16.8 react introduces react hooks
    - are javascript functions.
    - allow us to use state and other react features without implementing class.
    - is function let dev hook into React state and lifecycle features from function components.
    - wont work inside class style component.
    - only called at top level, dont call hooks inside loops, conditions, or nested functions
    - only call hooks from React function  components and custom hooks.
    1. useState 
        -   is a hook, called in side functional component to use local state
        -   return a pair value, the current state value and function to update state value
        -   argument of this function is initial state value, value can be any types, 
                only being used at first render
        -   can use multiple state hooks in single component
        -   array destructuring syntax is used to name different state with different name
    2. useEffect 
        -   is a hook, used to perform data fetching, subscriptions, or manually change DOM from 
            React components, these operations are called side effects   
        -   has same purpose as componentDidMount, componentDidUpdate, componentWillUnmount in React classes
        -   will receive a function as argument and the function will be call when component's 
          componentDidMount, componentDidUpdate, is called
        -   the argument function could return another function which could be called when 
           component's componentWillUnmount is called
        -   if the value is passed to the returned function of the argument function do not change, 
           React will skip calling the function.
    3. custom hook
        -   
    4. useContext
       -
    5. useReducer
       -