Frontend Developer interview prep for Meta.

To avoid main-thread UI freezing from thousands of DOM updates per second, decouple incoming network streaming from visual rendering. Receive comments over a WebSocket or Server-Sent Events (SSE) connection and buffer them in an in-memory queue. Rather than triggering a React re-render on every incoming comment, flush the buffer periodically (e.g., every 100–200ms or synchronized with requestAnimationFrame). Render comments using a virtualized list capping DOM nodes to only those visible in the viewport. Offload text sanitization or filtering to a Web Worker, and drop low-priority animations if the browser frame budget drops below 60 FPS.

When a user triggers an action (e.g., liking a post), an optimistic updater immediately updates the normalized client store with the predicted new state (e.g., `viewer_does_like: true`, `like_count: count + 1`), updating the UI in 0ms before the network request finishes. Relay assigns a temporary client mutation ID. If the server responds with an error, Relay automatically rolls back the store transaction to its prior snapshot and triggers a visual error toast or retry affordance, ensuring consistent state without manual DOM patching.