var reducers = {  
      totalInEuros : function(state, item) {
        return state.euros += item.price * 0.897424392;
      },
      totalInYen : function(state, item) {
        return state.yens += item.price * 113.852;
      }
    };

    // reducers 是上面reducers 對象
    var manageReducers = function(reducers) {
      
 
// return 是下面 items.reduce 匿名函數
      return function(state, item) {
          console.log(state, item); // state:{euros: 0} item:{price: 10}
        return Object.keys(reducers).reduce(
          function(nextState, key) {
              // 此處為state 進行計算後再返回
            reducers[key](state, item);
            
 
return state;
          },
          {}
        );
      }
    };

    var bigTotalPriceReducer = manageReducers(reducers);

    var initialState = {euros:0, yens: 0};

    var items = [{price: 10}, {price: 120}, {price: 1000}];

    var totals = items.reduce(bigTotalPriceReducer, initialState);

    console.log(totals);
 

對reducers 理解