[React 19] Allow access of default error callbacks in onCaughtError and onUncaughtError

[AbhiPrasad]

Hey React team! We at Sentry have been working on adding support for the new onCaughtError and onUncaughtError root options in React 19 for error reporting.

Here's a PR I'm iterating on atm: getsentry/sentry-javascript#12147

The API we are looking to add looks something like so:

import { reactErrorHandler } from '@sentry/react';
import { hydrateRoot } from "react-dom/client";

ReactDOM.hydrateRoot(
  document.getElementById("root") as HTMLElement,
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  {
    onUncaughtError: reactErrorHandler(),
    onCaughtError: reactErrorHandler((error, errorInfo) => {
      // optional callback if users want more/custom config in addition?
    }),
  }
);

The only issue with Sentry.reactErrorHandler in the current form is that setting the onUncaughtError or onCaughtError options overrides the defaults for onUncaughtError and onCaughtError, which means you lose out on the dev mode logging like so:

react/packages/react-reconciler/src/ReactFiberErrorLogger.js

Lines 36 to 51 in 3ac551e

	if (__DEV__) {
	const componentStack =
	errorInfo.componentStack != null ? errorInfo.componentStack : '';
	const componentNameMessage = componentName
	? `An error occurred in the <${componentName}> component:`
	: 'An error occurred in one of your React components:';
	console['warn'](
	'%s\n%s\n\n%s',
	componentNameMessage,
	componentStack || '',
	'Consider adding an error boundary to your tree to customize error handling behavior.\n' +
	'Visit https://react.dev/link/error-boundaries to learn more about error boundaries.',
	);
	}

In this case there is no way for us to replicate the An error occurred in the <${componentName}> component: message (because componentName is private API), so asking users to add Sentry.reactErrorHandler degrades their local development experience.

This means if someone adds Sentry, or their own error logging via these hooks, they lose out on useful messaging in dev mode. We could recommend to users to make this production-only, but folks also use Sentry error reporting locally via Spotlight, Sentry for development.

Is there a way we can get access to the original onCaughtError or onUncaughtError handler so we can execute that, while sending the error to Sentry at the same time? Or is there are alternate way we can design this API to make that happen?

Thanks!

[rickhanlonii]

Yeah, we're actually probably going to remove the componentName from the default. It's often wrong, and doesn't account for ignored frames. What you really want, is to traverse the component stack, filtering out any ignored frames (which the app has full visibility into, but React does not). So the recommendation is to show the error + the component stack (with the filtering when available), and not have it in the message at all.

After removing it, the behavior will be the same, so there's no need to call the defaults.

[AbhiPrasad]

Music to my ears - gonna proceed with my implementation then. Thank you @rickhanlonii!