1.useRef -- is a way to create ref in function component
a.store element node and focus
b.store a component instance
2.useRef is used to store special value which could cross lifey cycle
3.useImperativeHandle
a.forwardRef() -- is a higher order component used when parent component want to control elements in the child component
b.useImperativeHandle limit the control of parent component , limit what parent component can do upon child component
