createRoot

const root = createRoot(domNode, options?)

• Viite
  • createRoot(domNode, options?)
  • root.render(reactNode)
  • root.unmount()
• Käyttö
  • Rendering an app fully built with React
  • Rendering a page partially built with React
  • Updating a root component
• Vianmääritys
  • I’ve created a root, but nothing is displayed
  • I’m getting an error: “Target container is not a DOM element”
  • I’m getting an error: “Functions are not valid as a React child.”
  • My server-rendered HTML gets re-created from scratch

Viite

createRoot(domNode, options?)

Call createRoot to create a React root for displaying content inside a browser DOM element.

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

const domNode = document.getElementById('root');
const root = createRoot(domNode);

React will create a root for the domNode, and take over managing the DOM inside it. After you’ve created a root, you need to call root.render to display a React component inside of it:

root.render(<App />);

An app fully built with React will usually only have one createRoot call for its root component. A page that uses “sprinkles” of React for parts of the page may have as many separate roots as needed.

See more examples below.

Parametrit

• domNode: A DOM element. React will create a root for this DOM element and allow you to call functions on the root, such as render to display rendered React content.

• 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.

Palautukset

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

Rajoitukset

• If your app is server-rendered, using createRoot() is not supported. Use hydrateRoot() instead.
• You’ll likely have only one createRoot call in your app. If you use a framework, it might do this call for you.
• When you want to render a piece of JSX in a different part of the DOM tree that isn’t a child of your component (for example, a modal or a tooltip), use createPortal instead of createRoot.

root.render(reactNode)

Call root.render to display a piece of JSX (“React node”) into the React root’s browser DOM node.

root.render(<App />);

React will display <App /> in the root, and take over managing the DOM inside it.

See more examples below.

Parametrit

• reactNode: A React node that you want to display. 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.

Palautukset

root.render returns undefined.

Rajoitukset

• The first time you call root.render, React will clear all the existing HTML content inside the React root before rendering the React component into it.

• If your root’s DOM node contains HTML generated by React on the server or during the build, use hydrateRoot() instead, which attaches the event handlers to the existing HTML.

• If you call render on the same root more than once, React will update the DOM as necessary to reflect the latest JSX you passed. React will decide which parts of the DOM can be reused and which need to be recreated by “matching it up” with the previously rendered tree. Calling render on the same root again is similar to calling the set function on the root component: React avoids unnecessary DOM updates.

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. In that case, 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 know to clean up and free up global 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.

Parametrit

root.unmount does not accept any parameters.

Palautukset

root.unmount returns undefined.

Rajoitukset

• 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 same root. Attempting to call root.render on an unmounted root will throw a “Cannot update an unmounted root” error. However, you can create a new root for the same DOM node after the previous root for that node has been unmounted.

Käyttö

Rendering an app fully built with React

If your app is fully built with React, create a single root for your entire app.

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

const root = createRoot(document.getElementById('root'));
root.render(<App />);

Usually, you only need to run this code once at startup. It will:

1. Find the browser DOM node defined in your HTML.
2. Display the React component for your app inside.

If your app is fully built with React, you shouldn’t need to create any more roots, or to call root.render again.

From this point on, React will manage the DOM of your entire app. To add more components, nest them inside the App component. When you need to update the UI, each of your components can do this by using state. When you need to display extra content like a modal or a tooltip outside the DOM node, render it with a portal.

<div id="root"></div>

Rendering a page partially built with React

If your page isn’t fully built with React, you can call createRoot multiple times to create a root for each top-level piece of UI managed by React. You can display different content in each root by calling root.render.

Here, two different React components are rendered into two DOM nodes defined in the index.html file:

You could also create a new DOM node with document.createElement() and add it to the document manually.

const domNode = document.createElement('div');
const root = createRoot(domNode); 
root.render(<Comment />);
document.body.appendChild(domNode); // You can add it anywhere in the document

To remove the React tree from the DOM node and clean up all the resources used by it, call root.unmount.

root.unmount();

This is mostly useful if your React components are inside an app written in a different framework.

Updating a root component

You can call render more than once on the same root. As long as 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:

It is uncommon to call render multiple times. Usually, your components will update state instead.

Vianmääritys

I’ve created a root, but nothing is displayed

Make sure you haven’t forgotten to actually render your app into the root:

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

const root = createRoot(document.getElementById('root'));
root.render(<App />);

Until you do that, nothing is displayed.

I’m getting an error: “Target container is not a DOM element”

This error means that whatever you’re passing to createRoot is not a DOM node.

If you’re not sure what’s happening, try logging it:

const domNode = document.getElementById('root');
console.log(domNode); // ???
const root = createRoot(domNode);
root.render(<App />);

For example, if domNode is null, it means that getElementById returned null. This will happen if there is no node in the document with the given ID at the time of your call. There may be a few reasons for it:

1. The ID you’re looking for might differ from the ID you used in the HTML file. Check for typos!
2. Your bundle’s <script> tag cannot “see” any DOM nodes that appear after it in the HTML.

Another common way to get this error is to write createRoot(<App />) instead of createRoot(domNode).

I’m getting an error: “Functions are not valid as a React child.”

This error means that whatever you’re passing to root.render is not a React component.

This may happen if you call root.render with Component instead of <Component />:

// 🚩 Wrong: App is a function, not a Component.
root.render(App);

// ✅ Correct: <App /> is a component.
root.render(<App />);

Or if you pass a function to root.render, instead of the result of calling it:

// 🚩 Wrong: createApp is a function, not a component.
root.render(createApp);

// ✅ Correct: call createApp to return a component.
root.render(createApp());

My server-rendered HTML gets re-created from scratch

If your app is server-rendered and includes the initial HTML generated by React, you might notice that creating a root and calling root.render deletes all that HTML, and then re-creates all the DOM nodes from scratch. This can be slower, resets focus and scroll positions, and may lose other user input.

Server-rendered apps must use hydrateRoot instead of createRoot:

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

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

Note that its API is different. In particular, usually there will be no further root.render call.