As we have already learned, the set data structure does not allow duplicated elements. However, in mathematics, there is a concept called multi-set, which allows the same element to be inserted into the set even if the element has already been inserted before. Multisets (or bags) can be very useful for counting how many times the element is present in a set. It is also largely applied in database systems.
We will not cover the bag data structure in this book. However, you can check the source code and examples by downloading the code bundle of this book or accessing https://github.com/loiane/javascript-datastructures-algorithms.
