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In Redux, an action returns a new state via what's called a reducer. As you're about to learn, Reducers are a big part of Redux and perform most of the work in Redux.
New Terms
Reducer: A Redux construct that is responsible for maintaining a specific portion of the Redux store, which holds the app's state. In JavaScript, a reducer is implemented as a pure function that takes two arguments, the current state and the action being taken, and produces the next state. In order for Redux to work properly, Reducers must not mutate the current state. In other words, the state for a reducer must be treated as immutable.
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