Answer the following conceptual questions about JavaScript and React: 1. **JavaScript and multithreading** - How does JavaScript's execution model work in the browser or Node.js? - Is JavaScript itself multithreaded? If not, how does it achieve concurrency? - What mechanisms (if any) exist to use multiple threads (e.g., Web Workers or worker threads), and what are their limitations? 2. **React `useState` vs `useRef`** - What is the purpose of the `useState` hook, and how does it behave with respect to component re-renders? - What is the purpose of the `useRef` hook? How is its behavior different from `useState`? - In what situations would you choose `useState` vs `useRef`? Give a few concrete examples. Explain in enough detail that a junior engineer could understand the key differences and trade-offs.