From d310d654a7c7aab6c8213da84ef36dfba82711b0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Markb=C3=A5ge?= <sebastian@calyptus.eu>
Date: Tue, 14 Mar 2023 21:00:22 -0400
Subject: [PATCH] Avoid meta programming to initialize functions in module
 scope  (#26388)

I'm trying to get rid of all meta programming in the module scope so
that closure can do a better job figuring out cyclic dependencies and
ability to reorder.

This is converting a lot of the patterns that assign functions
conditionally to using function declarations instead.

```
let fn;
if (__DEV__) {
  fn = function() {
    ...
  };
}
```
->
```
function fn() {
  if (__DEV__) {
    ...
  }
}
```
---
 .../domEnvironment.js                         |   2 +-
 .../src/client/ReactDOMComponent.js           |  66 +-
 .../createMicrosoftUnsafeLocalFunction.js     |  25 -
 .../src/client/setInnerHTML.js                |  37 +-
 .../src/client/setTextContent.js              |   4 +-
 .../src/client/validateDOMNesting.js          | 716 +++++++++---------
 .../src/shared/ReactDOMUnknownPropertyHook.js |  26 +-
 .../src/shared/warnValidStyle.js              |  74 +-
 .../src/test-utils/ReactTestUtils.js          |   4 +-
 .../src/ReactNativeEventEmitter.js            |   8 +-
 .../src/ReactNativeFiberInspector.js          | 194 +++--
 .../Libraries/ReactPrivate/deepDiffer.js      |   4 +-
 .../deepFreezeAndThrowOnMutationInDev.js      |   2 +-
 .../Libraries/ReactPrivate/flattenStyle.js    |   2 +-
 .../src/legacy-events/EventBatching.js        |   8 +-
 .../src/legacy-events/EventPluginUtils.js     |   7 +-
 .../src/legacy-events/ResponderEventPlugin.js |   4 +-
 packages/react-reconciler/src/ReactFiber.js   |   4 +-
 .../src/ReactFiberClassComponent.js           |  56 +-
 .../src/ReactFiberCommitWork.js               |   7 +-
 .../src/ReactFizzClassComponent.js            |  18 +-
 packages/react/src/ReactElement.js            |   4 +-
 packages/react/src/jsx/ReactJSXElement.js     |   4 +-
 .../forks/invokeGuardedCallbackImpl.www.js    |   4 +-
 24 files changed, 628 insertions(+), 652 deletions(-)
 delete mode 100644 packages/react-dom-bindings/src/client/createMicrosoftUnsafeLocalFunction.js

diff --git a/packages/dom-event-testing-library/domEnvironment.js b/packages/dom-event-testing-library/domEnvironment.js
index 3ec9e016bff90..185b0b5ace349 100644
--- a/packages/dom-event-testing-library/domEnvironment.js
+++ b/packages/dom-event-testing-library/domEnvironment.js
@@ -13,7 +13,7 @@
  * Change environment support for PointerEvent.
  */
 
-const emptyFunction = function () {};
+function emptyFunction() {}
 
 export function hasPointerEvent() {
   return global != null && global.PointerEvent != null;
diff --git a/packages/react-dom-bindings/src/client/ReactDOMComponent.js b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
index 8cb2e95accb9f..037d787197c85 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMComponent.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
@@ -94,15 +94,8 @@ const HTML = '__html';
 let warnedUnknownTags: {
   [key: string]: boolean,
 };
-
-let validatePropertiesInDevelopment;
-let warnForPropDifference;
-let warnForExtraAttributes;
-let warnForInvalidEventListener;
 let canDiffStyleForHydrationWarning;
 
-let normalizeHTML;
-
 if (__DEV__) {
   warnedUnknownTags = {
     // There are working polyfills for <dialog>. Let people use it.
@@ -115,15 +108,6 @@ if (__DEV__) {
     webview: true,
   };
 
-  validatePropertiesInDevelopment = function (type: string, props: any) {
-    validateARIAProperties(type, props);
-    validateInputProperties(type, props);
-    validateUnknownProperties(type, props, {
-      registrationNameDependencies,
-      possibleRegistrationNames,
-    });
-  };
-
   // IE 11 parses & normalizes the style attribute as opposed to other
   // browsers. It adds spaces and sorts the properties in some
   // non-alphabetical order. Handling that would require sorting CSS
@@ -133,12 +117,25 @@ if (__DEV__) {
   // in that browser completely in favor of doing all that work.
   // See https://github.com/facebook/react/issues/11807
   canDiffStyleForHydrationWarning = canUseDOM && !document.documentMode;
+}
 
-  warnForPropDifference = function (
-    propName: string,
-    serverValue: mixed,
-    clientValue: mixed,
-  ) {
+function validatePropertiesInDevelopment(type: string, props: any) {
+  if (__DEV__) {
+    validateARIAProperties(type, props);
+    validateInputProperties(type, props);
+    validateUnknownProperties(type, props, {
+      registrationNameDependencies,
+      possibleRegistrationNames,
+    });
+  }
+}
+
+function warnForPropDifference(
+  propName: string,
+  serverValue: mixed,
+  clientValue: mixed,
+) {
+  if (__DEV__) {
     if (didWarnInvalidHydration) {
       return;
     }
@@ -156,9 +153,11 @@ if (__DEV__) {
       JSON.stringify(normalizedServerValue),
       JSON.stringify(normalizedClientValue),
     );
-  };
+  }
+}
 
-  warnForExtraAttributes = function (attributeNames: Set<string>) {
+function warnForExtraAttributes(attributeNames: Set<string>) {
+  if (__DEV__) {
     if (didWarnInvalidHydration) {
       return;
     }
@@ -168,12 +167,11 @@ if (__DEV__) {
       names.push(name);
     });
     console.error('Extra attributes from the server: %s', names);
-  };
+  }
+}
 
-  warnForInvalidEventListener = function (
-    registrationName: string,
-    listener: any,
-  ) {
+function warnForInvalidEventListener(registrationName: string, listener: any) {
+  if (__DEV__) {
     if (listener === false) {
       console.error(
         'Expected `%s` listener to be a function, instead got `false`.\n\n' +
@@ -190,11 +188,13 @@ if (__DEV__) {
         typeof listener,
       );
     }
-  };
+  }
+}
 
-  // Parse the HTML and read it back to normalize the HTML string so that it
-  // can be used for comparison.
-  normalizeHTML = function (parent: Element, html: string) {
+// Parse the HTML and read it back to normalize the HTML string so that it
+// can be used for comparison.
+function normalizeHTML(parent: Element, html: string) {
+  if (__DEV__) {
     // We could have created a separate document here to avoid
     // re-initializing custom elements if they exist. But this breaks
     // how <noscript> is being handled. So we use the same document.
@@ -208,7 +208,7 @@ if (__DEV__) {
           );
     testElement.innerHTML = html;
     return testElement.innerHTML;
-  };
+  }
 }
 
 // HTML parsing normalizes CR and CRLF to LF.
diff --git a/packages/react-dom-bindings/src/client/createMicrosoftUnsafeLocalFunction.js b/packages/react-dom-bindings/src/client/createMicrosoftUnsafeLocalFunction.js
deleted file mode 100644
index 0d6332f3cc903..0000000000000
--- a/packages/react-dom-bindings/src/client/createMicrosoftUnsafeLocalFunction.js
+++ /dev/null
@@ -1,25 +0,0 @@
-/**
- * Copyright (c) Meta Platforms, Inc. and affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- */
-
-/* globals MSApp */
-
-/**
- * Create a function which has 'unsafe' privileges (required by windows8 apps)
- */
-const createMicrosoftUnsafeLocalFunction = function (func) {
-  if (typeof MSApp !== 'undefined' && MSApp.execUnsafeLocalFunction) {
-    return function (arg0, arg1, arg2, arg3) {
-      MSApp.execUnsafeLocalFunction(function () {
-        return func(arg0, arg1, arg2, arg3);
-      });
-    };
-  } else {
-    return func;
-  }
-};
-
-export default createMicrosoftUnsafeLocalFunction;
diff --git a/packages/react-dom-bindings/src/client/setInnerHTML.js b/packages/react-dom-bindings/src/client/setInnerHTML.js
index f8a82fde34ef0..5f0d630fd38b5 100644
--- a/packages/react-dom-bindings/src/client/setInnerHTML.js
+++ b/packages/react-dom-bindings/src/client/setInnerHTML.js
@@ -7,24 +7,15 @@
  * @flow
  */
 
+/* globals MSApp */
+
 import {SVG_NAMESPACE} from './DOMNamespaces';
-import createMicrosoftUnsafeLocalFunction from './createMicrosoftUnsafeLocalFunction';
 import {enableTrustedTypesIntegration} from 'shared/ReactFeatureFlags';
 
 // SVG temp container for IE lacking innerHTML
 let reusableSVGContainer: HTMLElement;
 
-/**
- * Set the innerHTML property of a node
- *
- * @param {DOMElement} node
- * @param {string} html
- * @internal
- */
-const setInnerHTML: (
-  node: Element,
-  html: {valueOf(): {toString(): string, ...}, ...},
-) => void = createMicrosoftUnsafeLocalFunction(function (
+function setInnerHTMLImpl(
   node: Element,
   html: {valueOf(): {toString(): string, ...}, ...},
 ): void {
@@ -66,6 +57,26 @@ const setInnerHTML: (
     }
   }
   node.innerHTML = (html: any);
-});
+}
+
+let setInnerHTML: (
+  node: Element,
+  html: {valueOf(): {toString(): string, ...}, ...},
+) => void = setInnerHTMLImpl;
+// $FlowFixMe[cannot-resolve-name]
+if (typeof MSApp !== 'undefined' && MSApp.execUnsafeLocalFunction) {
+  /**
+   * Create a function which has 'unsafe' privileges (required by windows8 apps)
+   */
+  setInnerHTML = function (
+    node: Element,
+    html: {valueOf(): {toString(): string, ...}, ...},
+  ): void {
+    // $FlowFixMe[cannot-resolve-name]
+    return MSApp.execUnsafeLocalFunction(function () {
+      return setInnerHTMLImpl(node, html);
+    });
+  };
+}
 
 export default setInnerHTML;
diff --git a/packages/react-dom-bindings/src/client/setTextContent.js b/packages/react-dom-bindings/src/client/setTextContent.js
index e98575adcf967..12d1f6f82bb68 100644
--- a/packages/react-dom-bindings/src/client/setTextContent.js
+++ b/packages/react-dom-bindings/src/client/setTextContent.js
@@ -18,7 +18,7 @@ import {TEXT_NODE} from './HTMLNodeType';
  * @param {string} text
  * @internal
  */
-const setTextContent = function (node: Element, text: string): void {
+function setTextContent(node: Element, text: string): void {
   if (text) {
     const firstChild = node.firstChild;
 
@@ -32,6 +32,6 @@ const setTextContent = function (node: Element, text: string): void {
     }
   }
   node.textContent = text;
-};
+}
 
 export default setTextContent;
diff --git a/packages/react-dom-bindings/src/client/validateDOMNesting.js b/packages/react-dom-bindings/src/client/validateDOMNesting.js
index facc9278d23d9..4ded143a9f1ad 100644
--- a/packages/react-dom-bindings/src/client/validateDOMNesting.js
+++ b/packages/react-dom-bindings/src/client/validateDOMNesting.js
@@ -7,12 +7,6 @@
  * @flow
  */
 
-type ValidateDOMNesting = (?string, ?string, AncestorInfoDev) => void;
-let validateDOMNesting: ValidateDOMNesting = (() => {}: any);
-
-type UpdatedAncestorInfoDev = (?AncestorInfoDev, string) => AncestorInfoDev;
-let updatedAncestorInfoDev: UpdatedAncestorInfoDev = (() => {}: any);
-
 type Info = {tag: string};
 export type AncestorInfoDev = {
   current: ?Info,
@@ -30,156 +24,159 @@ export type AncestorInfoDev = {
   containerTagInScope: ?Info,
 };
 
-if (__DEV__) {
-  // This validation code was written based on the HTML5 parsing spec:
-  // https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
-  //
-  // Note: this does not catch all invalid nesting, nor does it try to (as it's
-  // not clear what practical benefit doing so provides); instead, we warn only
-  // for cases where the parser will give a parse tree differing from what React
-  // intended. For example, <b><div></div></b> is invalid but we don't warn
-  // because it still parses correctly; we do warn for other cases like nested
-  // <p> tags where the beginning of the second element implicitly closes the
-  // first, causing a confusing mess.
-
-  // https://html.spec.whatwg.org/multipage/syntax.html#special
-  const specialTags = [
-    'address',
-    'applet',
-    'area',
-    'article',
-    'aside',
-    'base',
-    'basefont',
-    'bgsound',
-    'blockquote',
-    'body',
-    'br',
-    'button',
-    'caption',
-    'center',
-    'col',
-    'colgroup',
-    'dd',
-    'details',
-    'dir',
-    'div',
-    'dl',
-    'dt',
-    'embed',
-    'fieldset',
-    'figcaption',
-    'figure',
-    'footer',
-    'form',
-    'frame',
-    'frameset',
-    'h1',
-    'h2',
-    'h3',
-    'h4',
-    'h5',
-    'h6',
-    'head',
-    'header',
-    'hgroup',
-    'hr',
-    'html',
-    'iframe',
-    'img',
-    'input',
-    'isindex',
-    'li',
-    'link',
-    'listing',
-    'main',
-    'marquee',
-    'menu',
-    'menuitem',
-    'meta',
-    'nav',
-    'noembed',
-    'noframes',
-    'noscript',
-    'object',
-    'ol',
-    'p',
-    'param',
-    'plaintext',
-    'pre',
-    'script',
-    'section',
-    'select',
-    'source',
-    'style',
-    'summary',
-    'table',
-    'tbody',
-    'td',
-    'template',
-    'textarea',
-    'tfoot',
-    'th',
-    'thead',
-    'title',
-    'tr',
-    'track',
-    'ul',
-    'wbr',
-    'xmp',
-  ];
-
-  // https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
-  const inScopeTags = [
-    'applet',
-    'caption',
-    'html',
-    'table',
-    'td',
-    'th',
-    'marquee',
-    'object',
-    'template',
-
-    // https://html.spec.whatwg.org/multipage/syntax.html#html-integration-point
-    // TODO: Distinguish by namespace here -- for <title>, including it here
-    // errs on the side of fewer warnings
-    'foreignObject',
-    'desc',
-    'title',
-  ];
-
-  // https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-button-scope
-  const buttonScopeTags = inScopeTags.concat(['button']);
-
-  // https://html.spec.whatwg.org/multipage/syntax.html#generate-implied-end-tags
-  const impliedEndTags = [
-    'dd',
-    'dt',
-    'li',
-    'option',
-    'optgroup',
-    'p',
-    'rp',
-    'rt',
-  ];
-
-  const emptyAncestorInfoDev: AncestorInfoDev = {
-    current: null,
-
-    formTag: null,
-    aTagInScope: null,
-    buttonTagInScope: null,
-    nobrTagInScope: null,
-    pTagInButtonScope: null,
-
-    listItemTagAutoclosing: null,
-    dlItemTagAutoclosing: null,
-
-    containerTagInScope: null,
-  };
-
-  updatedAncestorInfoDev = function (oldInfo: ?AncestorInfoDev, tag: string) {
+// This validation code was written based on the HTML5 parsing spec:
+// https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
+//
+// Note: this does not catch all invalid nesting, nor does it try to (as it's
+// not clear what practical benefit doing so provides); instead, we warn only
+// for cases where the parser will give a parse tree differing from what React
+// intended. For example, <b><div></div></b> is invalid but we don't warn
+// because it still parses correctly; we do warn for other cases like nested
+// <p> tags where the beginning of the second element implicitly closes the
+// first, causing a confusing mess.
+
+// https://html.spec.whatwg.org/multipage/syntax.html#special
+const specialTags = [
+  'address',
+  'applet',
+  'area',
+  'article',
+  'aside',
+  'base',
+  'basefont',
+  'bgsound',
+  'blockquote',
+  'body',
+  'br',
+  'button',
+  'caption',
+  'center',
+  'col',
+  'colgroup',
+  'dd',
+  'details',
+  'dir',
+  'div',
+  'dl',
+  'dt',
+  'embed',
+  'fieldset',
+  'figcaption',
+  'figure',
+  'footer',
+  'form',
+  'frame',
+  'frameset',
+  'h1',
+  'h2',
+  'h3',
+  'h4',
+  'h5',
+  'h6',
+  'head',
+  'header',
+  'hgroup',
+  'hr',
+  'html',
+  'iframe',
+  'img',
+  'input',
+  'isindex',
+  'li',
+  'link',
+  'listing',
+  'main',
+  'marquee',
+  'menu',
+  'menuitem',
+  'meta',
+  'nav',
+  'noembed',
+  'noframes',
+  'noscript',
+  'object',
+  'ol',
+  'p',
+  'param',
+  'plaintext',
+  'pre',
+  'script',
+  'section',
+  'select',
+  'source',
+  'style',
+  'summary',
+  'table',
+  'tbody',
+  'td',
+  'template',
+  'textarea',
+  'tfoot',
+  'th',
+  'thead',
+  'title',
+  'tr',
+  'track',
+  'ul',
+  'wbr',
+  'xmp',
+];
+
+// https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
+const inScopeTags = [
+  'applet',
+  'caption',
+  'html',
+  'table',
+  'td',
+  'th',
+  'marquee',
+  'object',
+  'template',
+
+  // https://html.spec.whatwg.org/multipage/syntax.html#html-integration-point
+  // TODO: Distinguish by namespace here -- for <title>, including it here
+  // errs on the side of fewer warnings
+  'foreignObject',
+  'desc',
+  'title',
+];
+
+// https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-button-scope
+const buttonScopeTags = __DEV__ ? inScopeTags.concat(['button']) : [];
+
+// https://html.spec.whatwg.org/multipage/syntax.html#generate-implied-end-tags
+const impliedEndTags = [
+  'dd',
+  'dt',
+  'li',
+  'option',
+  'optgroup',
+  'p',
+  'rp',
+  'rt',
+];
+
+const emptyAncestorInfoDev: AncestorInfoDev = {
+  current: null,
+
+  formTag: null,
+  aTagInScope: null,
+  buttonTagInScope: null,
+  nobrTagInScope: null,
+  pTagInButtonScope: null,
+
+  listItemTagAutoclosing: null,
+  dlItemTagAutoclosing: null,
+
+  containerTagInScope: null,
+};
+
+function updatedAncestorInfoDev(
+  oldInfo: ?AncestorInfoDev,
+  tag: string,
+): AncestorInfoDev {
+  if (__DEV__) {
     const ancestorInfo = {...(oldInfo || emptyAncestorInfoDev)};
     const info = {tag};
 
@@ -234,213 +231,214 @@ if (__DEV__) {
     }
 
     return ancestorInfo;
-  };
-
-  /**
-   * Returns whether
-   */
-  const isTagValidWithParent = function (
-    tag: string,
-    parentTag: ?string,
-  ): boolean {
-    // 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
-      case 'select':
-        return tag === 'option' || tag === 'optgroup' || tag === '#text';
-      case 'optgroup':
-        return tag === 'option' || tag === '#text';
-      // Strictly speaking, seeing an <option> doesn't mean we're in a <select>
-      // but
-      case 'option':
-        return tag === '#text';
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intd
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-incaption
-      // No special behavior since these rules fall back to "in body" mode for
-      // all except special table nodes which cause bad parsing behavior anyway.
-
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intr
-      case 'tr':
-        return (
-          tag === 'th' ||
-          tag === 'td' ||
-          tag === 'style' ||
-          tag === 'script' ||
-          tag === 'template'
-        );
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intbody
-      case 'tbody':
-      case 'thead':
-      case 'tfoot':
-        return (
-          tag === 'tr' ||
-          tag === 'style' ||
-          tag === 'script' ||
-          tag === 'template'
-        );
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-incolgroup
-      case 'colgroup':
-        return tag === 'col' || tag === 'template';
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intable
-      case 'table':
-        return (
-          tag === 'caption' ||
-          tag === 'colgroup' ||
-          tag === 'tbody' ||
-          tag === 'tfoot' ||
-          tag === 'thead' ||
-          tag === 'style' ||
-          tag === 'script' ||
-          tag === 'template'
-        );
-      // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-inhead
-      case 'head':
-        return (
-          tag === 'base' ||
-          tag === 'basefont' ||
-          tag === 'bgsound' ||
-          tag === 'link' ||
-          tag === 'meta' ||
-          tag === 'title' ||
-          tag === 'noscript' ||
-          tag === 'noframes' ||
-          tag === 'style' ||
-          tag === 'script' ||
-          tag === 'template'
-        );
-      // https://html.spec.whatwg.org/multipage/semantics.html#the-html-element
-      case 'html':
-        return tag === 'head' || tag === 'body' || tag === 'frameset';
-      case 'frameset':
-        return tag === 'frame';
-      case '#document':
-        return tag === 'html';
-    }
-
-    // Probably in the "in body" parsing mode, so we outlaw only tag combos
-    // where the parsing rules cause implicit opens or closes to be added.
-    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-inbody
-    switch (tag) {
-      case 'h1':
-      case 'h2':
-      case 'h3':
-      case 'h4':
-      case 'h5':
-      case 'h6':
-        return (
-          parentTag !== 'h1' &&
-          parentTag !== 'h2' &&
-          parentTag !== 'h3' &&
-          parentTag !== 'h4' &&
-          parentTag !== 'h5' &&
-          parentTag !== 'h6'
-        );
+  } else {
+    return (null: any);
+  }
+}
 
-      case 'rp':
-      case 'rt':
-        return impliedEndTags.indexOf(parentTag) === -1;
-
-      case 'body':
-      case 'caption':
-      case 'col':
-      case 'colgroup':
-      case 'frameset':
-      case 'frame':
-      case 'head':
-      case 'html':
-      case 'tbody':
-      case 'td':
-      case 'tfoot':
-      case 'th':
-      case 'thead':
-      case 'tr':
-        // These tags are only valid with a few parents that have special child
-        // parsing rules -- if we're down here, then none of those matched and
-        // so we allow it only if we don't know what the parent is, as all other
-        // cases are invalid.
-        return parentTag == null;
-    }
+/**
+ * Returns whether
+ */
+function isTagValidWithParent(tag: string, parentTag: ?string): boolean {
+  // 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
+    case 'select':
+      return tag === 'option' || tag === 'optgroup' || tag === '#text';
+    case 'optgroup':
+      return tag === 'option' || tag === '#text';
+    // Strictly speaking, seeing an <option> doesn't mean we're in a <select>
+    // but
+    case 'option':
+      return tag === '#text';
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intd
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-incaption
+    // No special behavior since these rules fall back to "in body" mode for
+    // all except special table nodes which cause bad parsing behavior anyway.
+
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intr
+    case 'tr':
+      return (
+        tag === 'th' ||
+        tag === 'td' ||
+        tag === 'style' ||
+        tag === 'script' ||
+        tag === 'template'
+      );
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intbody
+    case 'tbody':
+    case 'thead':
+    case 'tfoot':
+      return (
+        tag === 'tr' ||
+        tag === 'style' ||
+        tag === 'script' ||
+        tag === 'template'
+      );
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-incolgroup
+    case 'colgroup':
+      return tag === 'col' || tag === 'template';
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-intable
+    case 'table':
+      return (
+        tag === 'caption' ||
+        tag === 'colgroup' ||
+        tag === 'tbody' ||
+        tag === 'tfoot' ||
+        tag === 'thead' ||
+        tag === 'style' ||
+        tag === 'script' ||
+        tag === 'template'
+      );
+    // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-inhead
+    case 'head':
+      return (
+        tag === 'base' ||
+        tag === 'basefont' ||
+        tag === 'bgsound' ||
+        tag === 'link' ||
+        tag === 'meta' ||
+        tag === 'title' ||
+        tag === 'noscript' ||
+        tag === 'noframes' ||
+        tag === 'style' ||
+        tag === 'script' ||
+        tag === 'template'
+      );
+    // https://html.spec.whatwg.org/multipage/semantics.html#the-html-element
+    case 'html':
+      return tag === 'head' || tag === 'body' || tag === 'frameset';
+    case 'frameset':
+      return tag === 'frame';
+    case '#document':
+      return tag === 'html';
+  }
+
+  // Probably in the "in body" parsing mode, so we outlaw only tag combos
+  // where the parsing rules cause implicit opens or closes to be added.
+  // https://html.spec.whatwg.org/multipage/syntax.html#parsing-main-inbody
+  switch (tag) {
+    case 'h1':
+    case 'h2':
+    case 'h3':
+    case 'h4':
+    case 'h5':
+    case 'h6':
+      return (
+        parentTag !== 'h1' &&
+        parentTag !== 'h2' &&
+        parentTag !== 'h3' &&
+        parentTag !== 'h4' &&
+        parentTag !== 'h5' &&
+        parentTag !== 'h6'
+      );
 
-    return true;
-  };
-
-  /**
-   * Returns whether
-   */
-  const findInvalidAncestorForTag = function (
-    tag: string,
-    ancestorInfo: AncestorInfoDev,
-  ): ?Info {
-    switch (tag) {
-      case 'address':
-      case 'article':
-      case 'aside':
-      case 'blockquote':
-      case 'center':
-      case 'details':
-      case 'dialog':
-      case 'dir':
-      case 'div':
-      case 'dl':
-      case 'fieldset':
-      case 'figcaption':
-      case 'figure':
-      case 'footer':
-      case 'header':
-      case 'hgroup':
-      case 'main':
-      case 'menu':
-      case 'nav':
-      case 'ol':
-      case 'p':
-      case 'section':
-      case 'summary':
-      case 'ul':
-      case 'pre':
-      case 'listing':
-      case 'table':
-      case 'hr':
-      case 'xmp':
-      case 'h1':
-      case 'h2':
-      case 'h3':
-      case 'h4':
-      case 'h5':
-      case 'h6':
-        return ancestorInfo.pTagInButtonScope;
-
-      case 'form':
-        return ancestorInfo.formTag || ancestorInfo.pTagInButtonScope;
-
-      case 'li':
-        return ancestorInfo.listItemTagAutoclosing;
-
-      case 'dd':
-      case 'dt':
-        return ancestorInfo.dlItemTagAutoclosing;
-
-      case 'button':
-        return ancestorInfo.buttonTagInScope;
-
-      case 'a':
-        // Spec says something about storing a list of markers, but it sounds
-        // equivalent to this check.
-        return ancestorInfo.aTagInScope;
-
-      case 'nobr':
-        return ancestorInfo.nobrTagInScope;
-    }
+    case 'rp':
+    case 'rt':
+      return impliedEndTags.indexOf(parentTag) === -1;
+
+    case 'body':
+    case 'caption':
+    case 'col':
+    case 'colgroup':
+    case 'frameset':
+    case 'frame':
+    case 'head':
+    case 'html':
+    case 'tbody':
+    case 'td':
+    case 'tfoot':
+    case 'th':
+    case 'thead':
+    case 'tr':
+      // These tags are only valid with a few parents that have special child
+      // parsing rules -- if we're down here, then none of those matched and
+      // so we allow it only if we don't know what the parent is, as all other
+      // cases are invalid.
+      return parentTag == null;
+  }
+
+  return true;
+}
 
-    return null;
-  };
+/**
+ * Returns whether
+ */
+function findInvalidAncestorForTag(
+  tag: string,
+  ancestorInfo: AncestorInfoDev,
+): ?Info {
+  switch (tag) {
+    case 'address':
+    case 'article':
+    case 'aside':
+    case 'blockquote':
+    case 'center':
+    case 'details':
+    case 'dialog':
+    case 'dir':
+    case 'div':
+    case 'dl':
+    case 'fieldset':
+    case 'figcaption':
+    case 'figure':
+    case 'footer':
+    case 'header':
+    case 'hgroup':
+    case 'main':
+    case 'menu':
+    case 'nav':
+    case 'ol':
+    case 'p':
+    case 'section':
+    case 'summary':
+    case 'ul':
+    case 'pre':
+    case 'listing':
+    case 'table':
+    case 'hr':
+    case 'xmp':
+    case 'h1':
+    case 'h2':
+    case 'h3':
+    case 'h4':
+    case 'h5':
+    case 'h6':
+      return ancestorInfo.pTagInButtonScope;
+
+    case 'form':
+      return ancestorInfo.formTag || ancestorInfo.pTagInButtonScope;
+
+    case 'li':
+      return ancestorInfo.listItemTagAutoclosing;
+
+    case 'dd':
+    case 'dt':
+      return ancestorInfo.dlItemTagAutoclosing;
+
+    case 'button':
+      return ancestorInfo.buttonTagInScope;
+
+    case 'a':
+      // Spec says something about storing a list of markers, but it sounds
+      // equivalent to this check.
+      return ancestorInfo.aTagInScope;
+
+    case 'nobr':
+      return ancestorInfo.nobrTagInScope;
+  }
+
+  return null;
+}
 
-  const didWarn: {[string]: boolean} = {};
+const didWarn: {[string]: boolean} = {};
 
-  validateDOMNesting = function (
-    childTag: ?string,
-    childText: ?string,
-    ancestorInfo: AncestorInfoDev,
-  ) {
+function validateDOMNesting(
+  childTag: ?string,
+  childText: ?string,
+  ancestorInfo: AncestorInfoDev,
+): void {
+  if (__DEV__) {
     ancestorInfo = ancestorInfo || emptyAncestorInfoDev;
     const parentInfo = ancestorInfo.current;
     const parentTag = parentInfo && parentInfo.tag;
@@ -517,7 +515,7 @@ if (__DEV__) {
         ancestorTag,
       );
     }
-  };
+  }
 }
 
 export {updatedAncestorInfoDev, validateDOMNesting};
diff --git a/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js b/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
index f87b256b39b36..1c5580b74c5e0 100644
--- a/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
+++ b/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
@@ -16,16 +16,18 @@ import isCustomComponent from './isCustomComponent';
 import possibleStandardNames from './possibleStandardNames';
 import hasOwnProperty from 'shared/hasOwnProperty';
 
-let validateProperty = () => {};
+const warnedProperties = {};
+const EVENT_NAME_REGEX = /^on./;
+const INVALID_EVENT_NAME_REGEX = /^on[^A-Z]/;
+const rARIA = __DEV__
+  ? new RegExp('^(aria)-[' + ATTRIBUTE_NAME_CHAR + ']*$')
+  : null;
+const rARIACamel = __DEV__
+  ? new RegExp('^(aria)[A-Z][' + ATTRIBUTE_NAME_CHAR + ']*$')
+  : null;
 
-if (__DEV__) {
-  const warnedProperties = {};
-  const EVENT_NAME_REGEX = /^on./;
-  const INVALID_EVENT_NAME_REGEX = /^on[^A-Z]/;
-  const rARIA = new RegExp('^(aria)-[' + ATTRIBUTE_NAME_CHAR + ']*$');
-  const rARIACamel = new RegExp('^(aria)[A-Z][' + ATTRIBUTE_NAME_CHAR + ']*$');
-
-  validateProperty = function (tagName, name, value, eventRegistry) {
+function validateProperty(tagName, name, value, eventRegistry) {
+  if (__DEV__) {
     if (hasOwnProperty.call(warnedProperties, name) && warnedProperties[name]) {
       return true;
     }
@@ -234,10 +236,10 @@ if (__DEV__) {
     }
 
     return true;
-  };
+  }
 }
 
-const warnUnknownProperties = function (type, props, eventRegistry) {
+function warnUnknownProperties(type, props, eventRegistry) {
   if (__DEV__) {
     const unknownProps = [];
     for (const key in props) {
@@ -268,7 +270,7 @@ const warnUnknownProperties = function (type, props, eventRegistry) {
       );
     }
   }
-};
+}
 
 export function validateProperties(type, props, eventRegistry) {
   if (isCustomComponent(type, props)) {
diff --git a/packages/react-dom-bindings/src/shared/warnValidStyle.js b/packages/react-dom-bindings/src/shared/warnValidStyle.js
index 646082360a52b..6970669a86bce 100644
--- a/packages/react-dom-bindings/src/shared/warnValidStyle.js
+++ b/packages/react-dom-bindings/src/shared/warnValidStyle.js
@@ -5,29 +5,27 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-let warnValidStyle = () => {};
-
-if (__DEV__) {
-  // 'msTransform' is correct, but the other prefixes should be capitalized
-  const badVendoredStyleNamePattern = /^(?:webkit|moz|o)[A-Z]/;
-  const msPattern = /^-ms-/;
-  const hyphenPattern = /-(.)/g;
-
-  // style values shouldn't contain a semicolon
-  const badStyleValueWithSemicolonPattern = /;\s*$/;
-
-  const warnedStyleNames = {};
-  const warnedStyleValues = {};
-  let warnedForNaNValue = false;
-  let warnedForInfinityValue = false;
-
-  const camelize = function (string) {
-    return string.replace(hyphenPattern, function (_, character) {
-      return character.toUpperCase();
-    });
-  };
+// 'msTransform' is correct, but the other prefixes should be capitalized
+const badVendoredStyleNamePattern = /^(?:webkit|moz|o)[A-Z]/;
+const msPattern = /^-ms-/;
+const hyphenPattern = /-(.)/g;
+
+// style values shouldn't contain a semicolon
+const badStyleValueWithSemicolonPattern = /;\s*$/;
+
+const warnedStyleNames = {};
+const warnedStyleValues = {};
+let warnedForNaNValue = false;
+let warnedForInfinityValue = false;
+
+function camelize(string) {
+  return string.replace(hyphenPattern, function (_, character) {
+    return character.toUpperCase();
+  });
+}
 
-  const warnHyphenatedStyleName = function (name) {
+function warnHyphenatedStyleName(name) {
+  if (__DEV__) {
     if (warnedStyleNames.hasOwnProperty(name) && warnedStyleNames[name]) {
       return;
     }
@@ -41,9 +39,11 @@ if (__DEV__) {
       // is converted to lowercase `ms`.
       camelize(name.replace(msPattern, 'ms-')),
     );
-  };
+  }
+}
 
-  const warnBadVendoredStyleName = function (name) {
+function warnBadVendoredStyleName(name) {
+  if (__DEV__) {
     if (warnedStyleNames.hasOwnProperty(name) && warnedStyleNames[name]) {
       return;
     }
@@ -54,9 +54,11 @@ if (__DEV__) {
       name,
       name.charAt(0).toUpperCase() + name.slice(1),
     );
-  };
+  }
+}
 
-  const warnStyleValueWithSemicolon = function (name, value) {
+function warnStyleValueWithSemicolon(name, value) {
+  if (__DEV__) {
     if (warnedStyleValues.hasOwnProperty(value) && warnedStyleValues[value]) {
       return;
     }
@@ -68,9 +70,11 @@ if (__DEV__) {
       name,
       value.replace(badStyleValueWithSemicolonPattern, ''),
     );
-  };
+  }
+}
 
-  const warnStyleValueIsNaN = function (name, value) {
+function warnStyleValueIsNaN(name, value) {
+  if (__DEV__) {
     if (warnedForNaNValue) {
       return;
     }
@@ -80,9 +84,11 @@ if (__DEV__) {
       '`NaN` is an invalid value for the `%s` css style property.',
       name,
     );
-  };
+  }
+}
 
-  const warnStyleValueIsInfinity = function (name, value) {
+function warnStyleValueIsInfinity(name, value) {
+  if (__DEV__) {
     if (warnedForInfinityValue) {
       return;
     }
@@ -92,9 +98,11 @@ if (__DEV__) {
       '`Infinity` is an invalid value for the `%s` css style property.',
       name,
     );
-  };
+  }
+}
 
-  warnValidStyle = function (name, value) {
+function warnValidStyle(name, value) {
+  if (__DEV__) {
     if (name.indexOf('-') > -1) {
       warnHyphenatedStyleName(name);
     } else if (badVendoredStyleNamePattern.test(name)) {
@@ -110,7 +118,7 @@ if (__DEV__) {
         warnStyleValueIsInfinity(name, value);
       }
     }
-  };
+  }
 }
 
 export default warnValidStyle;
diff --git a/packages/react-dom/src/test-utils/ReactTestUtils.js b/packages/react-dom/src/test-utils/ReactTestUtils.js
index 2efe739241cc5..04a69bcd81a69 100644
--- a/packages/react-dom/src/test-utils/ReactTestUtils.js
+++ b/packages/react-dom/src/test-utils/ReactTestUtils.js
@@ -392,7 +392,7 @@ function executeDispatchesInOrder(event) {
  * @param {?object} event Synthetic event to be dispatched.
  * @private
  */
-const executeDispatchesAndRelease = function (event /* ReactSyntheticEvent */) {
+function executeDispatchesAndRelease(event /* ReactSyntheticEvent */) {
   if (event) {
     executeDispatchesInOrder(event);
 
@@ -400,7 +400,7 @@ const executeDispatchesAndRelease = function (event /* ReactSyntheticEvent */) {
       event.constructor.release(event);
     }
   }
-};
+}
 
 function isInteractive(tag) {
   return (
diff --git a/packages/react-native-renderer/src/ReactNativeEventEmitter.js b/packages/react-native-renderer/src/ReactNativeEventEmitter.js
index c740e085e863d..208254c4cb0df 100644
--- a/packages/react-native-renderer/src/ReactNativeEventEmitter.js
+++ b/packages/react-native-renderer/src/ReactNativeEventEmitter.js
@@ -44,13 +44,13 @@ const EMPTY_NATIVE_EVENT = (({}: any): AnyNativeEvent);
  * @return {Array<Touch>} Subsequence of touch objects.
  */
 // $FlowFixMe[missing-local-annot]
-const touchSubsequence = function (touches, indices) {
+function touchSubsequence(touches, indices) {
   const ret = [];
   for (let i = 0; i < indices.length; i++) {
     ret.push(touches[indices[i]]);
   }
   return ret;
-};
+}
 
 /**
  * TODO: Pool all of this.
@@ -63,7 +63,7 @@ const touchSubsequence = function (touches, indices) {
  * @param {Array<number>} indices Indices to remove from `touches`.
  * @return {Array<Touch>} Subsequence of removed touch objects.
  */
-const removeTouchesAtIndices = function (
+function removeTouchesAtIndices(
   touches: Array<Object>,
   indices: Array<number>,
 ): Array<Object> {
@@ -85,7 +85,7 @@ const removeTouchesAtIndices = function (
   }
   temp.length = fillAt;
   return rippedOut;
-};
+}
 
 /**
  * Internal version of `receiveEvent` in terms of normalized (non-tag)
diff --git a/packages/react-native-renderer/src/ReactNativeFiberInspector.js b/packages/react-native-renderer/src/ReactNativeFiberInspector.js
index 696e26baef800..57d9b0542c37b 100644
--- a/packages/react-native-renderer/src/ReactNativeFiberInspector.js
+++ b/packages/react-native-renderer/src/ReactNativeFiberInspector.js
@@ -28,49 +28,39 @@ if (__DEV__) {
   Object.freeze(emptyObject);
 }
 
-let createHierarchy;
-let getHostNode;
-let getHostProps;
-let lastNonHostInstance;
-let getInspectorDataForInstance: (
-  closestInstance: Fiber | null,
-) => InspectorData;
-let getOwnerHierarchy;
-let traverseOwnerTreeUp: (hierarchy: Array<$FlowFixMe>, instance: any) => void;
-
-if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
-  // $FlowFixMe[missing-local-annot]
-  createHierarchy = function (fiberHierarchy) {
-    return fiberHierarchy.map(fiber => ({
-      name: getComponentNameFromType(fiber.type),
-      getInspectorData: findNodeHandle => {
-        return {
-          props: getHostProps(fiber),
-          source: fiber._debugSource,
-          measure: callback => {
-            // If this is Fabric, we'll find a shadow node and use that to measure.
-            const hostFiber = findCurrentHostFiber(fiber);
-            const node =
-              hostFiber != null &&
-              hostFiber.stateNode !== null &&
-              hostFiber.stateNode.node;
+// $FlowFixMe[missing-local-annot]
+function createHierarchy(fiberHierarchy) {
+  return fiberHierarchy.map(fiber => ({
+    name: getComponentNameFromType(fiber.type),
+    getInspectorData: findNodeHandle => {
+      return {
+        props: getHostProps(fiber),
+        source: fiber._debugSource,
+        measure: callback => {
+          // If this is Fabric, we'll find a shadow node and use that to measure.
+          const hostFiber = findCurrentHostFiber(fiber);
+          const node =
+            hostFiber != null &&
+            hostFiber.stateNode !== null &&
+            hostFiber.stateNode.node;
 
-            if (node) {
-              nativeFabricUIManager.measure(node, callback);
-            } else {
-              return UIManager.measure(
-                getHostNode(fiber, findNodeHandle),
-                callback,
-              );
-            }
-          },
-        };
-      },
-    }));
-  };
+          if (node) {
+            nativeFabricUIManager.measure(node, callback);
+          } else {
+            return UIManager.measure(
+              getHostNode(fiber, findNodeHandle),
+              callback,
+            );
+          }
+        },
+      };
+    },
+  }));
+}
 
-  // $FlowFixMe[missing-local-annot]
-  getHostNode = function (fiber: Fiber | null, findNodeHandle) {
+// $FlowFixMe[missing-local-annot]
+function getHostNode(fiber: Fiber | null, findNodeHandle) {
+  if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     let hostNode;
     // look for children first for the hostNode
     // as composite fibers do not have a hostNode
@@ -84,20 +74,22 @@ if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
       fiber = fiber.child;
     }
     return null;
-  };
+  }
+}
 
-  // $FlowFixMe[missing-local-annot]
-  getHostProps = function (fiber) {
-    const host = findCurrentHostFiber(fiber);
-    if (host) {
-      return host.memoizedProps || emptyObject;
-    }
-    return emptyObject;
-  };
+// $FlowFixMe[missing-local-annot]
+function getHostProps(fiber) {
+  const host = findCurrentHostFiber(fiber);
+  if (host) {
+    return host.memoizedProps || emptyObject;
+  }
+  return emptyObject;
+}
 
-  getInspectorDataForInstance = function (
-    closestInstance: Fiber | null,
-  ): InspectorData {
+function getInspectorDataForInstance(
+  closestInstance: Fiber | null,
+): InspectorData {
+  if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     // Handle case where user clicks outside of ReactNative
     if (!closestInstance) {
       return {
@@ -123,46 +115,44 @@ if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
       selectedIndex,
       source,
     };
-  };
+  } else {
+    return (null: any);
+  }
+}
 
-  getOwnerHierarchy = function (instance: any) {
-    const hierarchy: Array<$FlowFixMe> = [];
-    traverseOwnerTreeUp(hierarchy, instance);
-    return hierarchy;
-  };
+function getOwnerHierarchy(instance: any) {
+  const hierarchy: Array<$FlowFixMe> = [];
+  traverseOwnerTreeUp(hierarchy, instance);
+  return hierarchy;
+}
 
-  // $FlowFixMe[missing-local-annot]
-  lastNonHostInstance = function (hierarchy) {
-    for (let i = hierarchy.length - 1; i > 1; i--) {
-      const instance = hierarchy[i];
+// $FlowFixMe[missing-local-annot]
+function lastNonHostInstance(hierarchy) {
+  for (let i = hierarchy.length - 1; i > 1; i--) {
+    const instance = hierarchy[i];
 
-      if (instance.tag !== HostComponent) {
-        return instance;
-      }
+    if (instance.tag !== HostComponent) {
+      return instance;
     }
-    return hierarchy[0];
-  };
+  }
+  return hierarchy[0];
+}
 
-  // $FlowFixMe[missing-local-annot]
-  traverseOwnerTreeUp = function (hierarchy, instance: any): void {
+// $FlowFixMe[missing-local-annot]
+function traverseOwnerTreeUp(
+  hierarchy: Array<$FlowFixMe>,
+  instance: any,
+): void {
+  if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     if (instance) {
       hierarchy.unshift(instance);
       traverseOwnerTreeUp(hierarchy, instance._debugOwner);
     }
-  };
+  }
 }
 
-let getInspectorDataForViewTag: (viewTag: number) => Object;
-let getInspectorDataForViewAtPoint: (
-  findNodeHandle: (componentOrHandle: any) => ?number,
-  inspectedView: Object,
-  locationX: number,
-  locationY: number,
-  callback: (viewData: TouchedViewDataAtPoint) => mixed,
-) => void;
-
-if (__DEV__) {
-  getInspectorDataForViewTag = function (viewTag: number): Object {
+function getInspectorDataForViewTag(viewTag: number): Object {
+  if (__DEV__) {
     const closestInstance = getClosestInstanceFromNode(viewTag);
 
     // Handle case where user clicks outside of ReactNative
@@ -189,15 +179,21 @@ if (__DEV__) {
       selectedIndex,
       source,
     };
-  };
+  } else {
+    throw new Error(
+      'getInspectorDataForViewTag() is not available in production',
+    );
+  }
+}
 
-  getInspectorDataForViewAtPoint = function (
-    findNodeHandle: (componentOrHandle: any) => ?number,
-    inspectedView: Object,
-    locationX: number,
-    locationY: number,
-    callback: (viewData: TouchedViewDataAtPoint) => mixed,
-  ): void {
+function getInspectorDataForViewAtPoint(
+  findNodeHandle: (componentOrHandle: any) => ?number,
+  inspectedView: Object,
+  locationX: number,
+  locationY: number,
+  callback: (viewData: TouchedViewDataAtPoint) => mixed,
+): void {
+  if (__DEV__) {
     let closestInstance = null;
 
     const fabricInstanceHandle =
@@ -271,25 +267,11 @@ if (__DEV__) {
 
       return;
     }
-  };
-} else {
-  getInspectorDataForViewTag = () => {
-    throw new Error(
-      'getInspectorDataForViewTag() is not available in production',
-    );
-  };
-
-  getInspectorDataForViewAtPoint = (
-    findNodeHandle: (componentOrHandle: any) => ?number,
-    inspectedView: Object,
-    locationX: number,
-    locationY: number,
-    callback: (viewData: TouchedViewDataAtPoint) => mixed,
-  ): void => {
+  } else {
     throw new Error(
       'getInspectorDataForViewAtPoint() is not available in production.',
     );
-  };
+  }
 }
 
 export {
diff --git a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepDiffer.js b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepDiffer.js
index 64ed3eba715c2..5e8b3a309f9ca 100644
--- a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepDiffer.js
+++ b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepDiffer.js
@@ -15,7 +15,7 @@ type Options = {+unsafelyIgnoreFunctions?: boolean};
 /*
  * @returns {bool} true if different, false if equal
  */
-const deepDiffer = function (
+function deepDiffer(
   one: any,
   two: any,
   maxDepthOrOptions: Options | number = -1,
@@ -79,6 +79,6 @@ const deepDiffer = function (
     }
   }
   return false;
-};
+}
 
 module.exports = deepDiffer;
diff --git a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepFreezeAndThrowOnMutationInDev.js b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepFreezeAndThrowOnMutationInDev.js
index 2d61b723ceff4..28da2c86ae55e 100644
--- a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepFreezeAndThrowOnMutationInDev.js
+++ b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/deepFreezeAndThrowOnMutationInDev.js
@@ -9,6 +9,6 @@
 
 // TODO: move into react or fbjs
 
-const deepFreezeAndThrowOnMutationInDev = function () {};
+function deepFreezeAndThrowOnMutationInDev() {}
 
 module.exports = deepFreezeAndThrowOnMutationInDev;
diff --git a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/flattenStyle.js b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/flattenStyle.js
index 8112fe2d925ac..49bebf6f07597 100644
--- a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/flattenStyle.js
+++ b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/flattenStyle.js
@@ -9,6 +9,6 @@
 
 // TODO: Move flattenStyle into react
 
-const flattenStyle = function () {};
+function flattenStyle() {}
 
 module.exports = flattenStyle;
diff --git a/packages/react-native-renderer/src/legacy-events/EventBatching.js b/packages/react-native-renderer/src/legacy-events/EventBatching.js
index f56f22f2d9f21..cad3671723ed7 100644
--- a/packages/react-native-renderer/src/legacy-events/EventBatching.js
+++ b/packages/react-native-renderer/src/legacy-events/EventBatching.js
@@ -25,7 +25,7 @@ let eventQueue: ?(Array<ReactSyntheticEvent> | ReactSyntheticEvent) = null;
  * @param {?object} event Synthetic event to be dispatched.
  * @private
  */
-const executeDispatchesAndRelease = function (event: ReactSyntheticEvent) {
+function executeDispatchesAndRelease(event: ReactSyntheticEvent) {
   if (event) {
     executeDispatchesInOrder(event);
 
@@ -33,11 +33,11 @@ const executeDispatchesAndRelease = function (event: ReactSyntheticEvent) {
       event.constructor.release(event);
     }
   }
-};
+}
 // $FlowFixMe[missing-local-annot]
-const executeDispatchesAndReleaseTopLevel = function (e) {
+function executeDispatchesAndReleaseTopLevel(e) {
   return executeDispatchesAndRelease(e);
-};
+}
 
 export function runEventsInBatch(
   events: Array<ReactSyntheticEvent> | ReactSyntheticEvent | null,
diff --git a/packages/react-native-renderer/src/legacy-events/EventPluginUtils.js b/packages/react-native-renderer/src/legacy-events/EventPluginUtils.js
index f4538e644dafa..c47192d785fa0 100644
--- a/packages/react-native-renderer/src/legacy-events/EventPluginUtils.js
+++ b/packages/react-native-renderer/src/legacy-events/EventPluginUtils.js
@@ -30,9 +30,8 @@ export function setComponentTree(
   }
 }
 
-let validateEventDispatches;
-if (__DEV__) {
-  validateEventDispatches = function (event) {
+function validateEventDispatches(event) {
+  if (__DEV__) {
     const dispatchListeners = event._dispatchListeners;
     const dispatchInstances = event._dispatchInstances;
 
@@ -53,7 +52,7 @@ if (__DEV__) {
     if (instancesIsArr !== listenersIsArr || instancesLen !== listenersLen) {
       console.error('EventPluginUtils: Invalid `event`.');
     }
-  };
+  }
 }
 
 /**
diff --git a/packages/react-native-renderer/src/legacy-events/ResponderEventPlugin.js b/packages/react-native-renderer/src/legacy-events/ResponderEventPlugin.js
index 2d59de4ff026a..21b0dde0ab3a0 100644
--- a/packages/react-native-renderer/src/legacy-events/ResponderEventPlugin.js
+++ b/packages/react-native-renderer/src/legacy-events/ResponderEventPlugin.js
@@ -44,7 +44,7 @@ let responderInst = null;
  */
 let trackedTouchCount = 0;
 
-const changeResponder = function (nextResponderInst, blockHostResponder) {
+function changeResponder(nextResponderInst, blockHostResponder) {
   const oldResponderInst = responderInst;
   responderInst = nextResponderInst;
   if (ResponderEventPlugin.GlobalResponderHandler !== null) {
@@ -54,7 +54,7 @@ const changeResponder = function (nextResponderInst, blockHostResponder) {
       blockHostResponder,
     );
   }
-};
+}
 
 const eventTypes = {
   /**
diff --git a/packages/react-reconciler/src/ReactFiber.js b/packages/react-reconciler/src/ReactFiber.js
index e3fc26659c45c..ed5706094d689 100644
--- a/packages/react-reconciler/src/ReactFiber.js
+++ b/packages/react-reconciler/src/ReactFiber.js
@@ -224,7 +224,7 @@ function FiberNode(
 //    is faster.
 // 5) It should be easy to port this to a C struct and keep a C implementation
 //    compatible.
-const createFiber = function (
+function createFiber(
   tag: WorkTag,
   pendingProps: mixed,
   key: null | string,
@@ -232,7 +232,7 @@ const createFiber = function (
 ): Fiber {
   // $FlowFixMe: the shapes are exact here but Flow doesn't like constructors
   return new FiberNode(tag, pendingProps, key, mode);
-};
+}
 
 function shouldConstruct(Component: Function) {
   const prototype = Component.prototype;
diff --git a/packages/react-reconciler/src/ReactFiberClassComponent.js b/packages/react-reconciler/src/ReactFiberClassComponent.js
index 7fe8a87e0abe6..7aa3d934f3b12 100644
--- a/packages/react-reconciler/src/ReactFiberClassComponent.js
+++ b/packages/react-reconciler/src/ReactFiberClassComponent.js
@@ -85,11 +85,10 @@ let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
 let didWarnAboutLegacyLifecyclesAndDerivedState;
 let didWarnAboutUndefinedDerivedState;
-let warnOnUndefinedDerivedState;
-let warnOnInvalidCallback;
 let didWarnAboutDirectlyAssigningPropsToState;
 let didWarnAboutContextTypeAndContextTypes;
 let didWarnAboutInvalidateContextType;
+let didWarnOnInvalidCallback;
 
 if (__DEV__) {
   didWarnAboutStateAssignmentForComponent = new Set<string>();
@@ -100,10 +99,31 @@ if (__DEV__) {
   didWarnAboutUndefinedDerivedState = new Set<string>();
   didWarnAboutContextTypeAndContextTypes = new Set<string>();
   didWarnAboutInvalidateContextType = new Set<string>();
+  didWarnOnInvalidCallback = new Set<string>();
 
-  const didWarnOnInvalidCallback = new Set<string>();
+  // This is so gross but it's at least non-critical and can be removed if
+  // it causes problems. This is meant to give a nicer error message for
+  // ReactDOM15.unstable_renderSubtreeIntoContainer(reactDOM16Component,
+  // ...)) which otherwise throws a "_processChildContext is not a function"
+  // exception.
+  Object.defineProperty(fakeInternalInstance, '_processChildContext', {
+    enumerable: false,
+    value: function (): empty {
+      throw new Error(
+        '_processChildContext is not available in React 16+. This likely ' +
+          'means you have multiple copies of React and are attempting to nest ' +
+          'a React 15 tree inside a React 16 tree using ' +
+          "unstable_renderSubtreeIntoContainer, which isn't supported. Try " +
+          'to make sure you have only one copy of React (and ideally, switch ' +
+          'to ReactDOM.createPortal).',
+      );
+    },
+  });
+  Object.freeze(fakeInternalInstance);
+}
 
-  warnOnInvalidCallback = function (callback: mixed, callerName: string) {
+function warnOnInvalidCallback(callback: mixed, callerName: string) {
+  if (__DEV__) {
     if (callback === null || typeof callback === 'function') {
       return;
     }
@@ -117,9 +137,11 @@ if (__DEV__) {
         callback,
       );
     }
-  };
+  }
+}
 
-  warnOnUndefinedDerivedState = function (type: any, partialState: any) {
+function warnOnUndefinedDerivedState(type: any, partialState: any) {
+  if (__DEV__) {
     if (partialState === undefined) {
       const componentName = getComponentNameFromType(type) || 'Component';
       if (!didWarnAboutUndefinedDerivedState.has(componentName)) {
@@ -131,27 +153,7 @@ if (__DEV__) {
         );
       }
     }
-  };
-
-  // This is so gross but it's at least non-critical and can be removed if
-  // it causes problems. This is meant to give a nicer error message for
-  // ReactDOM15.unstable_renderSubtreeIntoContainer(reactDOM16Component,
-  // ...)) which otherwise throws a "_processChildContext is not a function"
-  // exception.
-  Object.defineProperty(fakeInternalInstance, '_processChildContext', {
-    enumerable: false,
-    value: function (): empty {
-      throw new Error(
-        '_processChildContext is not available in React 16+. This likely ' +
-          'means you have multiple copies of React and are attempting to nest ' +
-          'a React 15 tree inside a React 16 tree using ' +
-          "unstable_renderSubtreeIntoContainer, which isn't supported. Try " +
-          'to make sure you have only one copy of React (and ideally, switch ' +
-          'to ReactDOM.createPortal).',
-      );
-    },
-  });
-  Object.freeze(fakeInternalInstance);
+  }
 }
 
 function applyDerivedStateFromProps(
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.js b/packages/react-reconciler/src/ReactFiberCommitWork.js
index 795a2347762ca..eb0aa892a313c 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.js
@@ -253,10 +253,7 @@ export function reportUncaughtErrorInDEV(error: mixed) {
   }
 }
 
-const callComponentWillUnmountWithTimer = function (
-  current: Fiber,
-  instance: any,
-) {
+function callComponentWillUnmountWithTimer(current: Fiber, instance: any) {
   instance.props = current.memoizedProps;
   instance.state = current.memoizedState;
   if (shouldProfile(current)) {
@@ -269,7 +266,7 @@ const callComponentWillUnmountWithTimer = function (
   } else {
     instance.componentWillUnmount();
   }
-};
+}
 
 // Capture errors so they don't interrupt unmounting.
 function safelyCallComponentWillUnmount(
diff --git a/packages/react-server/src/ReactFizzClassComponent.js b/packages/react-server/src/ReactFizzClassComponent.js
index 818bc6672fe89..e624dfa941293 100644
--- a/packages/react-server/src/ReactFizzClassComponent.js
+++ b/packages/react-server/src/ReactFizzClassComponent.js
@@ -24,11 +24,10 @@ let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
 let didWarnAboutLegacyLifecyclesAndDerivedState;
 let didWarnAboutUndefinedDerivedState;
-let warnOnUndefinedDerivedState;
-let warnOnInvalidCallback;
 let didWarnAboutDirectlyAssigningPropsToState;
 let didWarnAboutContextTypeAndContextTypes;
 let didWarnAboutInvalidateContextType;
+let didWarnOnInvalidCallback;
 
 if (__DEV__) {
   didWarnAboutUninitializedState = new Set<string>();
@@ -38,10 +37,11 @@ if (__DEV__) {
   didWarnAboutUndefinedDerivedState = new Set<string>();
   didWarnAboutContextTypeAndContextTypes = new Set<mixed>();
   didWarnAboutInvalidateContextType = new Set<mixed>();
+  didWarnOnInvalidCallback = new Set<string>();
+}
 
-  const didWarnOnInvalidCallback = new Set<string>();
-
-  warnOnInvalidCallback = function (callback: mixed, callerName: string) {
+function warnOnInvalidCallback(callback: mixed, callerName: string) {
+  if (__DEV__) {
     if (callback === null || typeof callback === 'function') {
       return;
     }
@@ -55,9 +55,11 @@ if (__DEV__) {
         callback,
       );
     }
-  };
+  }
+}
 
-  warnOnUndefinedDerivedState = function (type: any, partialState: any) {
+function warnOnUndefinedDerivedState(type: any, partialState: any) {
+  if (__DEV__) {
     if (partialState === undefined) {
       const componentName = getComponentNameFromType(type) || 'Component';
       if (!didWarnAboutUndefinedDerivedState.has(componentName)) {
@@ -69,7 +71,7 @@ if (__DEV__) {
         );
       }
     }
-  };
+  }
 }
 
 function warnNoop(
diff --git a/packages/react/src/ReactElement.js b/packages/react/src/ReactElement.js
index 1857a6e484fa0..e9d721b92693a 100644
--- a/packages/react/src/ReactElement.js
+++ b/packages/react/src/ReactElement.js
@@ -145,7 +145,7 @@ function warnIfStringRefCannotBeAutoConverted(config) {
  * indicating filename, line number, and/or other information.
  * @internal
  */
-const ReactElement = function (type, key, ref, self, source, owner, props) {
+function ReactElement(type, key, ref, self, source, owner, props) {
   const element = {
     // This tag allows us to uniquely identify this as a React Element
     $$typeof: REACT_ELEMENT_TYPE,
@@ -199,7 +199,7 @@ const ReactElement = function (type, key, ref, self, source, owner, props) {
   }
 
   return element;
-};
+}
 
 /**
  * https://github.com/reactjs/rfcs/pull/107
diff --git a/packages/react/src/jsx/ReactJSXElement.js b/packages/react/src/jsx/ReactJSXElement.js
index 97c12bff5b8f1..e2d8fe0af4aeb 100644
--- a/packages/react/src/jsx/ReactJSXElement.js
+++ b/packages/react/src/jsx/ReactJSXElement.js
@@ -145,7 +145,7 @@ function defineRefPropWarningGetter(props, displayName) {
  * indicating filename, line number, and/or other information.
  * @internal
  */
-const ReactElement = function (type, key, ref, self, source, owner, props) {
+function ReactElement(type, key, ref, self, source, owner, props) {
   const element = {
     // This tag allows us to uniquely identify this as a React Element
     $$typeof: REACT_ELEMENT_TYPE,
@@ -199,7 +199,7 @@ const ReactElement = function (type, key, ref, self, source, owner, props) {
   }
 
   return element;
-};
+}
 
 /**
  * https://github.com/reactjs/rfcs/pull/107
diff --git a/packages/shared/forks/invokeGuardedCallbackImpl.www.js b/packages/shared/forks/invokeGuardedCallbackImpl.www.js
index ff588831b3935..b85c10e7a4c07 100644
--- a/packages/shared/forks/invokeGuardedCallbackImpl.www.js
+++ b/packages/shared/forks/invokeGuardedCallbackImpl.www.js
@@ -16,7 +16,7 @@ if (typeof ReactFbErrorUtils.invokeGuardedCallback !== 'function') {
   );
 }
 
-const invokeGuardedCallbackImpl = function <A, B, C, D, E, F, Context>(
+function invokeGuardedCallbackImpl<A, B, C, D, E, F, Context>(
   name: string | null,
   func: (a: A, b: B, c: C, d: D, e: E, f: F) => mixed,
   context: Context,
@@ -29,6 +29,6 @@ const invokeGuardedCallbackImpl = function <A, B, C, D, E, F, Context>(
 ) {
   // This will call `this.onError(err)` if an error was caught.
   ReactFbErrorUtils.invokeGuardedCallback.apply(this, arguments);
-};
+}
 
 export default invokeGuardedCallbackImpl;