diff --git a/.flowconfig b/.flowconfig
index b7aa19446d88dc..6c51ae9173407e 100644
--- a/.flowconfig
+++ b/.flowconfig
@@ -51,5 +51,7 @@ suppress_comment=\\(.\\|\n\\)*\\$FlowIssue\\((\\(<VERSION>\\)? *\\(site=[a-z,_]*
 suppress_comment=\\(.\\|\n\\)*\\$FlowFixedInNextDeploy
 suppress_comment=\\(.\\|\n\\)*\\$FlowExpectedError
 
+experimental.const_params=true
+
 [version]
 ^0.66.0
diff --git a/IntegrationTests/AsyncStorageTest.js b/IntegrationTests/AsyncStorageTest.js
index 3ce0265dd8ceb0..065e073f1329ec 100644
--- a/IntegrationTests/AsyncStorageTest.js
+++ b/IntegrationTests/AsyncStorageTest.js
@@ -56,10 +56,11 @@ function expectEqual(lhs, rhs, testname : string) {
 }
 
 function expectAsyncNoError(place, err) {
-  if (err instanceof Error) {
-    err = err.message;
+  let error = err;
+  if (error instanceof Error) {
+    error = error.message;
   }
-  expectTrue(err === null, 'Unexpected error in ' + place + ': ' + JSON.stringify(err));
+  expectTrue(error === null, 'Unexpected error in ' + place + ': ' + JSON.stringify(err));
 }
 
 function testSetAndGet() {
diff --git a/Libraries/Animated/src/AnimatedImplementation.js b/Libraries/Animated/src/AnimatedImplementation.js
index b23a44db0e4fc7..dc3e44a363ee41 100644
--- a/Libraries/Animated/src/AnimatedImplementation.js
+++ b/Libraries/Animated/src/AnimatedImplementation.js
@@ -127,7 +127,7 @@ const spring = function(
     configuration: SpringAnimationConfig,
     callback?: ?EndCallback,
   ): void {
-    callback = _combineCallbacks(callback, configuration);
+    let mergedCallbacks = _combineCallbacks(callback, configuration);
     const singleValue: any = animatedValue;
     const singleConfig: any = configuration;
     singleValue.stopTracking();
@@ -138,11 +138,11 @@ const spring = function(
           configuration.toValue,
           SpringAnimation,
           singleConfig,
-          callback,
+          mergedCallbacks,
         ),
       );
     } else {
-      singleValue.animate(new SpringAnimation(singleConfig), callback);
+      singleValue.animate(new SpringAnimation(singleConfig), mergedCallbacks);
     }
   };
   return (
@@ -180,7 +180,7 @@ const timing = function(
     configuration: TimingAnimationConfig,
     callback?: ?EndCallback,
   ): void {
-    callback = _combineCallbacks(callback, configuration);
+    let callbacks = _combineCallbacks(callback, configuration);
     const singleValue: any = animatedValue;
     const singleConfig: any = configuration;
     singleValue.stopTracking();
@@ -191,11 +191,11 @@ const timing = function(
           configuration.toValue,
           TimingAnimation,
           singleConfig,
-          callback,
+          callbacks,
         ),
       );
     } else {
-      singleValue.animate(new TimingAnimation(singleConfig), callback);
+      singleValue.animate(new TimingAnimation(singleConfig), callbacks);
     }
   };
 
@@ -234,11 +234,11 @@ const decay = function(
     configuration: DecayAnimationConfig,
     callback?: ?EndCallback,
   ): void {
-    callback = _combineCallbacks(callback, configuration);
+    let mergedCallbacks = _combineCallbacks(callback, configuration);
     const singleValue: any = animatedValue;
     const singleConfig: any = configuration;
     singleValue.stopTracking();
-    singleValue.animate(new DecayAnimation(singleConfig), callback);
+    singleValue.animate(new DecayAnimation(singleConfig), mergedCallbacks);
   };
 
   return (
diff --git a/Libraries/Animated/src/Easing.js b/Libraries/Animated/src/Easing.js
index f5c00fd813f7dc..f8d1ab5a4ad7f1 100644
--- a/Libraries/Animated/src/Easing.js
+++ b/Libraries/Animated/src/Easing.js
@@ -175,10 +175,11 @@ class Easing {
    * - http://tiny.cc/back_default (s = 1.70158, default)
    */
   static back(s: number): (t: number) => number {
-    if (s === undefined) {
-      s = 1.70158;
+    let ss = s;
+    if (ss === undefined) {
+      ss = 1.70158;
     }
-    return (t) => t * t * ((s + 1) * t - s);
+    return (t) => t * t * ((ss + 1) * t - ss);
   }
 
   /**
@@ -187,22 +188,23 @@ class Easing {
    * http://easings.net/#easeInBounce
    */
   static bounce(t: number): number {
-    if (t < 1 / 2.75) {
-      return 7.5625 * t * t;
+    let tt = t;
+    if (tt < 1 / 2.75) {
+      return 7.5625 * tt * tt;
     }
 
-    if (t < 2 / 2.75) {
-      t -= 1.5 / 2.75;
-      return 7.5625 * t * t + 0.75;
+    if (tt < 2 / 2.75) {
+      tt -= 1.5 / 2.75;
+      return 7.5625 * tt * tt + 0.75;
     }
 
-    if (t < 2.5 / 2.75) {
-      t -= 2.25 / 2.75;
-      return 7.5625 * t * t + 0.9375;
+    if (tt < 2.5 / 2.75) {
+      tt -= 2.25 / 2.75;
+      return 7.5625 * tt * tt + 0.9375;
     }
 
-    t -= 2.625 / 2.75;
-    return 7.5625 * t * t + 0.984375;
+    tt -= 2.625 / 2.75;
+    return 7.5625 * tt * tt + 0.984375;
   }
 
   /**
diff --git a/Libraries/Animated/src/nodes/AnimatedProps.js b/Libraries/Animated/src/nodes/AnimatedProps.js
index 78cd11189edfa7..b10cd03c183898 100644
--- a/Libraries/Animated/src/nodes/AnimatedProps.js
+++ b/Libraries/Animated/src/nodes/AnimatedProps.js
@@ -25,13 +25,14 @@ class AnimatedProps extends AnimatedNode {
 
   constructor(props: Object, callback: () => void) {
     super();
-    if (props.style) {
-      props = {
-        ...props,
-        style: new AnimatedStyle(props.style),
+    let propsTmp = props;
+    if (propsTmp.style) {
+      propsTmp = {
+        ...propsTmp,
+        style: new AnimatedStyle(propsTmp.style),
       };
     }
-    this._props = props;
+    this._props = propsTmp;
     this._callback = callback;
     this.__attach();
   }
diff --git a/Libraries/Animated/src/nodes/AnimatedStyle.js b/Libraries/Animated/src/nodes/AnimatedStyle.js
index c7bb24e3b42b14..e2fad696363b9e 100644
--- a/Libraries/Animated/src/nodes/AnimatedStyle.js
+++ b/Libraries/Animated/src/nodes/AnimatedStyle.js
@@ -22,14 +22,14 @@ class AnimatedStyle extends AnimatedWithChildren {
 
   constructor(style: any) {
     super();
-    style = flattenStyle(style) || {};
-    if (style.transform) {
-      style = {
-        ...style,
-        transform: new AnimatedTransform(style.transform),
+    let styleTmp = flattenStyle(style) || {};
+    if (styleTmp.transform) {
+      styleTmp = {
+        ...styleTmp,
+        transform: new AnimatedTransform(styleTmp.transform),
       };
     }
-    this._style = style;
+    this._style = styleTmp;
   }
 
   // Recursively get values for nested styles (like iOS's shadowOffset)
diff --git a/Libraries/BatchedBridge/NativeModules.js b/Libraries/BatchedBridge/NativeModules.js
index 324c4f6e995bdd..ff5bda3476a823 100644
--- a/Libraries/BatchedBridge/NativeModules.js
+++ b/Libraries/BatchedBridge/NativeModules.js
@@ -101,8 +101,8 @@ function genMethod(moduleID: number, methodID: number, type: MethodType) {
       const onSuccess = hasSuccessCallback ? lastArg : null;
       const onFail = hasErrorCallback ? secondLastArg : null;
       const callbackCount = hasSuccessCallback + hasErrorCallback;
-      args = args.slice(0, args.length - callbackCount);
-      BatchedBridge.enqueueNativeCall(moduleID, methodID, args, onFail, onSuccess);
+      let argsArray = args.slice(0, args.length - callbackCount);
+      BatchedBridge.enqueueNativeCall(moduleID, methodID, argsArray, onFail, onSuccess);
     };
   }
   fn.type = type;
diff --git a/Libraries/Blob/Blob.js b/Libraries/Blob/Blob.js
index eca13a6a309e8f..35b5814f2616d1 100644
--- a/Libraries/Blob/Blob.js
+++ b/Libraries/Blob/Blob.js
@@ -83,19 +83,21 @@ class Blob {
   slice(start?: number, end?: number): Blob {
     const BlobManager = require('BlobManager');
     let {offset, size} = this.data;
+    let startVal = start;
+    let endVal = end;
 
-    if (typeof start === 'number') {
-      if (start > size) {
-        start = size;
+    if (typeof startVal === 'number') {
+      if (startVal > size) {
+        startVal = size;
       }
-      offset += start;
-      size -= start;
+      offset += startVal;
+      size -= startVal;
 
-      if (typeof end === 'number') {
-        if (end < 0) {
-          end = this.size + end;
+      if (typeof endVal === 'number') {
+        if (endVal < 0) {
+          endVal = this.size + endVal;
         }
-        size = end - start;
+        size = endVal - startVal;
       }
     }
     return BlobManager.createFromOptions({
diff --git a/Libraries/CameraRoll/ImagePickerIOS.js b/Libraries/CameraRoll/ImagePickerIOS.js
index 24d1309d2e1b91..285615b7fddff3 100644
--- a/Libraries/CameraRoll/ImagePickerIOS.js
+++ b/Libraries/CameraRoll/ImagePickerIOS.js
@@ -19,19 +19,19 @@ var ImagePickerIOS = {
     return RCTImagePicker.canUseCamera(callback);
   },
   openCameraDialog: function(config: Object, successCallback: Function, cancelCallback: Function) {
-    config = {
+    let configTmp = {
       videoMode: false,
       ...config,
     };
-    return RCTImagePicker.openCameraDialog(config, successCallback, cancelCallback);
+    return RCTImagePicker.openCameraDialog(configTmp, successCallback, cancelCallback);
   },
   openSelectDialog: function(config: Object, successCallback: Function, cancelCallback: Function) {
-    config = {
+    let configTmp = {
       showImages: true,
       showVideos: false,
       ...config,
     };
-    return RCTImagePicker.openSelectDialog(config, successCallback, cancelCallback);
+    return RCTImagePicker.openSelectDialog(configTmp, successCallback, cancelCallback);
   },
 };
 
diff --git a/Libraries/Components/Navigation/NavigatorIOS.ios.js b/Libraries/Components/Navigation/NavigatorIOS.ios.js
index 66aa906f61a4fd..0f0b1bc8fca2da 100644
--- a/Libraries/Components/Navigation/NavigatorIOS.ios.js
+++ b/Libraries/Components/Navigation/NavigatorIOS.ios.js
@@ -732,11 +732,12 @@ var NavigatorIOS = createReactClass({
    */
   replaceAtIndex: function(route: Route, index: number) {
     invariant(!!route, 'Must supply route to replace');
-    if (index < 0) {
-      index += this.state.routeStack.length;
+    let idx = index;
+    if (idx < 0) {
+      idx += this.state.routeStack.length;
     }
 
-    if (this.state.routeStack.length <= index) {
+    if (this.state.routeStack.length <= idx) {
       return;
     }
 
diff --git a/Libraries/Components/ScrollResponder.js b/Libraries/Components/ScrollResponder.js
index 9d60c462ef1dc7..2d35acca95e3b4 100644
--- a/Libraries/Components/ScrollResponder.js
+++ b/Libraries/Components/ScrollResponder.js
@@ -414,15 +414,21 @@ const ScrollResponderMixin = {
     y?: number,
     animated?: boolean
   ) {
+    let params = {};
     if (typeof x === 'number') {
       console.warn('`scrollResponderScrollTo(x, y, animated)` is deprecated. Use `scrollResponderScrollTo({x: 5, y: 5, animated: true})` instead.');
-    } else {
-      ({x, y, animated} = x || {});
+      params.x = x || 0;
+      params.y = y || 0;
+      params.animated = animated !== false;
+    } else if(typeof x === 'object') {
+      params.x = x.x || 0;
+      params.y = x.y || 0;
+      params.animated = x.animated !== false;
     }
     UIManager.dispatchViewManagerCommand(
       nullthrows(this.scrollResponderGetScrollableNode()),
       UIManager.RCTScrollView.Commands.scrollTo,
-      [x || 0, y || 0, animated !== false],
+      [params.x, params.y, params.animated],
     );
   },
 
@@ -465,13 +471,14 @@ const ScrollResponderMixin = {
     animated?: boolean // deprecated, put this inside the rect argument instead
   ) {
     invariant(ScrollViewManager && ScrollViewManager.zoomToRect, 'zoomToRect is not implemented');
+    let isAnimated = animated;
     if ('animated' in rect) {
-      animated = rect.animated;
+      isAnimated = rect.animated;
       delete rect.animated;
-    } else if (typeof animated !== 'undefined') {
+    } else if (typeof isAnimated !== 'undefined') {
       console.warn('`scrollResponderZoomTo` `animated` argument is deprecated. Use `options.animated` instead');
     }
-    ScrollViewManager.zoomToRect(this.scrollResponderGetScrollableNode(), rect, animated !== false);
+    ScrollViewManager.zoomToRect(this.scrollResponderGetScrollableNode(), rect, isAnimated !== false);
   },
 
   /**
diff --git a/Libraries/Components/ScrollView/ScrollView.js b/Libraries/Components/ScrollView/ScrollView.js
index 354fd952b8191e..be2071f2e0dabf 100644
--- a/Libraries/Components/ScrollView/ScrollView.js
+++ b/Libraries/Components/ScrollView/ScrollView.js
@@ -567,15 +567,21 @@ const ScrollView = createReactClass({
     x?: number,
     animated?: boolean
   ) {
+    let params = {};
+    // animation is enabled by default
     if (typeof y === 'number') {
       console.warn('`scrollTo(y, x, animated)` is deprecated. Use `scrollTo({x: 5, y: 5, ' +
         'animated: true})` instead.');
-    } else {
-      ({x, y, animated} = y || {});
+      params.x = x || 0;
+      params.y = y || 0;
+      params.animated = animated !== false;
     }
-    this.getScrollResponder().scrollResponderScrollTo(
-      {x: x || 0, y: y || 0, animated: animated !== false}
-    );
+    else if(typeof y === 'object') {
+      params.x = y.x || 0;
+      params.y = y.y || 0;
+      params.animated = y.animated !== false;
+    }
+    this.getScrollResponder().scrollResponderScrollTo(params);
   },
 
   /**
diff --git a/Libraries/Components/StatusBar/StatusBar.js b/Libraries/Components/StatusBar/StatusBar.js
index 0562a315493853..7152d4183f67a1 100644
--- a/Libraries/Components/StatusBar/StatusBar.js
+++ b/Libraries/Components/StatusBar/StatusBar.js
@@ -193,10 +193,10 @@ class StatusBar extends React.Component<{
    *    changing the status bar hidden property.
    */
   static setHidden(hidden: boolean, animation?: StatusBarAnimation) {
-    animation = animation || 'none';
+    let isAnimation = animation || 'none';
     StatusBar._defaultProps.hidden.value = hidden;
     if (Platform.OS === 'ios') {
-      StatusBarManager.setHidden(hidden, animation);
+      StatusBarManager.setHidden(hidden, isAnimation);
     } else if (Platform.OS === 'android') {
       StatusBarManager.setHidden(hidden);
     }
@@ -208,10 +208,10 @@ class StatusBar extends React.Component<{
    * @param animated Animate the style change.
    */
   static setBarStyle(style: StatusBarStyle, animated?: boolean) {
-    animated = animated || false;
+    let isAnimated = animated || false;
     StatusBar._defaultProps.barStyle.value = style;
     if (Platform.OS === 'ios') {
-      StatusBarManager.setStyle(style, animated);
+      StatusBarManager.setStyle(style, isAnimated);
     } else if (Platform.OS === 'android') {
       StatusBarManager.setStyle(style);
     }
@@ -242,9 +242,9 @@ class StatusBar extends React.Component<{
       console.warn('`setBackgroundColor` is only available on Android');
       return;
     }
-    animated = animated || false;
+    let isAnimated = animated || false;
     StatusBar._defaultProps.backgroundColor.value = color;
-    StatusBarManager.setColor(processColor(color), animated);
+    StatusBarManager.setColor(processColor(color), isAnimated);
   }
 
   /**
diff --git a/Libraries/Core/ExceptionsManager.js b/Libraries/Core/ExceptionsManager.js
index 8a63ec3bff4bcc..81c2275a329bba 100644
--- a/Libraries/Core/ExceptionsManager.js
+++ b/Libraries/Core/ExceptionsManager.js
@@ -56,15 +56,16 @@ function handleException(e: Error, isFatal: boolean) {
   // Unfortunately there is no way to figure out the stacktrace in this
   // case, so if you ended up here trying to trace an error, look for
   // `throw '<error message>'` somewhere in your codebase.
-  if (!e.message) {
-    e = new Error(e);
+  let error = e;
+  if (!error.message) {
+    error = new Error(error);
   }
   if (console._errorOriginal) {
-    console._errorOriginal(e.message);
+    console._errorOriginal(error.message);
   } else {
-    console.error(e.message);
+    console.error(error.message);
   }
-  reportException(e, isFatal);
+  reportException(error, isFatal);
 }
 
 function reactConsoleErrorHandler() {
diff --git a/Libraries/Experimental/SwipeableRow/SwipeableRow.js b/Libraries/Experimental/SwipeableRow/SwipeableRow.js
index 80e627376e8482..2f09285b73194f 100644
--- a/Libraries/Experimental/SwipeableRow/SwipeableRow.js
+++ b/Libraries/Experimental/SwipeableRow/SwipeableRow.js
@@ -290,7 +290,7 @@ const SwipeableRow = createReactClass({
      * Ensure the speed is at least the set speed threshold to prevent a slow
      * swiping animation
      */
-    speed = (
+    let speedVal = (
       speed > HORIZONTAL_FULL_SWIPE_SPEED_THRESHOLD ?
       speed :
       HORIZONTAL_FULL_SWIPE_SPEED_THRESHOLD
@@ -299,7 +299,7 @@ const SwipeableRow = createReactClass({
      * Calculate the duration the row should take to swipe the remaining distance
      * at the same speed the user swiped (or the speed threshold)
      */
-    const duration = Math.abs((this.props.maxSwipeDistance - Math.abs(distMoved)) / speed);
+    const duration = Math.abs((this.props.maxSwipeDistance - Math.abs(distMoved)) / speedVal);
     const maxSwipeDistance = IS_RTL ? -this.props.maxSwipeDistance : this.props.maxSwipeDistance;
     this._animateTo(-maxSwipeDistance, duration);
   },
diff --git a/Libraries/Image/resolveAssetSource.js b/Libraries/Image/resolveAssetSource.js
index a5b6229d35d738..3c97e801817f5b 100644
--- a/Libraries/Image/resolveAssetSource.js
+++ b/Libraries/Image/resolveAssetSource.js
@@ -34,19 +34,20 @@ function getDevServerURL(): ?string {
 }
 
 function _coerceLocalScriptURL(scriptURL: ?string): ?string {
+  let scriptURLTmp = scriptURL;
   if (scriptURL) {
     if (scriptURL.startsWith('assets://')) {
       // android: running from within assets, no offline path to use
       return null;
     }
-    scriptURL = scriptURL.substring(0, scriptURL.lastIndexOf('/') + 1);
-    if (!scriptURL.includes('://')) {
+    scriptURLTmp = scriptURL.substring(0, scriptURL.lastIndexOf('/') + 1);
+    if (!scriptURLTmp.includes('://')) {
       // Add file protocol in case we have an absolute file path and not a URL.
       // This shouldn't really be necessary. scriptURL should be a URL.
-      scriptURL = 'file://' + scriptURL;
+      scriptURLTmp = 'file://' + scriptURLTmp;
     }
   }
-  return scriptURL;
+  return scriptURLTmp;
 }
 
 function getScriptURL(): ?string {
diff --git a/Libraries/Inspector/Inspector.js b/Libraries/Inspector/Inspector.js
index 90e30b346a9461..af615ad9a13cad 100644
--- a/Libraries/Inspector/Inspector.js
+++ b/Libraries/Inspector/Inspector.js
@@ -97,17 +97,18 @@ class Inspector extends React.Component<{
   attachToDevtools = (agent: Object) => {
     let _hideWait = null;
     const hlSub = agent.sub('highlight', ({node, name, props}) => {
+      let nodeTmp = node;
       /* $FlowFixMe(>=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. */
       clearTimeout(_hideWait);
 
-      if (typeof node !== 'number') {
+      if (typeof nodeTmp !== 'number') {
         // Fiber
-        node = ReactNative.findNodeHandle(node);
+        nodeTmp = ReactNative.findNodeHandle(nodeTmp);
       }
 
-      UIManager.measure(node, (x, y, width, height, left, top) => {
+      UIManager.measure(nodeTmp, (x, y, width, height, left, top) => {
         this.setState({
           hierarchy: [],
           inspected: {
diff --git a/Libraries/Lists/ViewabilityHelper.js b/Libraries/Lists/ViewabilityHelper.js
index 0c0a8356019075..2ae8c50b7ffa01 100644
--- a/Libraries/Lists/ViewabilityHelper.js
+++ b/Libraries/Lists/ViewabilityHelper.js
@@ -235,12 +235,12 @@ class ViewabilityHelper {
     createViewToken,
   ) {
     // Filter out indices that have gone out of view since this call was scheduled.
-    viewableIndicesToCheck = viewableIndicesToCheck.filter(ii =>
+    let toCheck = viewableIndicesToCheck.filter(ii =>
       this._viewableIndices.includes(ii),
     );
     const prevItems = this._viewableItems;
     const nextItems = new Map(
-      viewableIndicesToCheck.map(ii => {
+      toCheck.map(ii => {
         const viewable = createViewToken(ii, true);
         return [viewable.key, viewable];
       }),
diff --git a/Libraries/Lists/VirtualizedList.js b/Libraries/Lists/VirtualizedList.js
index f40b44e404c4f6..e859dda603e6db 100644
--- a/Libraries/Lists/VirtualizedList.js
+++ b/Libraries/Lists/VirtualizedList.js
@@ -656,8 +656,8 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     const stickyOffset = this.props.ListHeaderComponent ? 1 : 0;
     const end = getItemCount(data) - 1;
     let prevCellKey;
-    last = Math.min(end, last);
-    for (let ii = first; ii <= last; ii++) {
+    let lastIndex = Math.min(end, last);
+    for (let ii = first; ii <= lastIndex; ii++) {
       const item = getItem(data, ii);
       const key = keyExtractor(item, ii);
       this._indicesToKeys.set(ii, key);
diff --git a/Libraries/Lists/VirtualizedSectionList.js b/Libraries/Lists/VirtualizedSectionList.js
index 2bb196814abf13..cb71aee56b532e 100644
--- a/Libraries/Lists/VirtualizedSectionList.js
+++ b/Libraries/Lists/VirtualizedSectionList.js
@@ -309,15 +309,16 @@ class VirtualizedSectionList<SectionT: SectionBase> extends React.PureComponent<
     index: number,
     info?: ?Object,
   ): ?React.ComponentType<*> {
-    info = info || this._subExtractor(index);
-    if (!info) {
+    let infoTmp = info;
+    infoTmp = infoTmp || this._subExtractor(index);
+    if (!infoTmp) {
       return null;
     }
     const ItemSeparatorComponent =
-      info.section.ItemSeparatorComponent || this.props.ItemSeparatorComponent;
+      infoTmp.section.ItemSeparatorComponent || this.props.ItemSeparatorComponent;
     const {SectionSeparatorComponent} = this.props;
     const isLastItemInList = index === this.state.childProps.getItemCount() - 1;
-    const isLastItemInSection = info.index === info.section.data.length - 1;
+    const isLastItemInSection = infoTmp.index === infoTmp.section.data.length - 1;
     if (SectionSeparatorComponent && isLastItemInSection) {
       return SectionSeparatorComponent;
     }
diff --git a/Libraries/Performance/Systrace.js b/Libraries/Performance/Systrace.js
index 1abedee7c1734b..0c6cf72ea3df5d 100644
--- a/Libraries/Performance/Systrace.js
+++ b/Libraries/Performance/Systrace.js
@@ -134,9 +134,9 @@ const Systrace = {
   **/
   beginEvent(profileName?: any, args?: any) {
     if (_enabled) {
-      profileName = typeof profileName === 'function' ?
+      let name = typeof profileName === 'function' ?
         profileName() : profileName;
-      global.nativeTraceBeginSection(TRACE_TAG_REACT_APPS, profileName, args);
+      global.nativeTraceBeginSection(TRACE_TAG_REACT_APPS, name, args);
     }
   },
 
@@ -155,18 +155,18 @@ const Systrace = {
     const cookie = _asyncCookie;
     if (_enabled) {
       _asyncCookie++;
-      profileName = typeof profileName === 'function' ?
+      let name = typeof profileName === 'function' ?
         profileName() : profileName;
-      global.nativeTraceBeginAsyncSection(TRACE_TAG_REACT_APPS, profileName, cookie);
+      global.nativeTraceBeginAsyncSection(TRACE_TAG_REACT_APPS, name, cookie);
     }
     return cookie;
   },
 
   endAsyncEvent(profileName?: any, cookie?: any) {
     if (_enabled) {
-      profileName = typeof profileName === 'function' ?
+      let name = typeof profileName === 'function' ?
         profileName() : profileName;
-      global.nativeTraceEndAsyncSection(TRACE_TAG_REACT_APPS, profileName, cookie);
+      global.nativeTraceEndAsyncSection(TRACE_TAG_REACT_APPS, name, cookie);
     }
   },
 
@@ -175,10 +175,10 @@ const Systrace = {
   **/
   counterEvent(profileName?: any, value?: any) {
     if (_enabled) {
-      profileName = typeof profileName === 'function' ?
+      let name = typeof profileName === 'function' ?
         profileName() : profileName;
       global.nativeTraceCounter &&
-        global.nativeTraceCounter(TRACE_TAG_REACT_APPS, profileName, value);
+        global.nativeTraceCounter(TRACE_TAG_REACT_APPS, name, value);
     }
   },
 
@@ -188,12 +188,13 @@ const Systrace = {
   **/
   attachToRelayProfiler(relayProfiler: RelayProfiler) {
     relayProfiler.attachProfileHandler('*', (name, state?) => {
+      let nameTmp = name;
       if (state != null && state.queryName !== undefined) {
-        name += '_' + state.queryName;
+        nameTmp += '_' + state.queryName
       }
-      const cookie = Systrace.beginAsyncEvent(name);
+      const cookie = Systrace.beginAsyncEvent(nameTmp);
       return () => {
-        Systrace.endAsyncEvent(name, cookie);
+        Systrace.endAsyncEvent(nameTmp, cookie);
       };
     });
 
diff --git a/Libraries/ReactNative/requireNativeComponent.js b/Libraries/ReactNative/requireNativeComponent.js
index 2a6c7c8ca71e8d..907f36f0db8123 100644
--- a/Libraries/ReactNative/requireNativeComponent.js
+++ b/Libraries/ReactNative/requireNativeComponent.js
@@ -56,6 +56,7 @@ function requireNativeComponent(
       // as lazy view managers discovery is Android-specific.
       if (UIManager.ViewManagerNames) {
         // Lazy view managers enabled.
+        // $FlowFixMe because parameter mutation here seems intentional
         viewConfig = merge(viewConfig, UIManager.getDefaultEventTypes());
       } else {
         viewConfig.bubblingEventTypes = merge(
diff --git a/Libraries/Settings/Settings.ios.js b/Libraries/Settings/Settings.ios.js
index 08f1be6a1c8ae1..11717d1d64b38a 100644
--- a/Libraries/Settings/Settings.ios.js
+++ b/Libraries/Settings/Settings.ios.js
@@ -29,17 +29,18 @@ var Settings = {
   },
 
   watchKeys(keys: string | Array<string>, callback: Function): number {
-    if (typeof keys === 'string') {
-      keys = [keys];
+    let keysTmp = keys;
+    if (typeof keysTmp === 'string') {
+      keysTmp = [keysTmp];
     }
 
     invariant(
-      Array.isArray(keys),
+      Array.isArray(keysTmp),
       'keys should be a string or array of strings'
     );
 
     var sid = subscriptions.length;
-    subscriptions.push({keys: keys, callback: callback});
+    subscriptions.push({keys: keysTmp, callback: callback});
     return sid;
   },
 
diff --git a/Libraries/StyleSheet/normalizeColor.js b/Libraries/StyleSheet/normalizeColor.js
index 09c2e1525d5473..589107bc90f268 100755
--- a/Libraries/StyleSheet/normalizeColor.js
+++ b/Libraries/StyleSheet/normalizeColor.js
@@ -96,20 +96,21 @@ function normalizeColor(color: string | number): ?number {
 }
 
 function hue2rgb(p: number, q: number, t: number): number {
-  if (t < 0) {
-    t += 1;
+  let tt = t;
+  if (tt < 0) {
+    tt += 1;
   }
-  if (t > 1) {
-    t -= 1;
+  if (tt > 1) {
+    tt -= 1;
   }
-  if (t < 1 / 6) {
-    return p + (q - p) * 6 * t;
+  if (tt < 1 / 6) {
+    return p + (q - p) * 6 * tt;
   }
-  if (t < 1 / 2) {
+  if (tt < 1 / 2) {
     return q;
   }
-  if (t < 2 / 3) {
-    return p + (q - p) * (2 / 3 - t) * 6;
+  if (tt < 2 / 3) {
+    return p + (q - p) * (2 / 3 - tt) * 6;
   }
   return p;
 }
diff --git a/Libraries/StyleSheet/setNormalizedColorAlpha.js b/Libraries/StyleSheet/setNormalizedColorAlpha.js
index 309d86cea12e80..b3d33670caee66 100644
--- a/Libraries/StyleSheet/setNormalizedColorAlpha.js
+++ b/Libraries/StyleSheet/setNormalizedColorAlpha.js
@@ -15,15 +15,16 @@
  * alpha should be number between 0 and 1
  */
 function setNormalizedColorAlpha(input: number, alpha: number): number {
-  if (alpha < 0) {
-    alpha = 0;
-  } else if (alpha > 1) {
-    alpha = 1;
+  let alphaTemp = alpha;
+  if (alphaTemp < 0) {
+    alphaTemp = 0;
+  } else if (alphaTemp > 1) {
+    alphaTemp = 1;
   }
 
-  alpha = Math.round(alpha * 255);
+  alphaTemp = Math.round(alphaTemp * 255);
   // magic bitshift guarantees we return an unsigned int
-  return ((input & 0xffffff00) | alpha) >>> 0;
+  return ((input & 0xffffff00) | alphaTemp) >>> 0;
 }
 
 module.exports = setNormalizedColorAlpha;
diff --git a/Libraries/Utilities/Dimensions.js b/Libraries/Utilities/Dimensions.js
index d13b14f1bd18ea..d4754a2001b28a 100644
--- a/Libraries/Utilities/Dimensions.js
+++ b/Libraries/Utilities/Dimensions.js
@@ -29,12 +29,13 @@ class Dimensions {
     // We calculate the window dimensions in JS so that we don't encounter loss of
     // precision in transferring the dimensions (which could be non-integers) over
     // the bridge.
-    if (dims && dims.windowPhysicalPixels) {
+    let tempDims = dims;
+    if (tempDims && tempDims.windowPhysicalPixels) {
       // parse/stringify => Clone hack
-      dims = JSON.parse(JSON.stringify(dims));
+      tempDims = JSON.parse(JSON.stringify(tempDims));
 
-      const windowPhysicalPixels = dims.windowPhysicalPixels;
-      dims.window = {
+      const windowPhysicalPixels = tempDims.windowPhysicalPixels;
+      tempDims.window = {
         width: windowPhysicalPixels.width / windowPhysicalPixels.scale,
         height: windowPhysicalPixels.height / windowPhysicalPixels.scale,
         scale: windowPhysicalPixels.scale,
@@ -42,8 +43,8 @@ class Dimensions {
       };
       if (Platform.OS === 'android') {
         // Screen and window dimensions are different on android
-        const screenPhysicalPixels = dims.screenPhysicalPixels;
-        dims.screen = {
+        const screenPhysicalPixels = tempDims.screenPhysicalPixels;
+        tempDims.screen = {
           width: screenPhysicalPixels.width / screenPhysicalPixels.scale,
           height: screenPhysicalPixels.height / screenPhysicalPixels.scale,
           scale: screenPhysicalPixels.scale,
@@ -51,15 +52,15 @@ class Dimensions {
         };
 
         // delete so no callers rely on this existing
-        delete dims.screenPhysicalPixels;
+        delete tempDims.screenPhysicalPixels;
       } else {
-        dims.screen = dims.window;
+        tempDims.screen = tempDims.window;
       }
       // delete so no callers rely on this existing
-      delete dims.windowPhysicalPixels;
+      delete tempDims.windowPhysicalPixels;
     }
 
-    Object.assign(dimensions, dims);
+    Object.assign(dimensions, tempDims);
     if (dimensionsInitialized) {
       // Don't fire 'change' the first time the dimensions are set.
       eventEmitter.emit('change', {
diff --git a/Libraries/Utilities/HMRClient.js b/Libraries/Utilities/HMRClient.js
index ff897afda8bf99..1a96a754af2d00 100644
--- a/Libraries/Utilities/HMRClient.js
+++ b/Libraries/Utilities/HMRClient.js
@@ -30,13 +30,13 @@ const HMRClient = {
 
     const wsHostPort = port !== null && port !== '' ? `${host}:${port}` : host;
 
-    bundleEntry = bundleEntry.replace(/\.(bundle|delta)/, '.js');
+    const bundleEntryParam = bundleEntry.replace(/\.(bundle|delta)/, '.js');
 
     // Build the websocket url
     const wsUrl =
       `ws://${wsHostPort}/hot?` +
       `platform=${platform}&` +
-      `bundleEntry=${bundleEntry}`;
+      `bundleEntry=${bundleEntryParam}`;
 
     const hmrClient = new MetroHMRClient(wsUrl);
 
diff --git a/Libraries/Utilities/binaryToBase64.js b/Libraries/Utilities/binaryToBase64.js
index d17c4d403c8705..1f93c732883e31 100644
--- a/Libraries/Utilities/binaryToBase64.js
+++ b/Libraries/Utilities/binaryToBase64.js
@@ -15,16 +15,17 @@
 const base64 = require('base64-js');
 
 function binaryToBase64(data: ArrayBuffer | $ArrayBufferView) {
-  if (data instanceof ArrayBuffer) {
-    data = new Uint8Array(data);
+  let dataBuffer = data;
+  if (dataBuffer instanceof ArrayBuffer) {
+    dataBuffer = new Uint8Array(dataBuffer);
   }
-  if (data instanceof Uint8Array) {
-    return base64.fromByteArray(data);
+  if (dataBuffer instanceof Uint8Array) {
+    return base64.fromByteArray(dataBuffer);
   }
-  if (!ArrayBuffer.isView(data)) {
+  if (!ArrayBuffer.isView(dataBuffer)) {
     throw new Error('data must be ArrayBuffer or typed array');
   }
-  const {buffer, byteOffset, byteLength} = data;
+  const {buffer, byteOffset, byteLength} = dataBuffer;
   return base64.fromByteArray(new Uint8Array(buffer, byteOffset, byteLength));
 }
 
diff --git a/Libraries/Utilities/differ/insetsDiffer.js b/Libraries/Utilities/differ/insetsDiffer.js
index 1d497c0948e53a..c27b3149a82cef 100644
--- a/Libraries/Utilities/differ/insetsDiffer.js
+++ b/Libraries/Utilities/differ/insetsDiffer.js
@@ -27,13 +27,13 @@ var insetsDiffer = function(
   one: ?Inset,
   two: ?Inset
 ): bool {
-  one = one || dummyInsets;
-  two = two || dummyInsets;
-  return one !== two && (
-    one.top !== two.top ||
-    one.left !== two.left ||
-    one.right !== two.right ||
-    one.bottom !== two.bottom
+  var insetOne = one || dummyInsets;
+  var insetTwo = two || dummyInsets;
+  return insetOne !== insetTwo && (
+    insetOne.top !== insetTwo.top ||
+    insetOne.left !== insetTwo.left ||
+    insetOne.right !== insetTwo.right ||
+    insetOne.bottom !== insetTwo.bottom
   );
 };
 
diff --git a/Libraries/Utilities/differ/pointsDiffer.js b/Libraries/Utilities/differ/pointsDiffer.js
index cb41cec85494a4..4694c8a16e8114 100644
--- a/Libraries/Utilities/differ/pointsDiffer.js
+++ b/Libraries/Utilities/differ/pointsDiffer.js
@@ -17,11 +17,11 @@ type Point = {
 var dummyPoint = {x: undefined, y: undefined};
 
 var pointsDiffer = function(one: ?Point, two: ?Point): bool {
-  one = one || dummyPoint;
-  two = two || dummyPoint;
-  return one !== two && (
-    one.x !== two.x ||
-    one.y !== two.y
+  var ptOne = one || dummyPoint;
+  var ptTwo = two || dummyPoint;
+  return ptOne !== ptTwo && (
+    ptOne.x !== ptTwo.x ||
+    ptOne.y !== ptTwo.y
   );
 };
 
diff --git a/Libraries/Utilities/truncate.js b/Libraries/Utilities/truncate.js
index 6a1204f9c4c2fb..8e78d082f40244 100644
--- a/Libraries/Utilities/truncate.js
+++ b/Libraries/Utilities/truncate.js
@@ -27,17 +27,18 @@ const truncate = function(
   maxChars: number,
   options: truncateOptions
 ): ?string {
-  options = Object.assign({}, defaultOptions, options);
-  if (str && str.length &&
-      str.length - options.minDelta + options.elipsis.length >= maxChars) {
-    str = str.slice(0, maxChars - options.elipsis.length + 1);
-    if (options.breakOnWords) {
-      var ii = Math.max(str.lastIndexOf(' '), str.lastIndexOf('\n'));
-      str = str.slice(0, ii);
+  let opts =  Object.assign({}, defaultOptions, options);
+  let strArg = str;
+  if (strArg && strArg.length &&
+      strArg.length - opts.minDelta + opts.elipsis.length >= maxChars) {
+    strArg = strArg.slice(0, maxChars - opts.elipsis.length + 1);
+    if (opts.breakOnWords) {
+      var ii = Math.max(strArg.lastIndexOf(' '), strArg.lastIndexOf('\n'));
+      strArg = strArg.slice(0, ii);
     }
-    str = str.trim() + options.elipsis;
+    strArg = strArg.trim() + opts.elipsis;
   }
-  return str;
+  return strArg;
 };
 
 module.exports = truncate;
diff --git a/Libraries/Vibration/Vibration.js b/Libraries/Vibration/Vibration.js
index e01da3d4022663..bf5cc1f96fa896 100644
--- a/Libraries/Vibration/Vibration.js
+++ b/Libraries/Vibration/Vibration.js
@@ -15,7 +15,7 @@ var Platform = require('Platform');
 
 /**
  * Vibration API
- * 
+ *
  * See https://facebook.github.io/react-native/docs/vibration.html
  */
 
@@ -27,15 +27,16 @@ function vibrateByPattern(pattern: Array<number>, repeat: boolean = false) {
     return;
   }
   _vibrating = true;
-  if (pattern[0] === 0) {
+  var vibrationPattern = pattern;
+  if (vibrationPattern[0] === 0) {
     RCTVibration.vibrate();
-    pattern = pattern.slice(1);
+    vibrationPattern = vibrationPattern.slice(1);
   }
-  if (pattern.length === 0) {
+  if (vibrationPattern.length === 0) {
     _vibrating = false;
     return;
   }
-  setTimeout(() => vibrateScheduler(++_id, pattern, repeat, 1), pattern[0]);
+  setTimeout(() => vibrateScheduler(++_id, vibrationPattern, repeat, 1), vibrationPattern[0]);
 }
 
 function vibrateScheduler(id, pattern: Array<number>, repeat: boolean, nextIndex: number) {
@@ -43,21 +44,22 @@ function vibrateScheduler(id, pattern: Array<number>, repeat: boolean, nextIndex
     return;
   }
   RCTVibration.vibrate();
-  if (nextIndex >= pattern.length) {
+  var indexNext = nextIndex;
+  if (indexNext >= pattern.length) {
     if (repeat) {
-      nextIndex = 0;
+      indexNext = 0;
     } else {
       _vibrating = false;
       return;
     }
   }
-  setTimeout(() => vibrateScheduler(id, pattern, repeat, nextIndex + 1), pattern[nextIndex]);
+  setTimeout(() => vibrateScheduler(id, pattern, repeat, indexNext + 1), pattern[indexNext]);
 }
 
 var Vibration = {
   /**
    * Trigger a vibration with specified `pattern`.
-   * 
+   *
    * See https://facebook.github.io/react-native/docs/vibration.html#vibrate
    */
   vibrate: function(pattern: number | Array<number> = 400, repeat: boolean = false) {
@@ -84,7 +86,7 @@ var Vibration = {
   },
   /**
    * Stop vibration
-   * 
+   *
    * See https://facebook.github.io/react-native/docs/vibration.html#cancel
    */
   cancel: function() {
diff --git a/Libraries/WebSocket/WebSocket.js b/Libraries/WebSocket/WebSocket.js
index 13e614557d78b4..ebb2688eb149a5 100644
--- a/Libraries/WebSocket/WebSocket.js
+++ b/Libraries/WebSocket/WebSocket.js
@@ -97,8 +97,9 @@ class WebSocket extends EventTarget(...WEBSOCKET_EVENTS) {
 
   constructor(url: string, protocols: ?string | ?Array<string>, options: ?{headers?: {origin?: string}}) {
     super();
-    if (typeof protocols === 'string') {
-      protocols = [protocols];
+    let protocolList = protocols;
+    if (typeof protocolList === 'string') {
+      protocolList = [protocolList];
     }
 
     const {headers = {}, ...unrecognized} = options || {};
@@ -122,8 +123,8 @@ class WebSocket extends EventTarget(...WEBSOCKET_EVENTS) {
         + 'Did you mean to put these under `headers`?');
     }
 
-    if (!Array.isArray(protocols)) {
-      protocols = null;
+    if (!Array.isArray(protocolList)) {
+      protocolList = null;
     }
 
     if (!WebSocket.isAvailable) {
@@ -134,7 +135,7 @@ class WebSocket extends EventTarget(...WEBSOCKET_EVENTS) {
     this._eventEmitter = new NativeEventEmitter(WebSocketModule);
     this._socketId = nextWebSocketId++;
     this._registerEvents();
-    WebSocketModule.connect(url, protocols, { headers }, this._socketId);
+    WebSocketModule.connect(url, protocolList, { headers }, this._socketId);
   }
 
   get binaryType(): ?BinaryType {
diff --git a/RNTester/js/AnimatedGratuitousApp/AnExApp.js b/RNTester/js/AnimatedGratuitousApp/AnExApp.js
index 1116a84ee98254..d9f2d51e64782e 100644
--- a/RNTester/js/AnimatedGratuitousApp/AnExApp.js
+++ b/RNTester/js/AnimatedGratuitousApp/AnExApp.js
@@ -254,15 +254,16 @@ function moveToClosest({activeKey, keys, restLayouts}, position) {
   var minDist = Infinity;
   var newKeys = [];
   keys.forEach((key, idx) => {
-    var dist = distance(position, restLayouts[idx]);
+    var currentIdx = idx;
+    var dist = distance(position, restLayouts[currentIdx]);
     if (key === activeKey) {
-      idx = activeIdx;
+      currentIdx = activeIdx;
     } else {
       newKeys.push(key);
     }
     if (dist < minDist) {
       minDist = dist;
-      closestIdx = idx;
+      closestIdx = currentIdx;
     }
   });
   if (closestIdx === activeIdx) {
diff --git a/RNTester/js/TextInputExample.ios.js b/RNTester/js/TextInputExample.ios.js
index a55206355046da..5199d8386b8ecc 100644
--- a/RNTester/js/TextInputExample.ios.js
+++ b/RNTester/js/TextInputExample.ios.js
@@ -106,8 +106,8 @@ class RewriteExample extends React.Component<$FlowFixMeProps, any> {
           multiline={false}
           maxLength={limit}
           onChangeText={(text) => {
-            text = text.replace(/ /g, '_');
-            this.setState({text});
+            let textReplace = text.replace(/ /g, '_');
+            this.setState({textReplace});
           }}
           style={styles.default}
           value={this.state.text}
diff --git a/RNTester/js/websocket_test_server.js b/RNTester/js/websocket_test_server.js
index 83a48ed213f062..ebfbec2bbd951c 100755
--- a/RNTester/js/websocket_test_server.js
+++ b/RNTester/js/websocket_test_server.js
@@ -32,16 +32,17 @@ const respondWithBinary = process.argv.indexOf('--binary') !== -1;
 const server = new WebSocket.Server({port: 5555});
 server.on('connection', (ws) => {
   ws.on('message', (message) => {
-    console.log('Received message:', message);
+    let messageBuffer = message;
+    console.log('Received message:', messageBuffer);
     console.log('Cookie:', ws.upgradeReq.headers.cookie);
     if (respondWithBinary) {
-      message = new Buffer(message);
+      messageBuffer = new Buffer(messageBuffer);
     }
-    if (message === 'getImage') {
-      message = fs.readFileSync(path.resolve(__dirname, 'flux@3x.png'));
+    if (messageBuffer === 'getImage') {
+      messageBuffer = fs.readFileSync(path.resolve(__dirname, 'flux@3x.png'));
     }
-    console.log('Replying with:', message);
-    ws.send(message);
+    console.log('Replying with:', messageBuffer);
+    ws.send(messageBuffer);
   });
 
   console.log('Incoming connection!');