I followed the mdn docs for learning nodejs. In the code i get confused what is the callback argument passed in funtion and where it is comming from i have not writtern any callback named function
in the code we have book,and book_instance as key and we have function as value with callback as argument
Please study about ways you can define function.
Basically there are multiple ways to do it.
The 3rd approach is called defining anonymous functions (make function without giving them name but assign to a variable.
Read more: https://javascript.info/function-expressions