refactor(core): replace useDocusaurusContext().isClient by useIsBrowser() (#5349)

* extract separate useIsClient() hook

* for consistency, rename to `useIsBrowser`

* useless return

* improve doc for BrowserOnly

* update snapshot

* polish

packages/docusaurus-module-type-aliases/src/index.d.ts

@@ -179,6 +179,10 @@ declare module '@docusaurus/useDocusaurusContext' {
   export default function useDocusaurusContext(): DocusaurusContext;
 }
 
+declare module '@docusaurus/useIsBrowser' {
+  export default function useIsBrowser(): boolean;
+}
+
 declare module '@docusaurus/useBaseUrl' {
   export type BaseUrlOptions = {
     forcePrependBaseUrl?: boolean;

packages/docusaurus-theme-bootstrap/src/theme/ThemedImage/index.tsx

@@ -8,14 +8,14 @@
 import React from 'react';
 import clsx from 'clsx';
 
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 import useThemeContext from '@theme/hooks/useThemeContext';
 import type {Props} from '@theme/ThemedImage';
 
 import styles from './styles.module.css';
 
 const ThemedImage = (props: Props): JSX.Element => {
-  const {isClient} = useDocusaurusContext();
+  const isBrowser = useIsBrowser();
   const {isDarkTheme} = useThemeContext();
   const {sources, className, alt = '', ...propsRest} = props;
 
@@ -23,7 +23,7 @@ const ThemedImage = (props: Props): JSX.Element => {
 
   const clientThemes: SourceName[] = isDarkTheme ? ['dark'] : ['light'];
 
-  const renderedSourceNames: SourceName[] = isClient
+  const renderedSourceNames: SourceName[] = isBrowser
     ? clientThemes
     : // We need to render both images on the server to avoid flash
       // See https://github.com/facebook/docusaurus/pull/3730

packages/docusaurus-theme-classic/src/theme/DocPage/index.tsx

@@ -8,7 +8,6 @@
 import React, {ReactNode, useState, useCallback} from 'react';
 import {MDXProvider} from '@mdx-js/react';
 
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
 import renderRoutes from '@docusaurus/renderRoutes';
 import type {PropVersionMetadata} from '@docusaurus/plugin-content-docs-types';
 import Layout from '@theme/Layout';
@@ -37,7 +36,6 @@ function DocPageContent({
   versionMetadata,
   children,
 }: DocPageContentProps): JSX.Element {
-  const {isClient} = useDocusaurusContext();
   const {pluginId, version} = versionMetadata;
 
   const sidebarName = currentDocRoute.sidebar;
@@ -57,7 +55,6 @@ function DocPageContent({
 
   return (
     <Layout
-      key={`${isClient}`} // TODO seems suspicious
       wrapperClassName={ThemeClassNames.wrapper.docPages}
       pageClassName={ThemeClassNames.page.docPage}
       searchMetadatas={{

packages/docusaurus-theme-classic/src/theme/Logo/index.tsx

@@ -17,7 +17,6 @@ import {useThemeConfig} from '@docusaurus/theme-common';
 const Logo = (props: Props): JSX.Element => {
   const {
     siteConfig: {title},
-    isClient,
   } = useDocusaurusContext();
   const {
     navbar: {title: navbarTitle, logo = {src: ''}},
@@ -37,7 +36,6 @@ const Logo = (props: Props): JSX.Element => {
       {...(logo.target && {target: logo.target})}>
       {logo.src && (
         <ThemedImage
-          key={`${isClient}`} // TODO seems suspicious
           className={imageClassName}
           sources={sources}
           alt={logo.alt || navbarTitle || title}

packages/docusaurus-theme-classic/src/theme/ThemedImage/index.tsx

@@ -8,14 +8,14 @@
 import React from 'react';
 import clsx from 'clsx';
 
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 import useThemeContext from '@theme/hooks/useThemeContext';
 import type {Props} from '@theme/ThemedImage';
 
 import styles from './styles.module.css';
 
 const ThemedImage = (props: Props): JSX.Element => {
-  const {isClient} = useDocusaurusContext();
+  const isBrowser = useIsBrowser();
   const {isDarkTheme} = useThemeContext();
   const {sources, className, alt = '', ...propsRest} = props;
 
@@ -23,7 +23,7 @@ const ThemedImage = (props: Props): JSX.Element => {
 
   const clientThemes: SourceName[] = isDarkTheme ? ['dark'] : ['light'];
 
-  const renderedSourceNames: SourceName[] = isClient
+  const renderedSourceNames: SourceName[] = isBrowser
     ? clientThemes
     : // We need to render both images on the server to avoid flash
       // See https://github.com/facebook/docusaurus/pull/3730

packages/docusaurus-theme-classic/src/theme/Toggle/index.tsx

@@ -8,7 +8,7 @@
 import React, {useState, useRef, memo, CSSProperties} from 'react';
 import type {Props} from '@theme/Toggle';
 import {useThemeConfig} from '@docusaurus/theme-common';
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 
 import clsx from 'clsx';
 import './styles.css';
@@ -91,11 +91,11 @@ export default function (props: Props): JSX.Element {
       switchConfig: {darkIcon, darkIconStyle, lightIcon, lightIconStyle},
     },
   } = useThemeConfig();
-  const {isClient} = useDocusaurusContext();
+  const isBrowser = useIsBrowser();
 
   return (
     <Toggle
-      disabled={!isClient}
+      disabled={!isBrowser}
       icons={{
         checked: <Dark icon={darkIcon} style={darkIconStyle} />,
         unchecked: <Light icon={lightIcon} style={lightIconStyle} />,

packages/docusaurus-theme-classic/src/theme/hooks/useWindowSize.ts

@@ -47,10 +47,6 @@ function useWindowSize(): WindowSize {
   });
 
   useEffect(() => {
-    if (!ExecutionEnvironment.canUseDOM) {
-      return undefined;
-    }
-
     function updateWindowSize() {
       setWindowSize(getWindowSize());
     }

packages/docusaurus-theme-common/src/components/Details/index.tsx

@@ -6,7 +6,7 @@
  */
 
 import React, {ComponentProps, ReactElement, useRef, useState} from 'react';
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 import clsx from 'clsx';
 import {useCollapsible, Collapsible} from '../Collapsible';
 import styles from './styles.module.css';
@@ -30,7 +30,7 @@ export type DetailsProps = {
 } & ComponentProps<'details'>;
 
 const Details = ({summary, children, ...props}: DetailsProps): JSX.Element => {
-  const {isClient} = useDocusaurusContext();
+  const isBrowser = useIsBrowser();
   const detailsRef = useRef<HTMLDetailsElement>(null);
 
   const {collapsed, setCollapsed} = useCollapsible({
@@ -48,7 +48,7 @@ const Details = ({summary, children, ...props}: DetailsProps): JSX.Element => {
       data-collapsed={collapsed}
       className={clsx(
         styles.details,
-        {[styles.isClient]: isClient},
+        {[styles.isBrowser]: isBrowser},
         props.className,
       )}>
       {/* eslint-disable-next-line jsx-a11y/click-events-have-key-events */}

packages/docusaurus-theme-common/src/components/Details/styles.module.css

@@ -46,9 +46,9 @@ CSS variables, meant to be overriden by final theme
 }
 
 /* When JS disabled/failed to load: we use the open property for arrow animation: */
-.details[open]:not(.isClient) > summary:before,
+.details[open]:not(.isBrowser) > summary:before,
 /* When JS works: we use the data-attribute for arrow animation */
-.details[data-collapsed='false'].isClient > summary:before {
+.details[data-collapsed='false'].isBrowser > summary:before {
   transform: rotate(90deg);
 }
 

packages/docusaurus-theme-common/src/utils/announcementBarUtils.tsx

@@ -14,7 +14,7 @@ import React, {
   useContext,
   createContext,
 } from 'react';
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 import {createStorageSlot} from './storageUtils';
 import {useThemeConfig} from './useThemeConfig';
 
@@ -39,10 +39,10 @@ type AnnouncementBarAPI = {
 
 const useAnnouncementBarContextValue = (): AnnouncementBarAPI => {
   const {announcementBar} = useThemeConfig();
-  const {isClient} = useDocusaurusContext();
+  const isBrowser = useIsBrowser();
 
   const [isClosed, setClosed] = useState(() => {
-    return isClient
+    return isBrowser
       ? // On client navigation: init with localstorage value
         isDismissedInStorage()
       : // On server/hydration: always visible to prevent layout shifts (will be hidden with css if needed)

packages/docusaurus-theme-live-codeblock/src/theme/Playground/index.js

@@ -10,6 +10,7 @@ import {LiveProvider, LiveEditor, LiveError, LivePreview} from 'react-live';
 import clsx from 'clsx';
 import Translate from '@docusaurus/Translate';
 import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 import usePrismTheme from '@theme/hooks/usePrismTheme';
 import styles from './styles.module.css';
 
@@ -51,8 +52,8 @@ function EditorWithHeader() {
 }
 
 export default function Playground({children, transformCode, ...props}) {
+  const isBrowser = useIsBrowser();
   const {
-    isClient,
     siteConfig: {
       themeConfig: {
         liveCodeBlock: {playgroundPosition},
@@ -64,8 +65,8 @@ export default function Playground({children, transformCode, ...props}) {
   return (
     <div className={styles.playgroundContainer}>
       <LiveProvider
-        key={isClient}
-        code={isClient ? children.replace(/\n$/, '') : ''}
+        key={isBrowser}
+        code={isBrowser ? children.replace(/\n$/, '') : ''}
         transformCode={transformCode || ((code) => `${code};`)}
         theme={prismTheme}
         {...props}>

packages/docusaurus-types/src/index.d.ts

@@ -125,7 +125,10 @@ export interface DocusaurusContext {
   globalData: Record<string, unknown>;
   i18n: I18n;
   codeTranslations: Record<string, string>;
-  isClient: boolean;
+
+  // Don't put mutable values here, to avoid triggering re-renders
+  // We could reconsider that choice if context selectors are implemented
+  // isBrowser: boolean; // Not here on purpose!
 }
 
 export interface Preset {

packages/docusaurus/src/client/App.tsx

@@ -5,16 +5,12 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import React, {useEffect, useState} from 'react';
+import React from 'react';
 
 import routes from '@generated/routes';
-import siteConfig from '@generated/docusaurus.config';
-import globalData from '@generated/globalData';
-import i18n from '@generated/i18n';
-import codeTranslations from '@generated/codeTranslations';
-import siteMetadata from '@generated/site-metadata';
 import renderRoutes from './exports/renderRoutes';
-import DocusaurusContext from './exports/context';
+import {BrowserContextProvider} from './exports/browserContext';
+import {DocusaurusContextProvider} from './exports/docusaurusContext';
 import PendingNavigation from './PendingNavigation';
 import BaseUrlIssueBanner from './baseUrlIssueBanner/BaseUrlIssueBanner';
 import Root from '@theme/Root';
@@ -22,29 +18,17 @@ import Root from '@theme/Root';
 import './client-lifecycles-dispatcher';
 
 function App(): JSX.Element {
-  const [isClient, setIsClient] = useState(false);
-
-  useEffect(() => {
-    setIsClient(true);
-  }, []);
-
   return (
-    <DocusaurusContext.Provider
-      value={{
-        siteConfig,
-        siteMetadata,
-        globalData,
-        i18n,
-        codeTranslations,
-        isClient,
-      }}>
-      <Root>
-        <BaseUrlIssueBanner />
-        <PendingNavigation routes={routes}>
-          {renderRoutes(routes)}
-        </PendingNavigation>
-      </Root>
-    </DocusaurusContext.Provider>
+    <DocusaurusContextProvider>
+      <BrowserContextProvider>
+        <Root>
+          <BaseUrlIssueBanner />
+          <PendingNavigation routes={routes}>
+            {renderRoutes(routes)}
+          </PendingNavigation>
+        </Root>
+      </BrowserContextProvider>
+    </DocusaurusContextProvider>
   );
 }
 

packages/docusaurus/src/client/exports/BrowserOnly.tsx

@@ -6,8 +6,10 @@
  */
 
 import React from 'react';
-import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import useIsBrowser from '@docusaurus/useIsBrowser';
 
+// Similar comp to the one described here:
+// https://www.joshwcomeau.com/react/the-perils-of-rehydration/#abstractions
 function BrowserOnly({
   children,
   fallback,
@@ -15,9 +17,9 @@ function BrowserOnly({
   children?: () => JSX.Element;
   fallback?: JSX.Element;
 }): JSX.Element | null {
-  const {isClient} = useDocusaurusContext();
+  const isBrowser = useIsBrowser();
 
-  if (isClient && children != null) {
+  if (isBrowser && children != null) {
     return <>{children()}</>;
   }
 

packages/docusaurus/src/client/exports/browserContext.tsx

@@ -0,0 +1,32 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React, {ReactNode, useEffect, useState} from 'react';
+
+// Encapsulate the logic to avoid React hydration problems
+// See https://www.joshwcomeau.com/react/the-perils-of-rehydration/
+// On first client-side render, we need to render exactly as the server rendered
+// isBrowser is set to true only after a successful hydration
+
+// Note, isBrowser is not part of useDocusaurusContext() for perf reasons
+// Using useDocusaurusContext() (much more common need) should not trigger re-rendering after a successful hydration
+
+export const Context = React.createContext<boolean>(false);
+
+export function BrowserContextProvider({
+  children,
+}: {
+  children: ReactNode;
+}): JSX.Element {
+  const [isBrowser, setIsBrowser] = useState(false);
+
+  useEffect(() => {
+    setIsBrowser(true);
+  }, []);
+
+  return <Context.Provider value={isBrowser}>{children}</Context.Provider>;
+}

packages/docusaurus/src/client/exports/docusaurusContext.tsx

@@ -0,0 +1,35 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React, {ReactNode} from 'react';
+import {DocusaurusContext} from '@docusaurus/types';
+
+import siteConfig from '@generated/docusaurus.config';
+import globalData from '@generated/globalData';
+import i18n from '@generated/i18n';
+import codeTranslations from '@generated/codeTranslations';
+import siteMetadata from '@generated/site-metadata';
+
+// Static value on purpose: don't make it dynamic!
+// Using context is still useful for testability reasons.
+const contextValue: DocusaurusContext = {
+  siteConfig,
+  siteMetadata,
+  globalData,
+  i18n,
+  codeTranslations,
+};
+
+export const Context = React.createContext<DocusaurusContext>(contextValue);
+
+export function DocusaurusContextProvider({
+  children,
+}: {
+  children: ReactNode;
+}): JSX.Element {
+  return <Context.Provider value={contextValue}>{children}</Context.Provider>;
+}

packages/docusaurus/src/client/exports/useDocusaurusContext.ts

@@ -6,16 +6,11 @@
  */
 
 import {useContext} from 'react';
-import context from './context';
+import {Context} from './docusaurusContext';
 import {DocusaurusContext} from '@docusaurus/types';
 
 function useDocusaurusContext(): DocusaurusContext {
-  const docusaurusContext = useContext(context);
-  if (docusaurusContext === null) {
-    // should not happen normally
-    throw new Error('Docusaurus context not provided.');
-  }
-  return docusaurusContext;
+  return useContext(Context);
 }
 
 export default useDocusaurusContext;

packages/docusaurus/src/client/exports/useIsBrowser.ts

@@ -5,7 +5,9 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import React from 'react';
-import {DocusaurusContext} from '@docusaurus/types';
+import {useContext} from 'react';
+import {Context} from './browserContext';
 
-export default React.createContext<DocusaurusContext | null>(null);
+export default function useIsBrowser(): boolean {
+  return useContext(Context);
+}

packages/docusaurus/src/webpack/__tests__/__snapshots__/base.test.ts.snap

@@ -10,14 +10,16 @@ Object {
   "@docusaurus/Link": "../../../../client/exports/Link.tsx",
   "@docusaurus/Noop": "../../../../client/exports/Noop.ts",
   "@docusaurus/Translate": "../../../../client/exports/Translate.tsx",
+  "@docusaurus/browserContext": "../../../../client/exports/browserContext.tsx",
   "@docusaurus/constants": "../../../../client/exports/constants.ts",
-  "@docusaurus/context": "../../../../client/exports/context.ts",
+  "@docusaurus/docusaurusContext": "../../../../client/exports/docusaurusContext.tsx",
   "@docusaurus/isInternalUrl": "../../../../client/exports/isInternalUrl.ts",
   "@docusaurus/renderRoutes": "../../../../client/exports/renderRoutes.ts",
   "@docusaurus/router": "../../../../client/exports/router.ts",
   "@docusaurus/useBaseUrl": "../../../../client/exports/useBaseUrl.ts",
   "@docusaurus/useDocusaurusContext": "../../../../client/exports/useDocusaurusContext.ts",
   "@docusaurus/useGlobalData": "../../../../client/exports/useGlobalData.ts",
+  "@docusaurus/useIsBrowser": "../../../../client/exports/useIsBrowser.ts",
   "@generated": "../../../../../../..",
   "@site": "",
   "@theme-init/PluginThemeComponentOverridden": "pluginThemeFolder/PluginThemeComponentOverridden.js",
@@ -51,13 +53,15 @@ Object {
   "@docusaurus/Link": "../../client/exports/Link.tsx",
   "@docusaurus/Noop": "../../client/exports/Noop.ts",
   "@docusaurus/Translate": "../../client/exports/Translate.tsx",
+  "@docusaurus/browserContext": "../../client/exports/browserContext.tsx",
   "@docusaurus/constants": "../../client/exports/constants.ts",
-  "@docusaurus/context": "../../client/exports/context.ts",
+  "@docusaurus/docusaurusContext": "../../client/exports/docusaurusContext.tsx",
   "@docusaurus/isInternalUrl": "../../client/exports/isInternalUrl.ts",
   "@docusaurus/renderRoutes": "../../client/exports/renderRoutes.ts",
   "@docusaurus/router": "../../client/exports/router.ts",
   "@docusaurus/useBaseUrl": "../../client/exports/useBaseUrl.ts",
   "@docusaurus/useDocusaurusContext": "../../client/exports/useDocusaurusContext.ts",
   "@docusaurus/useGlobalData": "../../client/exports/useGlobalData.ts",
+  "@docusaurus/useIsBrowser": "../../client/exports/useIsBrowser.ts",
 }
 `;

website/docs/docusaurus-core.md

@@ -109,19 +109,56 @@ const Home = () => {
 
 ### `<BrowserOnly/>` {#browseronly}
 
-The `<BrowserOnly>` component accepts a `children` prop, a render function which will not be executed during the pre-rendering phase of the build process. This is useful for hiding code that is only meant to run in the browsers (e.g. where the `window`/`document` objects are being accessed). To improve SEO, you can also provide fallback content using the `fallback` prop, which will be prerendered until in the build process and replaced with the client-side only contents when viewed in the browser.
+The `<BrowserOnly>` component permits to render React components only in the browser, after the React app has hydrated.
 
-```jsx {1,5-10}
+:::tip
+
+Use it for integrating with code that can't run in Node.js, because `window` or `document` objects are being accessed.
+
+:::
+
+#### Props {#browseronly-props}
+
+- `children`: render function prop returning browser-only JSX. Will not be executed in Node.js
+- `fallback` (optional): JSX to render on the server (Node.js) and until React hydration completes.
+
+#### Example with code {#browseronly-example-code}
+
+```jsx
+// highlight-start
 import BrowserOnly from '@docusaurus/BrowserOnly';
+// highlight-end
 
 const MyComponent = () => {
   return (
-    <BrowserOnly
-      fallback={<div>The fallback content to display on prerendering</div>}>
+    // highlight-start
+    <BrowserOnly>
       {() => {
-        // Something that should be excluded during build process prerendering.
+        <span>page url = {window.location.href}</span>;
       }}
     </BrowserOnly>
+    // highlight-end
+  );
+};
+```
+
+#### Example with a library {#browseronly-example-library}
+
+```jsx
+// highlight-start
+import BrowserOnly from '@docusaurus/BrowserOnly';
+// highlight-end
+
+const MyComponent = (props) => {
+  return (
+    // highlight-start
+    <BrowserOnly fallback={<div>Loading...</div>}>
+      {() => {
+        const LibComponent = require('some-lib').LibComponent;
+        return <LibComponent {...props} />;
+      }}
+    </BrowserOnly>
+    // highlight-end
   );
 };
 ```
@@ -132,7 +169,7 @@ A simple interpolation component for text containing dynamic placeholders.
 
 The placeholders will be replaced with the provided dynamic values and JSX elements of your choice (strings, links, styled elements...).
 
-#### Props {#props}
+#### Props {#interpolate-props}
 
 - `children`: text containing interpolation placeholders like `{placeholderName}`
 - `values`: object containing interpolation placeholder values
@@ -175,7 +212,7 @@ Apart the `values` prop used for interpolation, it is **not possible to use vari
 
 :::
 
-#### Props {#props-1}
+#### Props {#translate-props}
 
 - `children`: untranslated string in the default site locale (can contain [interpolation placeholders](#interpolate))
 - `id`: optional value to use as key in JSON translation files
@@ -253,7 +290,6 @@ interface DocusaurusContext {
   globalData: Record<string, unknown>;
   i18n: I18n;
   codeTranslations: Record<string, string>;
-  isClient: boolean;
 }
 ```
 
@@ -275,6 +311,34 @@ const MyComponent = () => {
 };
 ```
 
+### `useIsBrowser` {#useIsBrowser}
+
+Returns `true` when the React app has successfully hydrated in the browser.
+
+:::caution
+
+Use this hook instead of `typeof windows !== 'undefined'` in React rendering logic.
+
+The first client-side render output (in the browser) **must be exactly the same** as the server-side render output (Node.js).
+
+Not following this rule can lead to unexpected hydration behaviors, as described in [The Perils of Rehydration](https://www.joshwcomeau.com/react/the-perils-of-rehydration/).
+
+:::
+
+Usage example:
+
+```jsx
+import React from 'react';
+import useIsBrowser from '@docusaurus/useIsBrowser';
+
+const MyComponent = () => {
+  // highlight-start
+  const isBrowser = useIsBrowser();
+  // highlight-end
+  return <div>{isBrowser ? 'Client' : 'Server'}</div>;
+};
+```
+
 ### `useBaseUrl` {#usebaseurl}
 
 React hook to prepend your site `baseUrl` to a string.
@@ -525,21 +589,27 @@ export default function Home() {
 
 ### `ExecutionEnvironment` {#executionenvironment}
 
-A module which exposes a few boolean variables to check the current rendering environment. Useful if you want to only run certain code on client/server or need to write server-side rendering compatible code.
+A module which exposes a few boolean variables to check the current rendering environment.
 
-```jsx {2,5}
-import React from 'react';
+:::caution
+
+For React rendering logic, use [`useIsBrowser()`](#useIsBrowser) or [`<BrowserOnly>`](#browseronly) instead.
+
+:::
+
+Example:
+
+```jsx
 import ExecutionEnvironment from '@docusaurus/ExecutionEnvironment';
 
-const MyPage = () => {
-  const location = ExecutionEnvironment.canUseDOM ? window.location.href : null;
-  return <div>{location}</div>;
-};
+if (ExecutionEnvironment.canUseDOM) {
+  require('lib-that-only-works-client-side');
+}
 ```
 
 | Field | Description |
 | --- | --- |
-| `ExecutionEnvironment.canUseDOM` | `true` if on client, `false` if prerendering. |
+| `ExecutionEnvironment.canUseDOM` | `true` if on client/browser, `false` on Node.js/prerendering. |
 | `ExecutionEnvironment.canUseEventListeners` | `true` if on client and has `window.addEventListener`. |
 | `ExecutionEnvironment.canUseIntersectionObserver` | `true` if on client and has `IntersectionObserver`. |
 | `ExecutionEnvironment.canUseViewport` | `true` if on client and has `window.screen`. |