hydrateRoot lets you display React components inside a browser DOM node whose HTML content was previously generated by react-dom/server.

const root = hydrateRoot(domNode, reactNode, options?)

Reference

hydrateRoot(domNode, options?)

Call hydrateRoot to “attach” React to existing HTML that was already rendered by React in a server environment.

import { hydrateRoot } from 'react-dom/client';

const domNode = document.getElementById('root');
const root = hydrateRoot(domNode, reactNode);

React will attach to the HTML that exists inside the domNode, and take over managing the DOM inside it. An app fully built with React will usually only have one hydrateRoot call with its root component.

See more examples below.

Parameters

  • domNode: A DOM element that was rendered as the root element on the server.

  • reactNode: The “React node” used to render the existing HTML. This will usually be a piece of JSX like <App /> which was rendered with a ReactDOM Server method such as renderToPipeableStream(<App />).

  • optional options: An object with options for this React root.

    • optional onRecoverableError: Callback called when React automatically recovers from errors.
    • optional identifierPrefix: A string prefix React uses for IDs generated by useId. Useful to avoid conflicts when using multiple roots on the same page. Must be the same prefix as used on the server.

Returns

hydrateRoot returns an object with two methods: render and unmount.

Caveats

  • hydrateRoot() expects the rendered content to be identical with the server-rendered content. You should treat mismatches as bugs and fix them.
  • In development mode, React warns about mismatches during hydration. There are no guarantees that attribute differences will be patched up in case of mismatches. This is important for performance reasons because in most apps, mismatches are rare, and so validating all markup would be prohibitively expensive.
  • You’ll likely have only one hydrateRoot call in your app. If you use a framework, it might do this call for you.
  • If your app is client-rendered with no HTML rendered already, using hydrateRoot() is not supported. Use createRoot() instead.

root.render(reactNode)

Call root.render to update a React component inside a hydrated React root for a browser DOM element.

root.render(<App />);

React will update <App /> in the hydrated root.

See more examples below.

Parameters

  • reactNode: A “React node” that you want to update. This will usually be a piece of JSX like <App />, but you can also pass a React element constructed with createElement(), a string, a number, null, or undefined.

Returns

root.render returns undefined.

Caveats

  • If you call root.render before the root has finished hydrating, React will clear the existing server-rendered HTML content and switch the entire root to client rendering.

root.unmount()

Call root.unmount to destroy a rendered tree inside a React root.

root.unmount();

An app fully built with React will usually not have any calls to root.unmount.

This is mostly useful if your React root’s DOM node (or any of its ancestors) may get removed from the DOM by some other code. For example, imagine a jQuery tab panel that removes inactive tabs from the DOM. If a tab gets removed, everything inside it (including the React roots inside) would get removed from the DOM as well. You need to tell React to “stop” managing the removed root’s content by calling root.unmount. Otherwise, the components inside the removed root won’t clean up and free up resources like subscriptions.

Calling root.unmount will unmount all the components in the root and “detach” React from the root DOM node, including removing any event handlers or state in the tree.

Parameters

root.unmount does not accept any parameters.

Returns

render returns null.

Caveats

  • Calling root.unmount will unmount all the components in the tree and “detach” React from the root DOM node.

  • Once you call root.unmount you cannot call root.render again on the root. Attempting to call root.render on an unmounted root will throw a “Cannot update an unmounted root” error.

Usage

Hydrating server-rendered HTML

If your app’s HTML was generated by react-dom/server, you need to hydrate it on the client.

import { hydrateRoot } from 'react-dom/client';

hydrateRoot(document.getElementById('root'), <App />);

This will hydrate the server HTML inside the browser DOM node with the React component for your app. Usually, you will do it once at startup. If you use a framework, it might do this behind the scenes for you.

To hydrate your app, React will “attach” your components’ logic to the initial generated HTML from the server. Hydration turns the initial HTML snapshot from the server into a fully interactive app that runs in the browser.

import './styles.css';
import { hydrateRoot } from 'react-dom/client';
import App from './App.js';

hydrateRoot(
  document.getElementById('root'),
  <App />
);

You shouldn’t need to call hydrateRoot again or to call it in more places. From this point on, React will be managing the DOM of your application. To update the UI, your components will use state instead.

Pitfall

The React tree you pass to hydrateRoot needs to produce the same output as it did on the server.

This is important for the user experience. The user will spend some time looking at the server-generated HTML before your JavaScript code loads. Server rendering creates an illusion that the app loads faster by showing the HTML snapshot of its output. Suddenly showing different content breaks that illusion. This is why the server render output must match the initial render output on the client.

The most common causes leading to hydration errors include:

  • Extra whitespace (like newlines) around the React-generated HTML inside the root node.
  • Using checks like typeof window !== 'undefined' in your rendering logic.
  • Using browser-only APIs like window.matchMedia in your rendering logic.
  • Rendering different data on the server and the client.

React recovers from some hydration errors, but you must fix them like other bugs. In the best case, they’ll lead to a slowdown; in the worst case, event handlers can get attached to the wrong elements.

Hydrating an entire document

Apps fully built with React can render the entire document as JSX, including the <html> tag:

function App() {
  return (
    <html>
      <head>
        <meta charSet="utf-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1" />
        <link rel="stylesheet" href="/styles.css"></link>
        <title>My app</title>
      </head>
      <body>
        <Router />
      </body>
    </html>
  );
}

To hydrate the entire document, pass the document global as the first argument to hydrateRoot:

import { hydrateRoot } from 'react-dom/client';
import App from './App.js';

hydrateRoot(document, <App />);

Suppressing unavoidable hydration mismatch errors

If a single element’s attribute or text content is unavoidably different between the server and the client (for example, a timestamp), you may silence the hydration mismatch warning.

To silence hydration warnings on an element, add suppressHydrationWarning={true}:

export default function App() {
  return (
    <h1 suppressHydrationWarning={true}>
      Current Date: {new Date().toLocaleDateString()}
    </h1>
  );
}

This only works one level deep, and is intended to be an escape hatch. Don’t overuse it. Unless it’s text content, React still won’t attempt to patch it up, so it may remain inconsistent until future updates.

Handling different client and server content

If you intentionally need to render something different on the server and the client, you can do a two-pass rendering. Components that render something different on the client can read a state variable like isClient, which you can set to true in an Effect:

import { useState, useEffect } from "react";

export default function App() {
  const [isClient, setIsClient] = useState(false);

  useEffect(() => {
    setIsClient(true);
  }, []);

  return (
    <h1>
      {isClient ? 'Is Client' : 'Is Server'}
    </h1>
  );
}

This way the initial render pass will render the same content as the server, avoiding mismatches, but an additional pass will happen synchronously right after hydration.

Pitfall

This approach makes hydration slower because your components have to render twice. Be mindful of the user experience on slow connections. The JavaScript code may load significantly later than the initial HTML render, so rendering a different UI immediately after hydration may also feel jarring to the user.

Updating a hydrated root component

After the root has finished hydrating, you can call root.render to update the root React component. Unlike with createRoot, you don’t usually need to do this because the initial content was already rendered as HTML.

If you call root.render at some point after hydration, and the component tree structure matches up with what was previously rendered, React will preserve the state. Notice how you can type in the input, which means that the updates from repeated render calls every second in this example are not destructive:

import { hydrateRoot } from 'react-dom/client';
import './styles.css';
import App from './App.js';

const root = hydrateRoot(
  document.getElementById('root'),
  <App counter={0} />
);

let i = 0;
setInterval(() => {
  root.render(<App counter={i} />);
  i++;
}, 1000);

It is uncommon to call root.render on a hydrated root. Usually, you’ll update state inside one of the components instead.

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