {
   }
 }
 
-export function checkFormFieldValueStringCoercion(value: mixed) {
+export function checkFormFieldValueStringCoercion(value: mixed): void | string {
   if (__DEV__) {
     if (willCoercionThrow(value)) {
       console.error(
diff --git a/packages/shared/ReactErrorUtils.js b/packages/shared/ReactErrorUtils.js
index ae344d302f441..5a33bfceae1c0 100644
--- a/packages/shared/ReactErrorUtils.js
+++ b/packages/shared/ReactErrorUtils.js
@@ -105,7 +105,7 @@ export function rethrowCaughtError() {
   }
 }
 
-export function hasCaughtError() {
+export function hasCaughtError(): boolean {
   return hasError;
 }
 
diff --git a/packages/shared/ReactSymbols.js b/packages/shared/ReactSymbols.js
index 50f4fba27d98a..c68e50d5c693e 100644
--- a/packages/shared/ReactSymbols.js
+++ b/packages/shared/ReactSymbols.js
@@ -12,28 +12,36 @@
 // Please consider also adding to 'react-devtools-shared/src/backend/ReactSymbols'
 
 // The Symbol used to tag the ReactElement-like types.
-export const REACT_ELEMENT_TYPE = Symbol.for('react.element');
-export const REACT_PORTAL_TYPE = Symbol.for('react.portal');
-export const REACT_FRAGMENT_TYPE = Symbol.for('react.fragment');
-export const REACT_STRICT_MODE_TYPE = Symbol.for('react.strict_mode');
-export const REACT_PROFILER_TYPE = Symbol.for('react.profiler');
-export const REACT_PROVIDER_TYPE = Symbol.for('react.provider');
-export const REACT_CONTEXT_TYPE = Symbol.for('react.context');
-export const REACT_SERVER_CONTEXT_TYPE = Symbol.for('react.server_context');
-export const REACT_FORWARD_REF_TYPE = Symbol.for('react.forward_ref');
-export const REACT_SUSPENSE_TYPE = Symbol.for('react.suspense');
-export const REACT_SUSPENSE_LIST_TYPE = Symbol.for('react.suspense_list');
-export const REACT_MEMO_TYPE = Symbol.for('react.memo');
-export const REACT_LAZY_TYPE = Symbol.for('react.lazy');
-export const REACT_SCOPE_TYPE = Symbol.for('react.scope');
-export const REACT_DEBUG_TRACING_MODE_TYPE = Symbol.for(
+export const REACT_ELEMENT_TYPE: symbol = Symbol.for('react.element');
+export const REACT_PORTAL_TYPE: symbol = Symbol.for('react.portal');
+export const REACT_FRAGMENT_TYPE: symbol = Symbol.for('react.fragment');
+export const REACT_STRICT_MODE_TYPE: symbol = Symbol.for('react.strict_mode');
+export const REACT_PROFILER_TYPE: symbol = Symbol.for('react.profiler');
+export const REACT_PROVIDER_TYPE: symbol = Symbol.for('react.provider');
+export const REACT_CONTEXT_TYPE: symbol = Symbol.for('react.context');
+export const REACT_SERVER_CONTEXT_TYPE: symbol = Symbol.for(
+  'react.server_context',
+);
+export const REACT_FORWARD_REF_TYPE: symbol = Symbol.for('react.forward_ref');
+export const REACT_SUSPENSE_TYPE: symbol = Symbol.for('react.suspense');
+export const REACT_SUSPENSE_LIST_TYPE: symbol = Symbol.for(
+  'react.suspense_list',
+);
+export const REACT_MEMO_TYPE: symbol = Symbol.for('react.memo');
+export const REACT_LAZY_TYPE: symbol = Symbol.for('react.lazy');
+export const REACT_SCOPE_TYPE: symbol = Symbol.for('react.scope');
+export const REACT_DEBUG_TRACING_MODE_TYPE: symbol = Symbol.for(
   'react.debug_trace_mode',
 );
-export const REACT_OFFSCREEN_TYPE = Symbol.for('react.offscreen');
-export const REACT_LEGACY_HIDDEN_TYPE = Symbol.for('react.legacy_hidden');
-export const REACT_CACHE_TYPE = Symbol.for('react.cache');
-export const REACT_TRACING_MARKER_TYPE = Symbol.for('react.tracing_marker');
-export const REACT_SERVER_CONTEXT_DEFAULT_VALUE_NOT_LOADED = Symbol.for(
+export const REACT_OFFSCREEN_TYPE: symbol = Symbol.for('react.offscreen');
+export const REACT_LEGACY_HIDDEN_TYPE: symbol = Symbol.for(
+  'react.legacy_hidden',
+);
+export const REACT_CACHE_TYPE: symbol = Symbol.for('react.cache');
+export const REACT_TRACING_MARKER_TYPE: symbol = Symbol.for(
+  'react.tracing_marker',
+);
+export const REACT_SERVER_CONTEXT_DEFAULT_VALUE_NOT_LOADED: symbol = Symbol.for(
   'react.default_value',
 );
 
diff --git a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
index 520b102b7d3fc..67812e36dae55 100644
--- a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
+++ b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
@@ -19,7 +19,7 @@ export const enableFilterEmptyStringAttributesDOM = __VARIANT__;
 export const enableLegacyFBSupport = __VARIANT__;
 export const skipUnmountedBoundaries = __VARIANT__;
 export const enableUseRefAccessWarning = __VARIANT__;
-export const deletedTreeCleanUpLevel = __VARIANT__ ? 3 : 1;
+export const deletedTreeCleanUpLevel: number = __VARIANT__ ? 3 : 1;
 export const enableProfilerNestedUpdateScheduledHook = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableLazyContextPropagation = __VARIANT__;
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index 2c59dde3a11b6..18a333e40f769 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -39,13 +39,13 @@ export const {
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.
 // It's not used anywhere in production yet.
 
-export const enableStrictEffects =
+export const enableStrictEffects: boolean =
   __DEV__ && dynamicFeatureFlags.enableStrictEffects;
 export const debugRenderPhaseSideEffectsForStrictMode = __DEV__;
 export const enableProfilerTimer = __PROFILE__;
 export const enableProfilerCommitHooks = __PROFILE__;
 export const enableProfilerNestedUpdatePhase = __PROFILE__;
-export const enableProfilerNestedUpdateScheduledHook =
+export const enableProfilerNestedUpdateScheduledHook: boolean =
   __PROFILE__ && dynamicFeatureFlags.enableProfilerNestedUpdateScheduledHook;
 export const enableUpdaterTracking = __PROFILE__;
 
@@ -57,7 +57,7 @@ export const enableUseHook = true;
 export const enableUseMemoCacheHook = true;
 
 // Logs additional User Timing API marks for use with an experimental profiling tool.
-export const enableSchedulingProfiler =
+export const enableSchedulingProfiler: boolean =
   __PROFILE__ && dynamicFeatureFlags.enableSchedulingProfiler;
 
 // Note: we'll want to remove this when we to userland implementation.
diff --git a/packages/shared/isValidElementType.js b/packages/shared/isValidElementType.js
index c0e4cdc769323..d2b4602de22b1 100644
--- a/packages/shared/isValidElementType.js
+++ b/packages/shared/isValidElementType.js
@@ -35,7 +35,7 @@ import {
 
 const REACT_MODULE_REFERENCE: symbol = Symbol.for('react.module.reference');
 
-export default function isValidElementType(type: mixed) {
+export default function isValidElementType(type: mixed): boolean {
   if (typeof type === 'string' || typeof type === 'function') {
     return true;
   }