Create ServerOnly component (#207)

Closes #206

README.md

@@ -156,6 +156,37 @@ The rendering flow will be:
 
 This component uses the `useHydrated` hook internally.
 
+### ServerOnly
+
+The ServerOnly component is the opposite of the ClientOnly component, it lets you render the children element only on the server-side, avoiding rendering it the client-side.
+
+You can provide a fallback component to be used on CSR, and while optional, it's highly recommended to provide one to avoid content layout shift issues, unless you only render visually hidden elements.
+
+```tsx
+import { ServerOnly } from "remix-utils";
+
+export default function Component() {
+  return (
+    <ServerOnly fallback={<ComplexComponentNeedingBrowserEnvironment />}>
+      {() => <SimplerStaticVersion />}
+    </ServerOnly>
+  );
+}
+```
+
+This component is handy to render some content only on the server-side, like a hidden input you can later use to know if JS has loaded.
+
+Consider it like the `<noscript>` HTML tag but it can work even if JS failed to load but it's enabled on the browser.
+
+The rendering flow will be:
+
+- SSR: Always render the children.
+- CSR First Render: Always render the children.
+- CSR Update: Update to render the fallback component (if defined).
+- CSR Future Renders: Always render the fallback component, don't bother to render the children.
+
+This component uses the `useHydrated` hook internally.
+
 ### CORS
 
 The CORS function let you implement CORS headers on your loaders and actions so you can use them as an API for other client-side applications.

src/react/server-only.tsx

@@ -0,0 +1,29 @@
+import { ReactNode } from "react";
+import { useHydrated } from "./use-hydrated";
+
+type Props = {
+  /**
+   * You are encouraged to add a fallback that is the same dimensions
+   * as the server rendered children. This will avoid content layout
+   * shift which is disgusting
+   */
+  children(): ReactNode;
+  fallback?: ReactNode;
+};
+
+/**
+ * Render the children only before the JS has loaded client-side. Use an
+ * optional fallback component for once the JS has loaded.
+ *
+ * Example: Render a hidden input to identify if the user has JS.
+ * ```tsx
+ * return (
+ *   <ServerOnly fallback={<FakeChart />}>
+ *     {() => <Chart />}
+ *   </ServerOnly>
+ * );
+ * ```
+ */
+export function ServerOnly({ children, fallback = null }: Props) {
+  return useHydrated() ? <>{fallback}</> : <>{children()}</>;
+}