Why are they called reducer functions?

Reducer functions get their name because they're similar to the callback function we pass to the Array.reduce() method.
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.

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Why are they called reducer functions?

Reducer functions get their name because they're similar to the callback function we pass to the Array.reduce() method.
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.