javascript - 如何在 Javascript 中区分和合并对象数组中的对象

Question

我有一个对象数组。

[
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
]

我想从对象中删除重复的年份属性，并将不同的月份数据推送到一个数组对象中。

我想得到以下输出:

[
  {
    "_id": {
      "year": 2020,
      "month": [1]
    }
  },
  {
    "_id": {
      "year": 2019,
      "month": [1, 2, 4, 5, 8, 9, 11]
    }
  }
]

我已经尝试过，但无法获得预期的输出:

let arr = [
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
];

let newArr= []; 
    arr.forEach(function (item) { 
        let isMatch = newArr.filter(function (elem) {
            return elem._id.year === item._id.year 
        })
        if (isMatch.length == 0) {
            newArr.push(item)
        }
        else {
            newArr.find(x => x._id.year === item._id.year)._id.month.push(...item._id.month);
        }
    })
    console.log(newArr);

Answer 1

您可以尝试使用Array.reduce :

let input = [
  { _id: { year: 2020, month: 1 } },
  { _id: { year: 2019, month: 1 } },
  { _id: { year: 2019, month: 2 } },
  { _id: { year: 2019, month: 4 } },
  { _id: { year: 2019, month: 5 } },
  { _id: { year: 2019, month: 8 } },
  { _id: { year: 2019, month: 9 } },
  { _id: { year: 2019, month: 11 } }
];

let result = input.reduce((state, current) => {
   let prev = state.find(x => x._id.year === current._id.year);
   if(prev){
      prev._id.months.push(current._id.month);
   }
   else{
      state.push({_id: { year: current._id.year, months: [ current._id.month ] } });
   }
   return state;
},[]);

console.log(result);