One of the biggest mental shifts that developers have to take when embracing redux is shifting from mutable operations to immutable operations. The problem with mutable operations is that they create side effects that involve anything else that is referencing the thing that is being mutated.
With immutable operations within a reducer, a new object is returned for every operation – completely bypassing the possibility that an existing reference is going to be compromised. It is then up to the consumer to decide how they want to get the new data and what to do with it.
In this lesson, we are going to flesh out the CRUD functionality of our bookmarks reducer, doing it first using mutable operations and then refactoring it to use immutable operations.