Reducers are pure functions that take the current state and an action, and return a new state. They are the key piece in Redux for updating the application's state.

Example of a reducer:

          {`function counter(state = 0, action) {
            switch (action.type) {
              case "INCREMENT":
                return state + 1;
              case "DECREMENT":
                return state - 1;
              default:
                return state;
            }
          }`}