Mastering JavaScript Functional Programming - Second Edition

Cloning and mutating

If mutating an object isn't allowed, then you must create a new object. For example, if you use Redux, a reducer is a function that receives the current state and an action (essentially, an object with new data) and produces the new state. Modifying the current state is totally forbidden and we could avoid this error by always working with frozen objects, as we saw in the previous section. To fulfill the reducer's requirements, we have to be able to clone the original state, as well as mutate it according to the received action. The resulting object will become the new state.

You may want to revisit the More general looping section of Chapter 5, Programming Declaratively – A Better Style, where we wrote a basic objCopy() ...