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.agents/skills/vercel-react-best-practices/rules/bundle-conditional.md

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+---
+title: Conditional Module Loading
+impact: HIGH
+impactDescription: loads large data only when needed
+tags: bundle, conditional-loading, lazy-loading
+---
+
+## Conditional Module Loading
+
+Load large data or modules only when a feature is activated.
+
+**Example (lazy-load animation frames):**
+
+```tsx
+function AnimationPlayer({
+  enabled,
+  setEnabled,
+}: {
+  enabled: boolean;
+  setEnabled: React.Dispatch<React.SetStateAction<boolean>>;
+}) {
+  const [frames, setFrames] = useState<Frame[] | null>(null);
+
+  useEffect(() => {
+    if (enabled && !frames && typeof window !== "undefined") {
+      import("./animation-frames.js")
+        .then((mod) => setFrames(mod.frames))
+        .catch(() => setEnabled(false));
+    }
+  }, [enabled, frames, setEnabled]);
+
+  if (!frames) return <Skeleton />;
+  return <Canvas frames={frames} />;
+}
+```
+
+The `typeof window !== 'undefined'` check prevents bundling this module for SSR, optimizing server bundle size and build speed.