This kind of method can also be applied to more common requirements, such as modifying an object. This is a very good idea for, say, Redux users: a reducer can be programmed that will receive the old state as a parameter and produce an updated version with the minimum needed changes, without altering the original state in any way.
Imagine you had an object as follows:
myObj = { a: ..., b: ..., c: ..., d: { e: ..., f: ..., g: { h: ..., i: ... } }};
If you wanted to modify myObj.d.f, and wanted to do it in a persistent way, you would create a new object, which would have several attributes in common with the previous object, but would define new ones for the modified ones. See figure 10.6:
Figure 10.6. A persistent way of ...
