title: React hooks queries
body: <p>I have the following queries -</p>
  <ol>
  <li>What's the difference between <code>useEffect</code>, <code>useMemo</code> and <code>useCallback</code> hooks ? I have gone through many examples and explanations but still am not clear on their difference. All I know is that each executes only when at least one of their dependencies change.</li>
  <li>If <code>useRef</code> hook allows us to persist with values between re-renders, why not use a simple variable (not a state) for the same ? I read somewhere that if not changed manually, <code>useRef</code> will have the same value all the time. Can't we achieve this with a simple variable ?</li>
  </ol>