React 19 RC Upgrade Guide

April 25, 2024 by Ricky Hanlon

In this post, we will guide you through the steps for upgrading to React 19:

• Installing
• Codemods
• Breaking changes
• New deprecations
• Notable changes
• TypeScript changes
• Changelog

If you’d like to help us test React 19, follow the steps in this upgrade guide and report any issues you encounter. For a list of new features added to React 19, see the React 19 release post.

Installing

To install the latest version of React and React DOM:

npm install react@rc react-dom@rc

If you’re using TypeScript, you also need to update the types. Once React 19 is released as stable, you can install the types as usual from @types/react and @types/react-dom. Until the stable release, the types are available in different packages which need to be enforced in your package.json:

{
  "dependencies": {
    "@types/react": "npm:types-react@rc",
    "@types/react-dom": "npm:types-react-dom@rc"
  },
  "overrides": {
    "@types/react": "npm:types-react@rc",
    "@types/react-dom": "npm:types-react-dom@rc"
  }
}

We’re also including a codemod for the most common replacements. See TypeScript changes below.

Codemods

To help with the upgrade, we’ve worked with the team at codemod.com to publish codemods that will automatically update your code to many of the new APIs and patterns in React 19.

All codemods are available in the react-codemod repo and the Codemod team have joined in helping maintain the codemods. To run these codemods, we recommend using the codemod command instead of the react-codemod because it runs faster, handles more complex code migrations, and provides better support for TypeScript.

npx codemod@latest react/19/migration-recipe

Changes that include a codemod include the command below.

For a list of all available codemods, see the react-codemod repo.

Breaking changes

Errors in render are not re-thrown

In previous versions of React, errors thrown during render were caught and rethrown. In DEV, we would also log to console.error, resulting in duplicate error logs.

In React 19, we’ve improved how errors are handled to reduce duplication by not re-throwing:

• Uncaught Errors: Errors that are not caught by an Error Boundary are reported to window.reportError.
• Caught Errors: Errors that are caught by an Error Boundary are reported to console.error.

This change should not impact most apps, but if your production error reporting relies on errors being re-thrown, you may need to update your error handling. To support this, we’ve added new methods to createRoot and hydrateRoot for custom error handling:

const root = createRoot(container, {
  onUncaughtError: (error, errorInfo) => {
    // ... log error report
  },
  onCaughtError: (error, errorInfo) => {
    // ... log error report
  }
});

For more info, see the docs for createRoot and hydrateRoot.

Removed deprecated React APIs

Removed: propTypes and defaultProps for functions

PropTypes were deprecated in April 2017 (v15.5.0).

In React 19, we’re removing the propType checks from the React package, and using them will be silently ignored. If you’re using propTypes, we recommend migrating to TypeScript or another type-checking solution.

We’re also removing defaultProps from function components in place of ES6 default parameters. Class components will continue to support defaultProps since there is no ES6 alternative.

// Before
import PropTypes from 'prop-types';

function Heading({text}) {
  return <h1>{text}</h1>;
}
Heading.propTypes = {
  text: PropTypes.string,
};
Heading.defaultProps = {
  text: 'Hello, world!',
};

// After
interface Props {
  text?: string;
}
function Heading({text = 'Hello, world!'}: Props) {
  return <h1>{text}</h1>;
}

npx codemod@latest react/prop-types-typescript

Removed: Legacy Context using contextTypes and getChildContext

Legacy Context was deprecated in October 2018 (v16.6.0).

Legacy Context was only available in class components using the APIs contextTypes and getChildContext, and was replaced with contextType due to subtle bugs that were easy to miss. In React 19, we’re removing Legacy Context to make React slightly smaller and faster.

If you’re still using Legacy Context in class components, you’ll need to migrate to the new contextType API:

// Before
import PropTypes from 'prop-types';

class Parent extends React.Component {
  static childContextTypes = {
    foo: PropTypes.string.isRequired,
  };

  getChildContext() {
    return { foo: 'bar' };
  }

  render() {
    return <Child />;
  }
}

class Child extends React.Component {
  static contextTypes = {
    foo: PropTypes.string.isRequired,
  };

  render() {
    return <div>{this.context.foo}</div>;
  }
}

// After
const FooContext = React.createContext();

class Parent extends React.Component {
  render() {
    return (
      <FooContext value='bar'>
        <Child />
      </FooContext>
    );
  }
}

class Child extends React.Component {
  static contextType = FooContext;

  render() {
    return <div>{this.context}</div>;
  }
}

Removed: string refs

String refs were deprecated in March, 2018 (v16.3.0).

Class components supported string refs before being replaced by ref callbacks due to multiple downsides. In React 19, we’re removing string refs to make React simpler and easier to understand.

If you’re still using string refs in class components, you’ll need to migrate to ref callbacks:

// Before
class MyComponent extends React.Component {
  componentDidMount() {
    this.refs.input.focus();
  }

  render() {
    return <input ref='input' />;
  }
}

// After
class MyComponent extends React.Component {
  componentDidMount() {
    this.input.focus();
  }

  render() {
    return <input ref={input => this.input = input} />;
  }
}

npx codemod@latest react/19/replace-string-ref

Removed: Module pattern factories

Module pattern factories were deprecated in August 2019 (v16.9.0).

This pattern was rarely used and supporting it causes React to be slightly larger and slower than necessary. In React 19, we’re removing support for module pattern factories, and you’ll need to migrate to regular functions:

// Before
function FactoryComponent() {
  return { render() { return <div />; } }
}

// After
function FactoryComponent() {
  return <div />;
}

Removed: React.createFactory

createFactory was deprecated in February 2020 (v16.13.0).

Using createFactory was common before broad support for JSX, but it’s rarely used today and can be replaced with JSX. In React 19, we’re removing createFactory and you’ll need to migrate to JSX:

// Before
import { createFactory } from 'react';

const button = createFactory('button');

// After
const button = <button />;

Removed: react-test-renderer/shallow

In React 18, we updated react-test-renderer/shallow to re-export react-shallow-renderer. In React 19, we’re removing react-test-render/shallow to prefer installing the package directly:

npm install react-shallow-renderer --save-dev

- import ShallowRenderer from 'react-test-renderer/shallow';
+ import ShallowRenderer from 'react-shallow-renderer';

Removed deprecated React DOM APIs

Removed: react-dom/test-utils

We’ve moved act from react-dom/test-utils to the react package:

To fix this warning, you can import act from react:

- import {act} from 'react-dom/test-utils'
+ import {act} from 'react';

All other test-utils functions have been removed. These utilities were uncommon, and made it too easy to depend on low level implementation details of your components and React. In React 19, these functions will error when called and their exports will be removed in a future version.

See the warning page for alternatives.

npx codemod@latest react/19/replace-act-import

Removed: ReactDOM.render

ReactDOM.render was deprecated in March 2022 (v18.0.0). In React 19, we’re removing ReactDOM.render and you’ll need to migrate to using ReactDOM.createRoot:

// Before
import {render} from 'react-dom';
render(<App />, document.getElementById('root'));

// After
import {createRoot} from 'react-dom/client';
const root = createRoot(document.getElementById('root'));
root.render(<App />);

npx codemod@latest react/19/replace-reactdom-render

Removed: ReactDOM.hydrate

ReactDOM.hydrate was deprecated in March 2022 (v18.0.0). In React 19, we’re removing ReactDOM.hydrate you’ll need to migrate to using ReactDOM.hydrateRoot,

// Before
import {hydrate} from 'react-dom';
hydrate(<App />, document.getElementById('root'));

// After
import {hydrateRoot} from 'react-dom/client';
hydrateRoot(document.getElementById('root'), <App />);

npx codemod@latest react/19/replace-reactdom-render

Removed: unmountComponentAtNode

ReactDOM.unmountComponentAtNode was deprecated in March 2022 (v18.0.0). In React 19, you’ll need to migrate to using root.unmount().

// Before
unmountComponentAtNode(document.getElementById('root'));

// After
root.unmount();

For more see root.unmount() for createRoot and hydrateRoot.

npx codemod@latest react/19/replace-reactdom-render

Removed: ReactDOM.findDOMNode

ReactDOM.findDOMNode was deprecated in October 2018 (v16.6.0).

We’re removing findDOMNode because it was a legacy escape hatch that was slow to execute, fragile to refactoring, only returned the first child, and broke abstraction levels (see more here). You can replace ReactDOM.findDOMNode with DOM refs:

// Before
import {findDOMNode} from 'react-dom';

function AutoselectingInput() {
  useEffect(() => {
    const input = findDOMNode(this);
    input.select()
  }, []);

  return <input defaultValue="Hello" />;
}

// After
function AutoselectingInput() {
  const ref = useRef(null);
  useEffect(() => {
    ref.current.select();
  }, []);

  return <input ref={ref} defaultValue="Hello" />
}

New deprecations

Deprecated: element.ref

React 19 supports ref as a prop, so we’re deprecating the element.ref in place of element.props.ref.

Accessing element.ref will warn:

Deprecated: react-test-renderer

We are deprecating react-test-renderer because it implements its own renderer environment that doesn’t match the environment users use, promotes testing implementation details, and relies on introspection of React’s internals.

The test renderer was created before there were more viable testing strategies available like React Testing Library, and we now recommend using a modern testing library instead.

In React 19, react-test-renderer logs a deprecation warning, and has switched to concurrent rendering. We recommend migrating your tests to @testing-library/react or @testing-library/react-native for a modern and well supported testing experience.

Notable changes

StrictMode changes

React 19 includes several fixes and improvements to Strict Mode.

When double rendering in Strict Mode in development, useMemo and useCallback will reuse the memoized results from the first render during the second render. Components that are already Strict Mode compatible should not notice a difference in behavior.

As with all Strict Mode behaviors, these features are designed to proactively surface bugs in your components during development so you can fix them before they are shipped to production. For example, during development, Strict Mode will double-invoke ref callback functions on initial mount, to simulate what happens when a mounted component is replaced by a Suspense fallback.

UMD builds removed

UMD was widely used in the past as a convenient way to load React without a build step. Now, there are modern alternatives for loading modules as scripts in HTML documents. Starting with React 19, React will no longer produce UMD builds to reduce the complexity of its testing and release process.

To load React 19 with a script tag, we recommend using an ESM-based CDN such as esm.sh.

<script type="module">
  import React from "https://esm.sh/react@19/?dev"
  import ReactDOMClient from "https://esm.sh/react-dom@19/client?dev"
  ...
</script>

Libraries depending on React internals may block upgrades

This release includes changes to React internals that may impact libraries that ignore our pleas to not use internals like SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED. These changes are necessary to land improvements in React 19, and will not break libraries that follow our guidelines.

Based on our Versioning Policy, these updates are not listed as breaking changes, and we are not including docs for how to upgrade them. The recommendation is to remove any code that depends on internals.

To reflect the impact of using internals, we have renamed the SECRET_INTERNALS suffix to:

_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE

In the future we will more aggressively block accessing internals from React to discourage usage and ensure users are not blocked from upgrading.

TypeScript changes

Removed deprecated TypeScript types

We’ve cleaned up the TypeScript types based on the removed APIs in React 19. Some of the removed have types been moved to more relevant packages, and others are no longer needed to describe React’s behavior.

npx types-react-codemod@latest preset-19 ./path-to-app

npx types-react-codemod@latest react-element-default-any-props ./path-to-your-react-ts-files

Check out types-react-codemod for a list of supported replacements. If you feel a codemod is missing, it can be tracked in the list of missing React 19 codemods.

ref cleanups required

This change is included in the react-19 codemod preset as no-implicit-ref-callback-return .

Due to the introduction of ref cleanup functions, returning anything else from a ref callback will now be rejected by TypeScript. The fix is usually to stop using implicit returns:

- <div ref={current => (instance = current)} />
+ <div ref={current => {instance = current}} />

The original code returned the instance of the HTMLDivElement and TypeScript wouldn’t know if this was supposed to be a cleanup function or not.

useRef requires an argument

This change is included in the react-19 codemod preset as refobject-defaults.

A long-time complaint of how TypeScript and React work has been useRef. We’ve changed the types so that useRef now requires an argument. This significantly simplifies its type signature. It’ll now behave more like createContext.

// @ts-expect-error: Expected 1 argument but saw none
useRef();
// Passes
useRef(undefined);
// @ts-expect-error: Expected 1 argument but saw none
createContext();
// Passes
createContext(undefined);

This now also means that all refs are mutable. You’ll no longer hit the issue where you can’t mutate a ref because you initialised it with null:

const ref = useRef<number>(null);

// Cannot assign to 'current' because it is a read-only property
ref.current = 1;

MutableRef is now deprecated in favor of a single RefObject type which useRef will always return:

interface RefObject<T> {
  current: T
}

declare function useRef<T>: RefObject<T>

useRef still has a convenience overload for useRef<T>(null) that automatically returns RefObject<T | null>. To ease migration due to the required argument for useRef, a convenience overload for useRef(undefined) was added that automatically returns RefObject<T | undefined>.

Check out [RFC] Make all refs mutable for prior discussions about this change.

Changes to the ReactElement TypeScript type

This change is included in the react-element-default-any-props codemod.

The props of React elements now default to unknown instead of any if the element is typed as ReactElement. This does not affect you if you pass a type argument to ReactElement:

type Example2 = ReactElement<{ id: string }>["props"];
//   ^? { id: string }

But if you relied on the default, you now have to handle unknown:

type Example = ReactElement["props"];
//   ^? Before, was 'any', now 'unknown'

You should only need it if you have a lot of legacy code relying on unsound access of element props. Element introspection only exists as an escape hatch, and you should make it explicit that your props access is unsound via an explicit any.

The JSX namespace in TypeScript

This change is included in the react-19 codemod preset as scoped-jsx

A long-time request is to remove the global JSX namespace from our types in favor of React.JSX. This helps prevent pollution of global types which prevents conflicts between different UI libraries that leverage JSX.

You’ll now need to wrap module augmentation of the JSX namespace in `declare module »…»:

// global.d.ts
+ declare module "react" {
    namespace JSX {
      interface IntrinsicElements {
        "my-element": {
          myElementProps: string;
        };
      }
    }
+ }

The exact module specifier depends on the JSX runtime you specified in the compilerOptions of your tsconfig.json:

• For "jsx": "react-jsx" it would be react/jsx-runtime.
• For "jsx": "react-jsxdev" it would be react/jsx-dev-runtime.
• For "jsx": "react" and "jsx": "preserve" it would be react.

Better useReducer typings

useReducer now has improved type inference thanks to @mfp22.

However, this required a breaking change where useReducer doesn’t accept the full reducer type as a type parameter but instead either needs none (and rely on contextual typing) or needs both the state and action type.

The new best practice is not to pass type arguments to useReducer.

- useReducer<React.Reducer<State, Action>>(reducer)
+ useReducer(reducer)

This may not work in edge cases where you can explicitly type the state and action, by passing in the Action in a tuple:

- useReducer<React.Reducer<State, Action>>(reducer)
+ useReducer<State, [Action]>(reducer)

If you define the reducer inline, we encourage to annotate the function parameters instead:

- useReducer<React.Reducer<State, Action>>((state, action) => state)
+ useReducer((state: State, action: Action) => state)

This is also what you’d also have to do if you move the reducer outside of the useReducer call:

const reducer = (state: State, action: Action) => state;

Changelog

Other breaking changes

• react-dom: Error for javascript URLs in src/href #26507
• react-dom: Remove errorInfo.digest from onRecoverableError #28222
• react-dom: Remove unstable_flushControlled #26397
• react-dom: Remove unstable_createEventHandle #28271
• react-dom: Remove unstable_renderSubtreeIntoContainer #28271
• react-dom: Remove unstable_runWithPrioirty #28271
• react-is: Remove deprecated methods from react-is 28224

Other notable changes

• react: Batch sync, default and continuous lanes #25700
• react: Don’t prerender siblings of suspended component #26380
• react: Detect infinite update loops caused by render phase updates #26625
• react-dom: Transitions in popstate are now synchronous #26025
• react-dom: Remove layout effect warning during SSR #26395
• react-dom: Warn and don’t set empty string for src/href (except anchor tags) #28124

We’ll publish the full changelog with the stable release of React 19.

Thanks to Andrew Clark, Eli White, Jack Pope, Jan Kassens, Josh Story, Matt Carroll, Noah Lemen, Sophie Alpert, and Sebastian Silbermann for reviewing and editing this post.