5th of 10 Questions.
Why are they called reducer functions?
They’re called “reducers” because the idea comes directly from the concept of a reduce operation—like the one in JavaScript’s Array.prototype.reduce.
A Redux reducer function is the same idea as this 'reduce callback' function! It takes a 'previous result' (the state), and the 'current item' (the action object), decides a new state value based on those arguments, and returns that new state.