[React 19] - `ReactElement` created from `React.createElement` are not renderable anymore

[SukkaW]

Summary

TL; DR

Using React.createElement in the JSON.parse's revive function to deserialize json into ReactElement, but the returned value can no longer be rendered with errors:

Error: Objects are not valid as a React child (found: object with keys {key, props, _owner, _store}). If you meant to render a collection of children, use an array instead.

---

I am maintaining a docs site powered by React & Next.js Pages Router.

The docs site copy-pastes a risk approach of rendering MDX on the server serializing MDX components on the server and deserializing on the client from the React docs site:

https://github.com/reactjs/react.dev/blob/39abc60fce1588b4e83cc45d52b522aa63c01bc2/src/utils/compileMDX.ts#L145C1-L160C4
https://github.com/reactjs/react.dev/blob/39abc60fce1588b4e83cc45d52b522aa63c01bc2/src/pages/%5B%5B...markdownPath%5D%5D.js#L104-L128

I have slightly modified the reviveNodeOnClient function to use React.createElement instead:

function reviveNodeOnClient(key: unknown, val: unknown) {
  if (Array.isArray(val) && val[0] === '$r') {
    // Assume it's a React element.
    let type = val[1];
    const key = val[2];
    let props = val[3];
    if (type === 'wrapper') {
      type = Fragment;
      props = { children: props.children };
    }
    if (type in MDXComponents) {
      type = MDXComponents[type];
    }
    if (!type) {
      if (process.env.NODE_ENV !== 'production') {
        // eslint-disable-next-line no-console -- log error
        console.warn(`Unknown type: ${type}`);
      }
      type = Fragment;
    }
-    return {
-      $$typeof: Symbol.for('react.element'),
-      type: type,
-      key: key,
-      ref: null,
-      props: props,
-      _owner: null,
-    };
+    return createElement(type, key ? Object.assign(props, { key }) : props);
  }
  return val;
}

Yet, I get this error:

Error: Objects are not valid as a React child (found: object with keys {key, props, _owner, _store}). If you meant to render a collection of children, use an array instead.

Reproduction

Repo SukkaW/mirrorz-help with branch housekeeping.

https://github.com/SukkaW/mirrorz-help/tree/housekeeping

[eps1lon]

You probably have different versions of React installed. Or a 3rd party library is bundling the JSX runtime of a different React version.

The repro is too big for us to check out. Can you trim it down to something minimal?

[SukkaW]

You probably have different versions of React installed

I don't know. Since it is using Next.js Pages Router, Next.js would most likely not use its bundled version of React.

Or a 3rd party library is bundling the JSX runtime of a different React version.

No, we do not use any CSS-in-JS that requires non-React jsx runtime. It uses style9, a similar AoT CSS-in-JS approach to Facebook's stylex.

The repro is too big for us to check out. Can you trim it down to something minimal?

Lemme give it a shot!

[SukkaW]

Update

I know what happened and it is not an issue of React.

So the serialization assumes the $$typeof would most likely be Symbol.for('react.element'), which is true in 2018 (when Dan implements that serialization process for the https://react.dev).

Years later, the coming React 19 is introducing new shapes of react element (#28813), and therefore objects with Symbol.for('react.transitional.element') are produced. The serialization process doesn't handle Symbol.for('react.transitional.element'), hence the crash.

Also, because the original $$typeof information is lost during the serialization process, the error A React Element from an older version of React was rendered was never reported.