key prop ​

The key prop is a necessary prop whenever we're rendering the components dynamically (e.g. using .map), as it is a basis for React to know how to match data with the components properly.

key has to be unique ​

Using index is not the best way of adding the key prop, as this way we're linking data to the place (or position) in the array of the components we're mapping. It will work only for appending the components to the array. If, for example, we will prepend components, the data will match wrong components.