React 19 support

[prichey]

Followup to #871

react-markdown relies on JSX being in the global namespace, which has been removed in @types/react^19.

Per @ChristianMurphy here:

A new issue would be better, we won't support React 19 RC, only GA.

React 19 is now GA, so this issue is to track.

[github-actions]

Hi! This was marked as ready to be worked on! Note that while this is ready to be worked on, nothing is said about priority: it may take a while for this to be solved.

Is this something you can and want to work on?

Team: please use the area/* (to describe the scope of the change), platform/* (if this is related to a specific one), and semver/* and type/* labels to annotate this. If this is first-timers friendly, add good first issue and if this could use help, add help wanted.

[prichey]

@wooorm If I upgrade to React 19 in this repo and npm run build, I get the following error:

> [email protected] build
> tsc --build --clean && tsc --build && type-coverage

node_modules/hast-util-to-jsx-runtime/lib/index.d.ts:12:62 - error TS2503: Cannot find namespace 'JSX'.

12 export function toJsxRuntime(tree: Nodes, options: Options): JSX.Element;

In syntax-tree/hast-util-to-jsx-runtime@ae638ba you provide the solution of declaring a global JSX namespace. Would you recommend doing that here?

[0x80]

I tried the global JSX namespace suggestion

import type { JSX as Jsx } from "react/jsx-runtime";

declare global {
  namespace JSX {
    type ElementClass = Jsx.ElementClass;
    type Element = Jsx.Element;
    type IntrinsicElements = Jsx.IntrinsicElements;
  }
}

... but it doesn't seem to make a difference for the error I'm getting with React 19:

'ReactMarkdown' cannot be used as a JSX component.
Its type '(options: Readonly) => Element' is not a valid JSX element type.
Type '(options: Readonly) => Element' is not assignable to type '(props: any) => ReactNode | Promise'.
Type 'Element' is not assignable to type 'ReactNode | Promise'.ts(2786)

[steamwings]

I tried the global JSX namespace suggestion

import type { JSX as Jsx } from "react/jsx-runtime";

declare global {
  namespace JSX {
    type ElementClass = Jsx.ElementClass;
    type Element = Jsx.Element;
    type IntrinsicElements = Jsx.IntrinsicElements;
  }
}

...

Actually, this suggestion worked for me. Upgraded to Next.js 15 and React 19 without errors.