nice and easy "did my React had hydration errors?" script https://x.com/JLarky/status/1908362375490867547

HydrationCounter.astro

---
declare global {
	interface Window {
		had_hydration_error?: string;
	}
}
---

<script>
	const prefix = 'Uncaught Error: ';
	let hadError = false;
	function isHydrationError(message: string) {
		if (message.startsWith(prefix + (import.meta.env.DEV ? 'Hydration failed' : 'Minified React error #418;'))) {
			hadError = true;
			return 'hydration failed';
		}
		if (
			message.startsWith(
				prefix +
					(import.meta.env.DEV
						? 'This Suspense boundary received an update before it finished hydrating'
						: 'Minified React error #421;')
			)
		) {
			return 'hydration failed because of lazy components';
		}
		return;
	}
	window.addEventListener('error', (event) => {
		if (hadError) return;
		const error = isHydrationError(event.message);
		console.log('new error', error, event.message);
		if (error) {
			window.had_hydration_error = error;
		}
	});
</script>