Question

我需要遍历对象数组，并对 unique _id的总数求和。想象一下这样的数据结构：

  [
      { firstName: "John",
        lastName: "Johnson",
        _id: 23
      },
      { firstName: "John",
        lastName: "Johnson",
        _id: 23
      },
      { firstName: "Mary",
        lastName: "Smith",
        _id: 24
      }
  ]

...对于上述数据集，我的totalUniqueIDs应该是2。

如果我只是遍历一个数组并获得“ _id”的和，我会这样做：

let customersArray = docs.map(doc => doc._id);
let customersArrayLength = customersArray.length
console.log(customersArrayLength); // 3

这当然会给我3个结果。

在这种情况下，我怎样才能得到 unique _id的总和？我是否首先将array转换为set，然后找到length或size？

Answer 1

您可以使用.map()来获取ids的数组，并使用Set对其进行重复数据删除：

const data = [{
    firstName: "John",
    lastName: "Johnson",
    _id: 23
  },
  {
    firstName: "John",
    lastName: "Johnson",
    _id: 23
  },
  {
    firstName: "Mary",
    lastName: "Smith",
    _id: 24
  }
]

const result = [... new Set(data.map(({_id}) => _id))]

console.log(result.length)

Answer 2

另一种选择是使用reduce，以_id作为键将数组汇总为一个对象。使用Object.values将对象转换回数组。

var arr = [{"firstName":"John","lastName":"Johnson","_id":23},{"firstName":"John","lastName":"Johnson","_id":23},{"firstName":"Mary","lastName":"Smith","_id":24}]

var result = Object.values(arr.reduce((c, v) => Object.assign(c, {[v._id]:v}), {}));

console.log(result.length);

另一种选择是使用new Set和size属性

var arr = [{"firstName":"John","lastName":"Johnson","_id":23},{"firstName":"John","lastName":"Johnson","_id":23},{"firstName":"Mary","lastName":"Smith","_id":24}]

var result = new Set(arr.map(o => o._id)).size;

console.log(result);

Answer 3

使用map()从对象数组中获取所有_id，并使用Set查找唯一的_id，最后使用size来获取多少id是唯一的吗？

设置对象可让您存储任何类型的唯一值
map（）方法创建一个新数组，其结果是在调用数组中的每个元素上调用提供的函数

var obj = [{
    "firstName": "John",
    "lastName": "Johnson",
    "_id": 23
  },
  {
    "firstName": "John",
    "lastName": "Johnson",
    "_id": 23
  },
  {
    "firstName": "Mary",
    "lastName": "Smith",
    "_id": 24
  }
];

function countUnique(iterable) {
  return new Set(iterable).size;
}

finalArray = obj.map(function(obj) {
  return obj._id;
});

console.log(countUnique(finalArray));

从数组获取唯一ID的总和

3 个答案: