Reducer

 function reducer(state, action) {
   if (action.type === "ADD") {
     return { ...state, count: state.count + action.by }
   } else if (action.type === "MINUS") {
     return { ...state, count: state.count - action.by }
   }
   return state
 }
 
 let initialState = { count: 0 }
 let actions = [
   { type: 'ADD', by: 2 },
   { type: 'MINUS', by: 4 },
   { type: 'ADD', by: 10 }
 ]
 
 console.log(actions.reduce(reducer, initialState))