Flow: types first in reconciler

packages/react-reconciler/src/ReactChildFiber.new.js

@@ -264,11 +264,18 @@ function resolveLazy(lazyType) {
   return init(payload);
 }
 
+type ChildReconciler = (
+  returnFiber: Fiber,
+  currentFirstChild: Fiber | null,
+  newChild: any,
+  lanes: Lanes,
+) => Fiber | null;
+
 // This wrapper function exists because I expect to clone the code in each path
 // to be able to optimize each path individually by branching early. This needs
 // a compiler or we can do it manually. Helpers that don't need this branching
 // live outside of this function.
-function ChildReconciler(shouldTrackSideEffects) {
+function createChildReconciler(shouldTrackSideEffects): ChildReconciler {
   function deleteChild(returnFiber: Fiber, childToDelete: Fiber): void {
     if (!shouldTrackSideEffects) {
       // Noop.
@@ -1352,8 +1359,10 @@ function ChildReconciler(shouldTrackSideEffects) {
   return reconcileChildFibers;
 }
 
-export const reconcileChildFibers = ChildReconciler(true);
-export const mountChildFibers = ChildReconciler(false);
+export const reconcileChildFibers: ChildReconciler = createChildReconciler(
+  true,
+);
+export const mountChildFibers: ChildReconciler = createChildReconciler(false);
 
 export function cloneChildFibers(
   current: Fiber | null,

packages/react-reconciler/src/ReactChildFiber.old.js

@@ -264,11 +264,18 @@ function resolveLazy(lazyType) {
   return init(payload);
 }
 
+type ChildReconciler = (
+  returnFiber: Fiber,
+  currentFirstChild: Fiber | null,
+  newChild: any,
+  lanes: Lanes,
+) => Fiber | null;
+
 // This wrapper function exists because I expect to clone the code in each path
 // to be able to optimize each path individually by branching early. This needs
 // a compiler or we can do it manually. Helpers that don't need this branching
 // live outside of this function.
-function ChildReconciler(shouldTrackSideEffects) {
+function createChildReconciler(shouldTrackSideEffects): ChildReconciler {
   function deleteChild(returnFiber: Fiber, childToDelete: Fiber): void {
     if (!shouldTrackSideEffects) {
       // Noop.
@@ -1352,8 +1359,10 @@ function ChildReconciler(shouldTrackSideEffects) {
   return reconcileChildFibers;
 }
 
-export const reconcileChildFibers = ChildReconciler(true);
-export const mountChildFibers = ChildReconciler(false);
+export const reconcileChildFibers: ChildReconciler = createChildReconciler(
+  true,
+);
+export const mountChildFibers: ChildReconciler = createChildReconciler(false);
 
 export function cloneChildFibers(
   current: Fiber | null,

packages/react-reconciler/src/ReactFiberClassComponent.new.js

@@ -72,7 +72,7 @@ const fakeInternalInstance = {};
 
 // React.Component uses a shared frozen object by default.
 // We'll use it to determine whether we need to initialize legacy refs.
-export const emptyRefsObject = new React.Component().refs;
+export const emptyRefsObject: $FlowFixMe = new React.Component().refs;
 
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;

packages/react-reconciler/src/ReactFiberClassComponent.old.js

@@ -72,7 +72,7 @@ const fakeInternalInstance = {};
 
 // React.Component uses a shared frozen object by default.
 // We'll use it to determine whether we need to initialize legacy refs.
-export const emptyRefsObject = new React.Component().refs;
+export const emptyRefsObject: $FlowFixMe = new React.Component().refs;
 
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;

packages/react-reconciler/src/ReactFiberContext.new.js

@@ -24,7 +24,8 @@ if (__DEV__) {
   warnedAboutMissingGetChildContext = {};
 }
 
-export const emptyContextObject = {};
+// $FlowFixMe[incompatible-exact]
+export const emptyContextObject: {} = {};
 if (__DEV__) {
   Object.freeze(emptyContextObject);
 }

packages/react-reconciler/src/ReactFiberContext.old.js

@@ -24,7 +24,8 @@ if (__DEV__) {
   warnedAboutMissingGetChildContext = {};
 }
 
-export const emptyContextObject = {};
+// $FlowFixMe[incompatible-exact]
+export const emptyContextObject: {} = {};
 if (__DEV__) {
   Object.freeze(emptyContextObject);
 }

packages/react-reconciler/src/ReactFiberHotReloading.js

@@ -63,13 +63,13 @@ export type FindHostInstancesForRefresh = (
 export const setRefreshHandler: (
   handler: RefreshHandler | null,
 ) => void = enableNewReconciler ? setRefreshHandler_new : setRefreshHandler_old;
-export const resolveFunctionForHotReloading = enableNewReconciler
+export const resolveFunctionForHotReloading: typeof resolveFunctionForHotReloading_new = enableNewReconciler
   ? resolveFunctionForHotReloading_new
   : resolveFunctionForHotReloading_old;
-export const resolveClassForHotReloading = enableNewReconciler
+export const resolveClassForHotReloading: typeof resolveClassForHotReloading_new = enableNewReconciler
   ? resolveClassForHotReloading_new
   : resolveClassForHotReloading_old;
-export const resolveForwardRefForHotReloading = enableNewReconciler
+export const resolveForwardRefForHotReloading: typeof resolveForwardRefForHotReloading_new = enableNewReconciler
   ? resolveForwardRefForHotReloading_new
   : resolveForwardRefForHotReloading_old;
 export const isCompatibleFamilyForHotReloading: (

packages/react-reconciler/src/ReactFiberReconciler.js

@@ -90,107 +90,113 @@ import {
   getCurrentUpdatePriority as getCurrentUpdatePriority_new,
 } from './ReactFiberReconciler.new';
 
-export const createContainer = enableNewReconciler
+export const createContainer: typeof createContainer_new = enableNewReconciler
   ? createContainer_new
   : createContainer_old;
-export const createHydrationContainer = enableNewReconciler
+export const createHydrationContainer: typeof createHydrationContainer_new = enableNewReconciler
   ? createHydrationContainer_new
   : createHydrationContainer_old;
-export const updateContainer = enableNewReconciler
+export const updateContainer: typeof updateContainer_new = enableNewReconciler
   ? updateContainer_new
   : updateContainer_old;
-export const batchedUpdates = enableNewReconciler
+export const batchedUpdates: typeof batchedUpdates_new = enableNewReconciler
   ? batchedUpdates_new
   : batchedUpdates_old;
-export const deferredUpdates = enableNewReconciler
+export const deferredUpdates: typeof deferredUpdates_new = enableNewReconciler
   ? deferredUpdates_new
   : deferredUpdates_old;
-export const discreteUpdates = enableNewReconciler
+export const discreteUpdates: typeof discreteUpdates_new = enableNewReconciler
   ? discreteUpdates_new
   : discreteUpdates_old;
-export const flushControlled = enableNewReconciler
+export const flushControlled: typeof flushControlled_new = enableNewReconciler
   ? flushControlled_new
   : flushControlled_old;
-export const flushSync = enableNewReconciler ? flushSync_new : flushSync_old;
-export const isAlreadyRendering = enableNewReconciler
+export const flushSync: typeof flushSync_new = enableNewReconciler
+  ? flushSync_new
+  : flushSync_old;
+export const isAlreadyRendering: typeof isAlreadyRendering_new = enableNewReconciler
   ? isAlreadyRendering_new
   : isAlreadyRendering_old;
-export const flushPassiveEffects = enableNewReconciler
+export const flushPassiveEffects: typeof flushPassiveEffects_new = enableNewReconciler
   ? flushPassiveEffects_new
   : flushPassiveEffects_old;
-export const getPublicRootInstance = enableNewReconciler
+export const getPublicRootInstance: typeof getPublicRootInstance_new = enableNewReconciler
   ? getPublicRootInstance_new
   : getPublicRootInstance_old;
-export const attemptSynchronousHydration = enableNewReconciler
+export const attemptSynchronousHydration: typeof attemptSynchronousHydration_new = enableNewReconciler
   ? attemptSynchronousHydration_new
   : attemptSynchronousHydration_old;
-export const attemptDiscreteHydration = enableNewReconciler
+export const attemptDiscreteHydration: typeof attemptDiscreteHydration_new = enableNewReconciler
   ? attemptDiscreteHydration_new
   : attemptDiscreteHydration_old;
-export const attemptContinuousHydration = enableNewReconciler
+export const attemptContinuousHydration: typeof attemptContinuousHydration_new = enableNewReconciler
   ? attemptContinuousHydration_new
   : attemptContinuousHydration_old;
-export const attemptHydrationAtCurrentPriority = enableNewReconciler
+export const attemptHydrationAtCurrentPriority: typeof attemptHydrationAtCurrentPriority_new = enableNewReconciler
   ? attemptHydrationAtCurrentPriority_new
   : attemptHydrationAtCurrentPriority_old;
-export const getCurrentUpdatePriority = enableNewReconciler
+export const getCurrentUpdatePriority: typeof getCurrentUpdatePriority_new = enableNewReconciler
   ? getCurrentUpdatePriority_new
-  : getCurrentUpdatePriority_old;
-export const findHostInstance = enableNewReconciler
+  : /* $FlowFixMe[incompatible-type] opaque types EventPriority from new and old
+     * are incompatible. */
+    getCurrentUpdatePriority_old;
+export const findHostInstance: typeof findHostInstance_new = enableNewReconciler
   ? findHostInstance_new
   : findHostInstance_old;
-export const findHostInstanceWithWarning = enableNewReconciler
+export const findHostInstanceWithWarning: typeof findHostInstanceWithWarning_new = enableNewReconciler
   ? findHostInstanceWithWarning_new
   : findHostInstanceWithWarning_old;
-export const findHostInstanceWithNoPortals = enableNewReconciler
+export const findHostInstanceWithNoPortals: typeof findHostInstanceWithNoPortals_new = enableNewReconciler
   ? findHostInstanceWithNoPortals_new
   : findHostInstanceWithNoPortals_old;
-export const shouldError = enableNewReconciler
+export const shouldError: typeof shouldError_new = enableNewReconciler
   ? shouldError_new
   : shouldError_old;
-export const shouldSuspend = enableNewReconciler
+export const shouldSuspend: typeof shouldSuspend_new = enableNewReconciler
   ? shouldSuspend_new
   : shouldSuspend_old;
-export const injectIntoDevTools = enableNewReconciler
+export const injectIntoDevTools: typeof injectIntoDevTools_new = enableNewReconciler
   ? injectIntoDevTools_new
   : injectIntoDevTools_old;
-export const createPortal = enableNewReconciler
+export const createPortal: typeof createPortal_new = enableNewReconciler
   ? createPortal_new
   : createPortal_old;
-export const createComponentSelector = enableNewReconciler
+export const createComponentSelector: typeof createComponentSelector_new = enableNewReconciler
   ? createComponentSelector_new
   : createComponentSelector_old;
 
-export const createHasPseudoClassSelector = enableNewReconciler
+export const createHasPseudoClassSelector: typeof createHasPseudoClassSelector_new = enableNewReconciler
   ? createHasPseudoClassSelector_new
   : createHasPseudoClassSelector_old;
-export const createRoleSelector = enableNewReconciler
+export const createRoleSelector: typeof createRoleSelector_new = enableNewReconciler
   ? createRoleSelector_new
   : createRoleSelector_old;
-export const createTextSelector = enableNewReconciler
+export const createTextSelector: typeof createTextSelector_new = enableNewReconciler
   ? createTextSelector_new
   : createTextSelector_old;
-export const createTestNameSelector = enableNewReconciler
+export const createTestNameSelector: typeof createTestNameSelector_new = enableNewReconciler
   ? createTestNameSelector_new
   : createTestNameSelector_old;
-export const getFindAllNodesFailureDescription = enableNewReconciler
+export const getFindAllNodesFailureDescription: typeof getFindAllNodesFailureDescription_new = enableNewReconciler
   ? getFindAllNodesFailureDescription_new
   : getFindAllNodesFailureDescription_old;
-export const findAllNodes = enableNewReconciler
+export const findAllNodes: typeof findAllNodes_new = enableNewReconciler
   ? findAllNodes_new
   : findAllNodes_old;
-export const findBoundingRects = enableNewReconciler
+export const findBoundingRects: typeof findBoundingRects_new = enableNewReconciler
   ? findBoundingRects_new
   : findBoundingRects_old;
-export const focusWithin = enableNewReconciler
+export const focusWithin: typeof focusWithin_new = enableNewReconciler
   ? focusWithin_new
   : focusWithin_old;
-export const observeVisibleRects = enableNewReconciler
+export const observeVisibleRects: typeof observeVisibleRects_new = enableNewReconciler
   ? observeVisibleRects_new
   : observeVisibleRects_old;
-export const registerMutableSourceForHydration = enableNewReconciler
+export const registerMutableSourceForHydration: typeof registerMutableSourceForHydration_new = enableNewReconciler
   ? registerMutableSourceForHydration_new
   : registerMutableSourceForHydration_old;
-export const runWithPriority = enableNewReconciler
+/* $FlowFixMe[incompatible-type] opaque types EventPriority from new and old
+ * are incompatible. */
+export const runWithPriority: typeof runWithPriority_new = enableNewReconciler
   ? runWithPriority_new
   : runWithPriority_old;

packages/react-reconciler/src/ReactFiberUnwindWork.new.js

@@ -62,7 +62,7 @@ function unwindWork(
   current: Fiber | null,
   workInProgress: Fiber,
   renderLanes: Lanes,
-) {
+): Fiber | null {
   // Note: This intentionally doesn't check if we're hydrating because comparing
   // to the current tree provider fiber is just as fast and less error-prone.
   // Ideally we would have a special version of the work loop only

packages/react-reconciler/src/ReactFiberUnwindWork.old.js

@@ -62,7 +62,7 @@ function unwindWork(
   current: Fiber | null,
   workInProgress: Fiber,
   renderLanes: Lanes,
-) {
+): Fiber | null {
   // Note: This intentionally doesn't check if we're hydrating because comparing
   // to the current tree provider fiber is just as fast and less error-prone.
   // Ideally we would have a special version of the work loop only

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -570,7 +570,7 @@ export function getWorkInProgressRootRenderLanes(): Lanes {
   return workInProgressRootRenderLanes;
 }
 
-export function requestEventTime() {
+export function requestEventTime(): number {
   if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {
     // We're inside React, so it's fine to read the actual time.
     return now();
@@ -585,7 +585,7 @@ export function requestEventTime() {
   return currentEventTime;
 }
 
-export function getCurrentTime() {
+export function getCurrentTime(): number {
   return now();
 }
 
@@ -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): void {
+export function flushSync<R>(fn: (() => R) | void): R | 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 (
@@ -1605,7 +1605,7 @@ export function isAlreadyRendering(): boolean {
   );
 }
 
-export function isInvalidExecutionContextForEventFunction() {
+export function isInvalidExecutionContextForEventFunction(): boolean {
   // Used to throw if certain APIs are called from the wrong context.
   return (executionContext & RenderContext) !== NoContext;
 }

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -570,7 +570,7 @@ export function getWorkInProgressRootRenderLanes(): Lanes {
   return workInProgressRootRenderLanes;
 }
 
-export function requestEventTime() {
+export function requestEventTime(): number {
   if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {
     // We're inside React, so it's fine to read the actual time.
     return now();
@@ -585,7 +585,7 @@ export function requestEventTime() {
   return currentEventTime;
 }
 
-export function getCurrentTime() {
+export function getCurrentTime(): number {
   return now();
 }
 
@@ -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): void {
+export function flushSync<R>(fn: (() => R) | void): R | 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 (
@@ -1605,7 +1605,7 @@ export function isAlreadyRendering(): boolean {
   );
 }
 
-export function isInvalidExecutionContextForEventFunction() {
+export function isInvalidExecutionContextForEventFunction(): boolean {
   // Used to throw if certain APIs are called from the wrong context.
   return (executionContext & RenderContext) !== NoContext;
 }

scripts/flow/config/flowconfig

@@ -73,7 +73,7 @@ well_formed_exports.includes=<PROJECT_ROOT>/packages/react-is
 ; well_formed_exports.includes=<PROJECT_ROOT>/packages/react-native-renderer
 well_formed_exports.includes=<PROJECT_ROOT>/packages/react-noop-renderer
 well_formed_exports.includes=<PROJECT_ROOT>/packages/react-pg
-; well_formed_exports.includes=<PROJECT_ROOT>/packages/react-reconciler
+well_formed_exports.includes=<PROJECT_ROOT>/packages/react-reconciler
 well_formed_exports.includes=<PROJECT_ROOT>/packages/react-refresh
 well_formed_exports.includes=<PROJECT_ROOT>/packages/react-server
 well_formed_exports.includes=<PROJECT_ROOT>/packages/react-server-dom-relay