diff --git a/packages/react-dom/src/client/ReactDOMFiberOption.js b/packages/react-dom/src/client/ReactDOMFiberOption.js
index fc130fb50052..63151ad0d9d4 100644
--- a/packages/react-dom/src/client/ReactDOMFiberOption.js
+++ b/packages/react-dom/src/client/ReactDOMFiberOption.js
@@ -11,7 +11,7 @@ import React from 'react';
 import warning from 'fbjs/lib/warning';
 
 function flattenChildren(children) {
-  var content = '';
+  let content = '';
 
   // Flatten children and warn if they aren't strings or numbers;
   // invalid types are ignored.
@@ -52,9 +52,8 @@ export function postMountWrapper(element: Element, props: Object) {
 }
 
 export function getHostProps(element: Element, props: Object) {
-  var hostProps = Object.assign({children: undefined}, props);
-
-  var content = flattenChildren(props.children);
+  const hostProps = {children: undefined, ...props};
+  const content = flattenChildren(props.children);
 
   if (content) {
     hostProps.children = content;
diff --git a/packages/react-dom/src/client/ReactDOMFiberTextarea.js b/packages/react-dom/src/client/ReactDOMFiberTextarea.js
index ec2b478805f0..d16716143528 100644
--- a/packages/react-dom/src/client/ReactDOMFiberTextarea.js
+++ b/packages/react-dom/src/client/ReactDOMFiberTextarea.js
@@ -14,8 +14,8 @@ import ReactDebugCurrentFiber from 'react-reconciler/src/ReactDebugCurrentFiber'
 
 import ReactControlledValuePropTypes from '../shared/ReactControlledValuePropTypes';
 
-var {getCurrentFiberStackAddendum} = ReactDebugCurrentFiber;
-var didWarnValDefaultVal = false;
+const {getCurrentFiberStackAddendum} = ReactDebugCurrentFiber;
+let didWarnValDefaultVal = false;
 
 type TextAreaWithWrapperState = HTMLTextAreaElement & {
   _wrapperState: {
@@ -40,7 +40,7 @@ type TextAreaWithWrapperState = HTMLTextAreaElement & {
  */
 
 export function getHostProps(element: Element, props: Object) {
-  var node = ((element: any): TextAreaWithWrapperState);
+  const node = ((element: any): TextAreaWithWrapperState);
   invariant(
     props.dangerouslySetInnerHTML == null,
     '`dangerouslySetInnerHTML` does not make sense on <textarea>.',
@@ -52,17 +52,18 @@ export function getHostProps(element: Element, props: Object) {
   // completely solve this IE9 bug), but Sebastian+Sophie seemed to like this
   // solution. The value can be a boolean or object so that's why it's forced
   // to be a string.
-  var hostProps = Object.assign({}, props, {
+  const hostProps = {
+    ...props,
     value: undefined,
     defaultValue: undefined,
     children: '' + node._wrapperState.initialValue,
-  });
+  };
 
   return hostProps;
 }
 
 export function initWrapperState(element: Element, props: Object) {
-  var node = ((element: any): TextAreaWithWrapperState);
+  const node = ((element: any): TextAreaWithWrapperState);
   if (__DEV__) {
     ReactControlledValuePropTypes.checkPropTypes(
       'textarea',
@@ -86,14 +87,13 @@ export function initWrapperState(element: Element, props: Object) {
     }
   }
 
-  var value = props.value;
-  var initialValue = value;
+  let initialValue = props.value;
 
   // Only bother fetching default value if we're going to use it
-  if (value == null) {
-    var defaultValue = props.defaultValue;
+  if (initialValue == null) {
+    let defaultValue = props.defaultValue;
     // TODO (yungsters): Remove support for children content in <textarea>.
-    var children = props.children;
+    let children = props.children;
     if (children != null) {
       if (__DEV__) {
         warning(
@@ -128,12 +128,12 @@ export function initWrapperState(element: Element, props: Object) {
 }
 
 export function updateWrapper(element: Element, props: Object) {
-  var node = ((element: any): TextAreaWithWrapperState);
-  var value = props.value;
+  const node = ((element: any): TextAreaWithWrapperState);
+  const value = props.value;
   if (value != null) {
     // Cast `value` to a string to ensure the value is set correctly. While
     // browsers typically do this as necessary, jsdom doesn't.
-    var newValue = '' + value;
+    const newValue = '' + value;
 
     // To avoid side effects (such as losing text selection), only set value if changed
     if (newValue !== node.value) {
@@ -149,10 +149,10 @@ export function updateWrapper(element: Element, props: Object) {
 }
 
 export function postMountWrapper(element: Element, props: Object) {
-  var node = ((element: any): TextAreaWithWrapperState);
+  const node = ((element: any): TextAreaWithWrapperState);
   // This is in postMount because we need access to the DOM node, which is not
   // available until after the component has mounted.
-  var textContent = node.textContent;
+  const textContent = node.textContent;
 
   // Only set node.value if textContent is equal to the expected
   // initial value. In IE10/IE11 there is a bug where the placeholder attribute
diff --git a/packages/react-dom/src/client/ReactDOMSelection.js b/packages/react-dom/src/client/ReactDOMSelection.js
index 6fc53edf4bc8..2fe966df8216 100644
--- a/packages/react-dom/src/client/ReactDOMSelection.js
+++ b/packages/react-dom/src/client/ReactDOMSelection.js
@@ -14,16 +14,13 @@ import {TEXT_NODE} from '../shared/HTMLNodeType';
  * @return {?object}
  */
 export function getOffsets(outerNode) {
-  var selection = window.getSelection && window.getSelection();
+  const selection = window.getSelection && window.getSelection();
 
   if (!selection || selection.rangeCount === 0) {
     return null;
   }
 
-  var anchorNode = selection.anchorNode;
-  var anchorOffset = selection.anchorOffset;
-  var focusNode = selection.focusNode;
-  var focusOffset = selection.focusOffset;
+  const {anchorNode, anchorOffset, focusNode, focusOffset} = selection;
 
   // In Firefox, anchorNode and focusNode can be "anonymous divs", e.g. the
   // up/down buttons on an <input type="number">. Anonymous divs do not seem to
@@ -157,21 +154,21 @@ export function setOffsets(node, offsets) {
     return;
   }
 
-  var selection = window.getSelection();
-  var length = node[getTextContentAccessor()].length;
-  var start = Math.min(offsets.start, length);
-  var end = offsets.end === undefined ? start : Math.min(offsets.end, length);
+  const selection = window.getSelection();
+  const length = node[getTextContentAccessor()].length;
+  let start = Math.min(offsets.start, length);
+  let end = offsets.end === undefined ? start : Math.min(offsets.end, length);
 
   // IE 11 uses modern selection, but doesn't support the extend method.
   // Flip backward selections, so we can set with a single range.
   if (!selection.extend && start > end) {
-    var temp = end;
+    let temp = end;
     end = start;
     start = temp;
   }
 
-  var startMarker = getNodeForCharacterOffset(node, start);
-  var endMarker = getNodeForCharacterOffset(node, end);
+  const startMarker = getNodeForCharacterOffset(node, start);
+  const endMarker = getNodeForCharacterOffset(node, end);
 
   if (startMarker && endMarker) {
     if (
@@ -183,7 +180,7 @@ export function setOffsets(node, offsets) {
     ) {
       return;
     }
-    var range = document.createRange();
+    const range = document.createRange();
     range.setStart(startMarker.node, startMarker.offset);
     selection.removeAllRanges();
 
diff --git a/packages/react-dom/src/client/setTextContent.js b/packages/react-dom/src/client/setTextContent.js
index cc246a52eb64..ffaa3c9b14ac 100644
--- a/packages/react-dom/src/client/setTextContent.js
+++ b/packages/react-dom/src/client/setTextContent.js
@@ -21,7 +21,7 @@ import {TEXT_NODE} from '../shared/HTMLNodeType';
  * @param {string} text
  * @internal
  */
-var setTextContent = function(node, text) {
+let setTextContent = function(node, text) {
   if (text) {
     let firstChild = node.firstChild;
 
diff --git a/packages/react-dom/src/client/validateDOMNesting.js b/packages/react-dom/src/client/validateDOMNesting.js
index b7cce32e324e..8e6165d5e991 100644
--- a/packages/react-dom/src/client/validateDOMNesting.js
+++ b/packages/react-dom/src/client/validateDOMNesting.js
@@ -11,7 +11,7 @@ import warning from 'fbjs/lib/warning';
 import ReactDebugCurrentFiber from 'react-reconciler/src/ReactDebugCurrentFiber';
 
 const {getCurrentFiberStackAddendum} = ReactDebugCurrentFiber;
-var validateDOMNesting = emptyFunction;
+let validateDOMNesting = emptyFunction;
 
 if (__DEV__) {
   // This validation code was written based on the HTML5 parsing spec:
@@ -26,7 +26,7 @@ if (__DEV__) {
   // first, causing a confusing mess.
 
   // https://html.spec.whatwg.org/multipage/syntax.html#special
-  var specialTags = [
+  const specialTags = [
     'address',
     'applet',
     'area',
@@ -113,7 +113,7 @@ if (__DEV__) {
   ];
 
   // https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
-  var inScopeTags = [
+  const inScopeTags = [
     'applet',
     'caption',
     'html',
@@ -133,10 +133,10 @@ if (__DEV__) {
   ];
 
   // https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-button-scope
-  var buttonScopeTags = inScopeTags.concat(['button']);
+  const buttonScopeTags = inScopeTags.concat(['button']);
 
   // https://html.spec.whatwg.org/multipage/syntax.html#generate-implied-end-tags
-  var impliedEndTags = [
+  const impliedEndTags = [
     'dd',
     'dt',
     'li',
@@ -147,7 +147,7 @@ if (__DEV__) {
     'rt',
   ];
 
-  var emptyAncestorInfo = {
+  const emptyAncestorInfo = {
     current: null,
 
     formTag: null,
@@ -160,9 +160,9 @@ if (__DEV__) {
     dlItemTagAutoclosing: null,
   };
 
-  var updatedAncestorInfo = function(oldInfo, tag, instance) {
-    var ancestorInfo = Object.assign({}, oldInfo || emptyAncestorInfo);
-    var info = {tag: tag, instance: instance};
+  const updatedAncestorInfo = function(oldInfo, tag, instance) {
+    let ancestorInfo = {...(oldInfo || emptyAncestorInfo)};
+    let info = {tag: tag, instance: instance};
 
     if (inScopeTags.indexOf(tag) !== -1) {
       ancestorInfo.aTagInScope = null;
@@ -215,7 +215,7 @@ if (__DEV__) {
   /**
    * Returns whether
    */
-  var isTagValidWithParent = function(tag, parentTag) {
+  const isTagValidWithParent = function(tag, parentTag) {
     // First, let's check if we're in an unusual parsing mode...
     switch (parentTag) {
       // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-inselect
@@ -337,7 +337,7 @@ if (__DEV__) {
   /**
    * Returns whether
    */
-  var findInvalidAncestorForTag = function(tag, ancestorInfo) {
+  const findInvalidAncestorForTag = function(tag, ancestorInfo) {
     switch (tag) {
       case 'address':
       case 'article':
@@ -401,7 +401,7 @@ if (__DEV__) {
     return null;
   };
 
-  var didWarn = {};
+  const didWarn = {};
 
   validateDOMNesting = function(childTag, childText, ancestorInfo) {
     ancestorInfo = ancestorInfo || emptyAncestorInfo;