Discussion questions on React.useEffect()

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1. When does a React component re-render? 
2. How can you replicate componentWillUnmount using useEffect ?
3. How can you replicate componentDidUpdate using useEffect ?
4. What does the React.useMemo hook do?

1- React renders whenever there is a change in the state or props
2-Return a callback in useEffect's callback argument and it will be called before unmounting.
3-useEffect is a very useful hook. It receives a callback function that executes when the component has mounted and every time it updates. So it works similarly to the old componentDidMount() and componentDidUpdate() methods for class components.
4-useMemo is a React hook that memorizes the output of a function. That is it. useMemo accepts two arguments: a function and a list of dependencies. useMemo will call the function and return its return value.