React19

React 19 in ONE sites

Islands, not pages

React components are islands. A .tsx only runs in the browser when an .astro mounts it with a directive:

<Counter client:load />     <!-- hydrate immediately -->
<Chart client:visible />    <!-- hydrate when scrolled into view -->

A .tsx with no client:* directive renders to static HTML — no interactivity.

Use @oneie/react hooks for ONE data

import { useSignal, useMark } from '@oneie/react'

function Likes({ pathId }: { pathId: string }) {
  const strength = useSignal(pathId, 'strength')
  const mark = useMark(pathId)
  return <button => mark()}>{strength} ♥</button>
}

Do not useEffect(() => fetch(...)) for ONE data — the hooks own subscription + the closed loop (mark/warn).

React 19 specifics

• Use Actions / use() / transitions for async UI; avoid manual loading-state useState ladders.
• Props named children are reserved slot content — never use children as a data prop on a client:only island (it silently renders nothing).

Don't

• Don't fetch ONE data in useEffect — use @oneie/react hooks.
• Don't ship a .tsx expecting interactivity without a client:* directive.