diff --git a/packages/react-reconciler/src/ReactFiber.new.js b/packages/react-reconciler/src/ReactFiber.new.js
index 484f954a6f4df..16297ef3bea4b 100644
--- a/packages/react-reconciler/src/ReactFiber.new.js
+++ b/packages/react-reconciler/src/ReactFiber.new.js
@@ -693,7 +693,7 @@ export function createFiberFromSuspense(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(SuspenseComponent, pendingProps, key, mode);
   fiber.elementType = REACT_SUSPENSE_TYPE;
   fiber.lanes = lanes;
@@ -705,7 +705,7 @@ export function createFiberFromSuspenseList(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(SuspenseListComponent, pendingProps, key, mode);
   fiber.elementType = REACT_SUSPENSE_LIST_TYPE;
   fiber.lanes = lanes;
@@ -717,7 +717,7 @@ export function createFiberFromOffscreen(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(OffscreenComponent, pendingProps, key, mode);
   fiber.elementType = REACT_OFFSCREEN_TYPE;
   fiber.lanes = lanes;
@@ -736,7 +736,7 @@ export function createFiberFromLegacyHidden(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(LegacyHiddenComponent, pendingProps, key, mode);
   fiber.elementType = REACT_LEGACY_HIDDEN_TYPE;
   fiber.lanes = lanes;
@@ -757,7 +757,7 @@ export function createFiberFromCache(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(CacheComponent, pendingProps, key, mode);
   fiber.elementType = REACT_CACHE_TYPE;
   fiber.lanes = lanes;
@@ -769,7 +769,7 @@ export function createFiberFromTracingMarker(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(TracingMarkerComponent, pendingProps, key, mode);
   fiber.elementType = REACT_TRACING_MARKER_TYPE;
   fiber.lanes = lanes;
diff --git a/packages/react-reconciler/src/ReactFiber.old.js b/packages/react-reconciler/src/ReactFiber.old.js
index 60d52c2d0b827..5599141eec8a0 100644
--- a/packages/react-reconciler/src/ReactFiber.old.js
+++ b/packages/react-reconciler/src/ReactFiber.old.js
@@ -693,7 +693,7 @@ export function createFiberFromSuspense(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(SuspenseComponent, pendingProps, key, mode);
   fiber.elementType = REACT_SUSPENSE_TYPE;
   fiber.lanes = lanes;
@@ -705,7 +705,7 @@ export function createFiberFromSuspenseList(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(SuspenseListComponent, pendingProps, key, mode);
   fiber.elementType = REACT_SUSPENSE_LIST_TYPE;
   fiber.lanes = lanes;
@@ -717,7 +717,7 @@ export function createFiberFromOffscreen(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(OffscreenComponent, pendingProps, key, mode);
   fiber.elementType = REACT_OFFSCREEN_TYPE;
   fiber.lanes = lanes;
@@ -736,7 +736,7 @@ export function createFiberFromLegacyHidden(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(LegacyHiddenComponent, pendingProps, key, mode);
   fiber.elementType = REACT_LEGACY_HIDDEN_TYPE;
   fiber.lanes = lanes;
@@ -757,7 +757,7 @@ export function createFiberFromCache(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(CacheComponent, pendingProps, key, mode);
   fiber.elementType = REACT_CACHE_TYPE;
   fiber.lanes = lanes;
@@ -769,7 +769,7 @@ export function createFiberFromTracingMarker(
   mode: TypeOfMode,
   lanes: Lanes,
   key: null | string,
-) {
+): Fiber {
   const fiber = createFiber(TracingMarkerComponent, pendingProps, key, mode);
   fiber.elementType = REACT_TRACING_MARKER_TYPE;
   fiber.lanes = lanes;
diff --git a/packages/react-reconciler/src/ReactFiberAct.new.js b/packages/react-reconciler/src/ReactFiberAct.new.js
index 53a4922ad5417..fbb12f4bd178b 100644
--- a/packages/react-reconciler/src/ReactFiberAct.new.js
+++ b/packages/react-reconciler/src/ReactFiberAct.new.js
@@ -37,7 +37,7 @@ export function isLegacyActEnvironment(fiber: Fiber): boolean {
   return false;
 }
 
-export function isConcurrentActEnvironment() {
+export function isConcurrentActEnvironment(): void | boolean {
   if (__DEV__) {
     const isReactActEnvironmentGlobal =
       // $FlowFixMe – Flow doesn't know about IS_REACT_ACT_ENVIRONMENT global
diff --git a/packages/react-reconciler/src/ReactFiberAct.old.js b/packages/react-reconciler/src/ReactFiberAct.old.js
index 32d3a0f270719..90f771b594059 100644
--- a/packages/react-reconciler/src/ReactFiberAct.old.js
+++ b/packages/react-reconciler/src/ReactFiberAct.old.js
@@ -37,7 +37,7 @@ export function isLegacyActEnvironment(fiber: Fiber): boolean {
   return false;
 }
 
-export function isConcurrentActEnvironment() {
+export function isConcurrentActEnvironment(): void | boolean {
   if (__DEV__) {
     const isReactActEnvironmentGlobal =
       // $FlowFixMe – Flow doesn't know about IS_REACT_ACT_ENVIRONMENT global