What runs on the server

The server render builds the tree once, then disposes it. Nothing is reactive there.

• Effects do not run: useEffect, useRenderEffect and onMount are no-ops. Do browser work in onMount (it is skipped on the server by design) instead of guarding with typeof window.
• Memos and component bodies run, once. A signal read gives its current value; writes to signals do not re-render anything.
• isServer from @shadow-js/core is true, isDev follows the build.
• Resources start their fetch. renderToString does not wait for them; their Suspense boundaries render the fallback. renderToStringAsync and renderToStream wait (below).
• Event handlers are dropped. ref, use: and on: directives are dropped.
• The root is disposed before renderToString returns, so onCleanup callbacks run.
• innerHTML is written as raw markup; everything else is escaped. Attribute names are validated and URL attributes are sanitized exactly as on the client (Security).
• Writes to signals created outside the render (module-level state) are allowed and are shared by every request in the process. Do not keep per-request data in module-level signals.

import { isServer, onCleanup, onMount, useEffect, useMemo, useStore } from "@shadow-js/core";
import { renderToString } from "@shadow-js/web/server";

const record = { effects: 0, setup: 0, cleanups: 0 };

function Probe() {
  const [n] = useStore(2);
  const doubled = useMemo(() => n() * 2);
  record.setup += 1;
  useEffect(() => {
    record.effects += 1;
  });
  onMount(() => {
    record.effects += 1;
  });
  onCleanup(() => {
    record.cleanups += 1;
  });
  return (
    <strong>
      {doubled()} {isServer ? "server" : "browser"}
      {isServer ? null : <i>client only</i>}
    </strong>
  );
}

console.log(renderToString(() => <Probe />));
console.log(`effects: ${record.effects} setup: ${record.setup} cleanups: ${record.cleanups}`);

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