diff --git a/IntegrationTests/TimersTest.js b/IntegrationTests/TimersTest.js
index 04496e0a943..3a74f97edf3 100644
--- a/IntegrationTests/TimersTest.js
+++ b/IntegrationTests/TimersTest.js
@@ -39,7 +39,7 @@ class TimersTest extends React.Component<Props, State> {
   };
 
   setTimeout(fn: () => void, time: number): TimeoutID {
-    const id = setTimeout(() => {
+    const id: TimeoutID = setTimeout(() => {
       this._timeoutIDs.delete(id);
       fn();
     }, time);
@@ -70,7 +70,7 @@ class TimersTest extends React.Component<Props, State> {
   }
 
   setImmediate(fn: () => void): ImmediateID {
-    const id = setImmediate(() => {
+    const id: any = setImmediate(() => {
       this._immediateIDs.delete(id);
       fn();
     });
@@ -81,7 +81,7 @@ class TimersTest extends React.Component<Props, State> {
   }
 
   requestAnimationFrame(fn: () => void): AnimationFrameID {
-    const id = requestAnimationFrame(() => {
+    const id: AnimationFrameID = requestAnimationFrame(() => {
       this._animationFrameIDs.delete(id);
       fn();
     });
diff --git a/Libraries/Animated/AnimatedImplementation.js b/Libraries/Animated/AnimatedImplementation.js
index 21f6c7069e1..7e25c65f2a4 100644
--- a/Libraries/Animated/AnimatedImplementation.js
+++ b/Libraries/Animated/AnimatedImplementation.js
@@ -415,7 +415,7 @@ const parallel = function (
       });
     },
 
-    _startNativeLoop: function () {
+    _startNativeLoop: function (): empty {
       throw new Error(
         'Loops run using the native driver cannot contain Animated.parallel animations',
       );
diff --git a/Libraries/BatchedBridge/MessageQueue.js b/Libraries/BatchedBridge/MessageQueue.js
index 65c7102ca72..a9810bc6f94 100644
--- a/Libraries/BatchedBridge/MessageQueue.js
+++ b/Libraries/BatchedBridge/MessageQueue.js
@@ -252,7 +252,7 @@ class MessageQueue {
       // folly-convertible.  As a special case, if a prop value is a
       // function it is permitted here, and special-cased in the
       // conversion.
-      const isValidArgument = (val: mixed) => {
+      const isValidArgument = (val: mixed): boolean => {
         switch (typeof val) {
           case 'undefined':
           case 'boolean':
diff --git a/Libraries/BugReporting/getReactData.js b/Libraries/BugReporting/getReactData.js
index 4440cc4e0d0..cc988ced109 100644
--- a/Libraries/BugReporting/getReactData.js
+++ b/Libraries/BugReporting/getReactData.js
@@ -165,7 +165,7 @@ function copyWithSetImpl(
   path: Array<string | number>,
   idx: number,
   value: any,
-) {
+): any {
   if (idx >= path.length) {
     return value;
   }
diff --git a/Libraries/Core/Timers/JSTimers.js b/Libraries/Core/Timers/JSTimers.js
index 8147ebb8e90..33f8a68d76b 100644
--- a/Libraries/Core/Timers/JSTimers.js
+++ b/Libraries/Core/Timers/JSTimers.js
@@ -242,7 +242,7 @@ const JSTimers = {
    * @param {function} func Callback to be invoked before the end of the
    * current JavaScript execution loop.
    */
-  queueReactNativeMicrotask: function (func: Function, ...args: any) {
+  queueReactNativeMicrotask: function (func: Function, ...args: any): number {
     const id = _allocateCallback(
       () => func.apply(undefined, args),
       'queueReactNativeMicrotask',
@@ -254,7 +254,7 @@ const JSTimers = {
   /**
    * @param {function} func Callback to be invoked every frame.
    */
-  requestAnimationFrame: function (func: Function) {
+  requestAnimationFrame: function (func: Function): any | number {
     const id = _allocateCallback(func, 'requestAnimationFrame');
     createTimer(id, 1, Date.now(), /* recurring */ false);
     return id;
@@ -265,16 +265,19 @@ const JSTimers = {
    * with time remaining in frame.
    * @param {?object} options
    */
-  requestIdleCallback: function (func: Function, options: ?Object) {
+  requestIdleCallback: function (
+    func: Function,
+    options: ?Object,
+  ): any | number {
     if (requestIdleCallbacks.length === 0) {
       setSendIdleEvents(true);
     }
 
     const timeout = options && options.timeout;
-    const id = _allocateCallback(
+    const id: number = _allocateCallback(
       timeout != null
         ? (deadline: any) => {
-            const timeoutId = requestIdleCallbackTimeouts[id];
+            const timeoutId: number = requestIdleCallbackTimeouts[id];
             if (timeoutId) {
               JSTimers.clearTimeout(timeoutId);
               delete requestIdleCallbackTimeouts[id];
@@ -287,8 +290,8 @@ const JSTimers = {
     requestIdleCallbacks.push(id);
 
     if (timeout != null) {
-      const timeoutId = JSTimers.setTimeout(() => {
-        const index = requestIdleCallbacks.indexOf(id);
+      const timeoutId: number = JSTimers.setTimeout(() => {
+        const index: number = requestIdleCallbacks.indexOf(id);
         if (index > -1) {
           requestIdleCallbacks.splice(index, 1);
           _callTimer(id, global.performance.now(), true);
@@ -345,7 +348,7 @@ const JSTimers = {
    * This is called from the native side. We are passed an array of timerIDs,
    * and
    */
-  callTimers: function (timersToCall: Array<number>) {
+  callTimers: function (timersToCall: Array<number>): any | void {
     invariant(
       timersToCall.length !== 0,
       'Cannot call `callTimers` with an empty list of IDs.',
diff --git a/Libraries/Lists/ViewabilityHelper.js b/Libraries/Lists/ViewabilityHelper.js
index 104f3931c0a..33a9811825a 100644
--- a/Libraries/Lists/ViewabilityHelper.js
+++ b/Libraries/Lists/ViewabilityHelper.js
@@ -234,7 +234,7 @@ class ViewabilityHelper {
     }
     this._viewableIndices = viewableIndices;
     if (this._config.minimumViewTime) {
-      const handle = setTimeout(() => {
+      const handle: TimeoutID = setTimeout(() => {
         /* $FlowFixMe[incompatible-call] (>=0.63.0 site=react_native_fb) This
          * comment suppresses an error found when Flow v0.63 was deployed. To
          * see the error delete this comment and run Flow. */
diff --git a/Libraries/LogBox/Data/LogBoxData.js b/Libraries/LogBox/Data/LogBoxData.js
index d868d89e8e9..6c5e160dfe0 100644
--- a/Libraries/LogBox/Data/LogBoxData.js
+++ b/Libraries/LogBox/Data/LogBoxData.js
@@ -68,7 +68,7 @@ const observers: Set<{observer: Observer, ...}> = new Set();
 const ignorePatterns: Set<IgnorePattern> = new Set();
 let appInfo: ?() => AppInfo = null;
 let logs: LogBoxLogs = new Set();
-let updateTimeout = null;
+let updateTimeout: $FlowFixMe | null = null;
 let _isDisabled = false;
 let _selectedIndex = -1;
 
diff --git a/Libraries/vendor/emitter/__tests__/EventEmitter-test.js b/Libraries/vendor/emitter/__tests__/EventEmitter-test.js
index 7ae26000cc6..99288ac3e2e 100644
--- a/Libraries/vendor/emitter/__tests__/EventEmitter-test.js
+++ b/Libraries/vendor/emitter/__tests__/EventEmitter-test.js
@@ -9,6 +9,8 @@
  * @oncall react_native
  */
 
+import type {EventSubscription} from '../EventEmitter';
+
 import EventEmitter from '../EventEmitter';
 
 describe('listeners', () => {
@@ -309,7 +311,7 @@ describe('event emission', () => {
     const listenerA = jest.fn(() => {
       results.push('A');
     });
-    const listenerB = jest.fn(() => {
+    const listenerB: JestMockFn<Array<mixed>, void> = jest.fn(() => {
       results.push('B');
       subscriptionB.remove();
     });
@@ -317,7 +319,10 @@ describe('event emission', () => {
       results.push('C');
     });
     emitter.addListener('A', listenerA);
-    const subscriptionB = emitter.addListener('A', listenerB);
+    const subscriptionB: EventSubscription = emitter.addListener(
+      'A',
+      listenerB,
+    );
     emitter.addListener('A', listenerC);
 
     emitter.emit('A');
diff --git a/packages/react-native-codegen/src/generators/modules/GenerateModuleObjCpp/serializeMethod.js b/packages/react-native-codegen/src/generators/modules/GenerateModuleObjCpp/serializeMethod.js
index ea84c731b2c..d0864ca5ea8 100644
--- a/packages/react-native-codegen/src/generators/modules/GenerateModuleObjCpp/serializeMethod.js
+++ b/packages/react-native-codegen/src/generators/modules/GenerateModuleObjCpp/serializeMethod.js
@@ -295,7 +295,7 @@ function getParamObjCType(
 function getReturnObjCType(
   methodName: string,
   nullableTypeAnnotation: Nullable<NativeModuleReturnTypeAnnotation>,
-) {
+): string {
   const [typeAnnotation, nullable] = unwrapNullable(nullableTypeAnnotation);
 
   function wrapIntoNullableIfNeeded(generatedType: string) {
diff --git a/packages/react-native-codegen/src/parsers/flow/components/events.js b/packages/react-native-codegen/src/parsers/flow/components/events.js
index a6a788a467d..8384edd789b 100644
--- a/packages/react-native-codegen/src/parsers/flow/components/events.js
+++ b/packages/react-native-codegen/src/parsers/flow/components/events.js
@@ -104,7 +104,11 @@ function findEventArgumentsAndType(
   types: TypeMap,
   bubblingType: void | 'direct' | 'bubble',
   paperName: ?$FlowFixMe,
-) {
+): {
+  argumentProps: $FlowFixMe,
+  bubblingType: ?('direct' | 'bubble'),
+  paperTopLevelNameDeprecated: ?$FlowFixMe,
+} {
   if (!typeAnnotation.id) {
     throw new Error("typeAnnotation of event doesn't have a name");
   }
diff --git a/packages/rn-tester/js/examples/ActivityIndicator/ActivityIndicatorExample.js b/packages/rn-tester/js/examples/ActivityIndicator/ActivityIndicatorExample.js
index 7be8bc4a5dd..2713fce143f 100644
--- a/packages/rn-tester/js/examples/ActivityIndicator/ActivityIndicatorExample.js
+++ b/packages/rn-tester/js/examples/ActivityIndicator/ActivityIndicatorExample.js
@@ -17,7 +17,7 @@ function ToggleAnimatingActivityIndicator() {
 
   const [animating, setAnimating] = useState(true);
 
-  const setToggleTimeout = useCallback(() => {
+  const setToggleTimeout: () => void = useCallback(() => {
     timer.current = setTimeout(() => {
       setAnimating(currentState => !currentState);
       setToggleTimeout();
diff --git a/packages/rn-tester/js/examples/AnimatedGratuitousApp/AnExTilt.js b/packages/rn-tester/js/examples/AnimatedGratuitousApp/AnExTilt.js
index 3f9552ca88d..3e9b54a338d 100644
--- a/packages/rn-tester/js/examples/AnimatedGratuitousApp/AnExTilt.js
+++ b/packages/rn-tester/js/examples/AnimatedGratuitousApp/AnExTilt.js
@@ -60,7 +60,7 @@ class AnExTilt extends React.Component<Object, any> {
           useNativeDriver: false,
         }).start();
         this.state.panX.removeAllListeners();
-        const id = this.state.panX.addListener(({value}) => {
+        const id: any = this.state.panX.addListener(({value}) => {
           // listen until offscreen
           if (Math.abs(value) > 400) {
             this.state.panX.removeListener(id); // offscreen, so stop listening