diff --git a/packages/shared/ReactDOMFrameScheduling.js b/packages/shared/ReactDOMFrameScheduling.js
index f50e0ed204c7..78041064678e 100644
--- a/packages/shared/ReactDOMFrameScheduling.js
+++ b/packages/shared/ReactDOMFrameScheduling.js
@@ -74,20 +74,20 @@ if (!ExecutionEnvironment.canUseDOM) {
 ) {
   // Polyfill requestIdleCallback and cancelIdleCallback
 
-  var scheduledRICCallback = null;
-  var isIdleScheduled = false;
-  var timeoutTime = -1;
+  let scheduledRICCallback = null;
+  let isIdleScheduled = false;
+  let timeoutTime = -1;
 
-  var isAnimationFrameScheduled = false;
+  let isAnimationFrameScheduled = false;
 
-  var frameDeadline = 0;
+  let frameDeadline = 0;
   // We start out assuming that we run at 30fps but then the heuristic tracking
   // will adjust this value to a faster fps if we get more frequent animation
   // frames.
-  var previousFrameTime = 33;
-  var activeFrameTime = 33;
+  let previousFrameTime = 33;
+  let activeFrameTime = 33;
 
-  var frameDeadlineObject;
+  let frameDeadlineObject;
   if (hasNativePerformanceNow) {
     frameDeadlineObject = {
       didTimeout: false,
@@ -110,12 +110,12 @@ if (!ExecutionEnvironment.canUseDOM) {
   }
 
   // We use the postMessage trick to defer idle work until after the repaint.
-  var messageKey =
+  const messageKey =
     '__reactIdleCallback$' +
     Math.random()
       .toString(36)
       .slice(2);
-  var idleTick = function(event) {
+  const idleTick = function(event) {
     if (event.source !== window || event.data !== messageKey) {
       return;
     }
@@ -146,7 +146,7 @@ if (!ExecutionEnvironment.canUseDOM) {
     }
 
     timeoutTime = -1;
-    var callback = scheduledRICCallback;
+    const callback = scheduledRICCallback;
     scheduledRICCallback = null;
     if (callback !== null) {
       callback(frameDeadlineObject);
@@ -156,9 +156,9 @@ if (!ExecutionEnvironment.canUseDOM) {
   // something better for old IE.
   window.addEventListener('message', idleTick, false);
 
-  var animationTick = function(rafTime) {
+  const animationTick = function(rafTime) {
     isAnimationFrameScheduled = false;
-    var nextFrameTime = rafTime - frameDeadline + activeFrameTime;
+    let nextFrameTime = rafTime - frameDeadline + activeFrameTime;
     if (
       nextFrameTime < activeFrameTime &&
       previousFrameTime < activeFrameTime
diff --git a/packages/shared/ReactFiberComponentTreeHook.js b/packages/shared/ReactFiberComponentTreeHook.js
index ff0648e8a145..8926ce342686 100644
--- a/packages/shared/ReactFiberComponentTreeHook.js
+++ b/packages/shared/ReactFiberComponentTreeHook.js
@@ -24,10 +24,10 @@ function describeFiber(fiber: Fiber): string {
     case FunctionalComponent:
     case ClassComponent:
     case HostComponent:
-      var owner = fiber._debugOwner;
-      var source = fiber._debugSource;
-      var name = getComponentName(fiber);
-      var ownerName = null;
+      const owner = fiber._debugOwner;
+      const source = fiber._debugSource;
+      const name = getComponentName(fiber);
+      let ownerName = null;
       if (owner) {
         ownerName = getComponentName(owner);
       }
@@ -43,8 +43,8 @@ function describeFiber(fiber: Fiber): string {
 export function getStackAddendumByWorkInProgressFiber(
   workInProgress: Fiber,
 ): string {
-  var info = '';
-  var node = workInProgress;
+  let info = '';
+  let node = workInProgress;
   do {
     info += describeFiber(node);
     // Otherwise this return pointer might point to the wrong tree:
diff --git a/packages/shared/ReactGlobalSharedState.js b/packages/shared/ReactGlobalSharedState.js
index 72213c141b8f..b911e90801ed 100644
--- a/packages/shared/ReactGlobalSharedState.js
+++ b/packages/shared/ReactGlobalSharedState.js
@@ -7,7 +7,7 @@
 
 import React from 'react';
 
-var ReactInternals = React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
+const ReactInternals = React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
 
 export const ReactCurrentOwner = ReactInternals.ReactCurrentOwner;
 export const ReactDebugCurrentFrame = __DEV__
diff --git a/packages/shared/ReactTreeTraversal.js b/packages/shared/ReactTreeTraversal.js
index e86ea1120939..7db43fdf2fc2 100644
--- a/packages/shared/ReactTreeTraversal.js
+++ b/packages/shared/ReactTreeTraversal.js
@@ -27,12 +27,12 @@ function getParent(inst) {
  * different trees.
  */
 export function getLowestCommonAncestor(instA, instB) {
-  var depthA = 0;
-  for (var tempA = instA; tempA; tempA = getParent(tempA)) {
+  let depthA = 0;
+  for (let tempA = instA; tempA; tempA = getParent(tempA)) {
     depthA++;
   }
-  var depthB = 0;
-  for (var tempB = instB; tempB; tempB = getParent(tempB)) {
+  let depthB = 0;
+  for (let tempB = instB; tempB; tempB = getParent(tempB)) {
     depthB++;
   }
 
@@ -49,7 +49,7 @@ export function getLowestCommonAncestor(instA, instB) {
   }
 
   // Walk in lockstep until we find a match.
-  var depth = depthA;
+  let depth = depthA;
   while (depth--) {
     if (instA === instB || instA === instB.alternate) {
       return instA;
@@ -84,12 +84,12 @@ export function getParentInstance(inst) {
  * Simulates the traversal of a two-phase, capture/bubble event dispatch.
  */
 export function traverseTwoPhase(inst, fn, arg) {
-  var path = [];
+  const path = [];
   while (inst) {
     path.push(inst);
     inst = getParent(inst);
   }
-  var i;
+  let i;
   for (i = path.length; i-- > 0; ) {
     fn(path[i], 'captured', arg);
   }
diff --git a/packages/shared/__tests__/ReactErrorUtils-test.internal.js b/packages/shared/__tests__/ReactErrorUtils-test.internal.js
index 9a064e4312af..b0a6b5813758 100644
--- a/packages/shared/__tests__/ReactErrorUtils-test.internal.js
+++ b/packages/shared/__tests__/ReactErrorUtils-test.internal.js
@@ -9,7 +9,7 @@
 
 'use strict';
 
-var ReactErrorUtils;
+let ReactErrorUtils;
 
 describe('ReactErrorUtils', () => {
   beforeEach(() => {
@@ -18,8 +18,8 @@ describe('ReactErrorUtils', () => {
   });
 
   it(`it should rethrow caught errors`, () => {
-    var err = new Error('foo');
-    var callback = function() {
+    const err = new Error('foo');
+    const callback = function() {
       throw err;
     };
     ReactErrorUtils.invokeGuardedCallbackAndCatchFirstError(
@@ -32,7 +32,7 @@ describe('ReactErrorUtils', () => {
   });
 
   it(`should call the callback the passed arguments`, () => {
-    var callback = jest.fn();
+    const callback = jest.fn();
     ReactErrorUtils.invokeGuardedCallback(
       'foo',
       callback,
@@ -44,7 +44,7 @@ describe('ReactErrorUtils', () => {
   });
 
   it(`should call the callback with the provided context`, () => {
-    var context = {didCall: false};
+    const context = {didCall: false};
     ReactErrorUtils.invokeGuardedCallback(
       'foo',
       function() {
@@ -72,7 +72,7 @@ describe('ReactErrorUtils', () => {
   });
 
   it(`should return false from clearCaughtError if no error was thrown`, () => {
-    var callback = jest.fn();
+    const callback = jest.fn();
     ReactErrorUtils.invokeGuardedCallback('foo', callback, null);
     expect(ReactErrorUtils.hasCaughtError()).toBe(false);
     expect(ReactErrorUtils.clearCaughtError).toThrow('no error was captured');
@@ -193,8 +193,8 @@ describe('ReactErrorUtils', () => {
     ReactErrorUtils = require('shared/ReactErrorUtils').default;
 
     try {
-      var err = new Error('foo');
-      var callback = function() {
+      const err = new Error('foo');
+      const callback = function() {
         throw err;
       };
       ReactErrorUtils.invokeGuardedCallbackAndCatchFirstError(
diff --git a/packages/shared/__tests__/reactProdInvariant-test.internal.js b/packages/shared/__tests__/reactProdInvariant-test.internal.js
index 8c8ce0bcd04b..2263241a503a 100644
--- a/packages/shared/__tests__/reactProdInvariant-test.internal.js
+++ b/packages/shared/__tests__/reactProdInvariant-test.internal.js
@@ -8,7 +8,7 @@
  */
 'use strict';
 
-var reactProdInvariant;
+let reactProdInvariant;
 
 describe('reactProdInvariant', () => {
   let globalErrorMock;
diff --git a/packages/shared/isTextInputElement.js b/packages/shared/isTextInputElement.js
index 3921bcc052c9..2671c3e3dde2 100644
--- a/packages/shared/isTextInputElement.js
+++ b/packages/shared/isTextInputElement.js
@@ -10,7 +10,7 @@
 /**
  * @see http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html#input-type-attr-summary
  */
-var supportedInputTypes: {[key: string]: true | void} = {
+const supportedInputTypes: {[key: string]: true | void} = {
   color: true,
   date: true,
   datetime: true,
@@ -29,7 +29,7 @@ var supportedInputTypes: {[key: string]: true | void} = {
 };
 
 function isTextInputElement(elem: ?HTMLElement): boolean {
-  var nodeName = elem && elem.nodeName && elem.nodeName.toLowerCase();
+  const nodeName = elem && elem.nodeName && elem.nodeName.toLowerCase();
 
   if (nodeName === 'input') {
     return !!supportedInputTypes[((elem: any): HTMLInputElement).type];
diff --git a/packages/shared/lowPriorityWarning.js b/packages/shared/lowPriorityWarning.js
index cfc1ef7cae9c..ba2e7ee98715 100644
--- a/packages/shared/lowPriorityWarning.js
+++ b/packages/shared/lowPriorityWarning.js
@@ -19,12 +19,12 @@
  * same logic and follow the same code paths.
  */
 
-var lowPriorityWarning = function() {};
+let lowPriorityWarning = function() {};
 
 if (__DEV__) {
   const printWarning = function(format, ...args) {
-    var argIndex = 0;
-    var message = 'Warning: ' + format.replace(/%s/g, () => args[argIndex++]);
+    let argIndex = 0;
+    const message = 'Warning: ' + format.replace(/%s/g, () => args[argIndex++]);
     if (typeof console !== 'undefined') {
       console.warn(message);
     }
diff --git a/packages/shared/reactProdInvariant.js b/packages/shared/reactProdInvariant.js
index e9d63051fccf..56adc77d94f6 100644
--- a/packages/shared/reactProdInvariant.js
+++ b/packages/shared/reactProdInvariant.js
@@ -14,16 +14,16 @@
  * It always throws.
  */
 function reactProdInvariant(code: string): void {
-  var argCount = arguments.length - 1;
+  const argCount = arguments.length - 1;
 
-  var message =
+  let message =
     'Minified React error #' +
     code +
     '; visit ' +
     'http://facebook.github.io/react/docs/error-decoder.html?invariant=' +
     code;
 
-  for (var argIdx = 0; argIdx < argCount; argIdx++) {
+  for (let argIdx = 0; argIdx < argCount; argIdx++) {
     message += '&args[]=' + encodeURIComponent(arguments[argIdx + 1]);
   }
 
@@ -31,7 +31,7 @@ function reactProdInvariant(code: string): void {
     ' for the full message or use the non-minified dev environment' +
     ' for full errors and additional helpful warnings.';
 
-  var error: Error & {framesToPop?: number} = new Error(message);
+  const error: Error & {framesToPop?: number} = new Error(message);
   error.name = 'Invariant Violation';
   error.framesToPop = 1; // we don't care about reactProdInvariant's own frame