My own personal preference for building reducers for Redux.
var makeReducer = require('create-redux-reducer-from-map')
const arithmeticReducer = makeReducer({
ADD: (state, action) => state + action.payload,
SUBTRACT: (state, action) => state - action.payload
}, 0)
makeReducer(mapOfFunctions[, initialState])
Returns a reducer appropriate for Redux. Takes a map of action types to functions taking (state, action).