Immutability and Side Effects
In a pure function context, the mutation of variables is considered a side effect and therefore a function where the mutation occurs, especially of variables that live beyond the execution of the function, is not pure.
It is possible to write entire sections of applications without using side effects. Any data manipulation is possible without side effects. Most applications, however, need to load the data so that it is displayed from somewhere and possibly save some of the data somewhere as well. These are side effects that need to be managed.
A Look at Redux Action Creators
Action creators create Redux actions. They're useful to abstract the constants and centralize what actions the Redux store supports.