对数组中的项目进行分组和计数的简洁方法

Question

我有一个这样的数组：

const arr = [ {name: 'Server 1', country: 'DE'}, {name: 'Server 2', country: 'PL'}, 
              {name: 'Server 3', country: 'US'}, {name: 'Server 4', country: 'DE'}, 
              {name: 'Server 5', country: 'US'}];

我想要的是 group and count 得到如下 输出：

 [
  {
    "country": "DE",
    "count": 2
  },
  {
    "country": "PL",
    "count": 1
  },
  {
    "country": "US",
    "count": 2
  }
]

目前，我正在使用 lodash，但我认为有更好的方法 （例如，使用 _groupBy 或其他方法像那样) 来解决吧?

我的代码在这里：

const arr = [ {name: 'Server 1', country: 'DE'}, {name: 'Server 2', country: 'PL'}, {name: 'Server 3', country: 'US'}, {name: 'Server 4', country: 'DE'}, {name: 'Server 5', country: 'US'}];

const objectGroupby = _.countBy(arr, 'country');
const result = Object.entries(objectGroupby).map(([key, value]) => ({country: key, count: value}));
console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.21/lodash.min.js"></script>

如您所见，_.countBy(arr, 'country')只是returns一个对象而不是数组。

{
  "DE": 2,
  "PL": 1,
  "US": 2
}

那我就得用Object.entries()&map来解决了

Answer 1

就性能而言，我认为我可以使用简单的 for loops as for loops are faster than .map(), .reduce() 等来编写性能更高的代码

只需要遍历一次原数组即可，

const arr = [ {name: 'Server 1', country: 'DE'}, {name: 'Server 2', country: 'PL'}, {name: 'Server 3', country: 'US'}, {name: 'Server 4', country: 'DE'}, {name: 'Server 5', country: 'US'}];

let mapObj = {};
let res = [];
let resIndex = 0;

for(let i = 0; i < arr.length; i++) {
  if(mapObj[arr[i].country] >= 0) {
    res[mapObj[arr[i].country]].count++;
  } else {
    res.push({country: arr[i].country, count: 1});
    mapObj[arr[i].country] = resIndex;
    resIndex++;
  }
}

console.log(res);

就优雅或更易读的代码而言，我认为使用 reduce 更具可读性。但可读性是主观的，因人而异。对我来说 reduce 会更具可读性。

const arr = [ {name: 'Server 1', country: 'DE'}, {name: 'Server 2', country: 'PL'}, {name: 'Server 3', country: 'US'}, {name: 'Server 4', country: 'DE'}, {name: 'Server 5', country: 'US'}];

res = arr.reduce((prev, curr) => {
  const index = prev.findIndex(item => item.country === curr.country);
  if(index > -1) {
    prev[index].count++;
  } else {
    prev.push({ country: curr.country, count: 1});
  }
  return prev;
}, []);

console.log(res);

更新： 使用 Lodash，

const arr = [ {name: 'Server 1', country: 'DE'}, {name: 'Server 2', country: 'PL'}, {name: 'Server 3', country: 'US'}, {name: 'Server 4', country: 'DE'}, {name: 'Server 5', country: 'US'}];

result = _.reduce(_.countBy(arr, 'country'), (result, value, key) => {
  result.push({ country: key, count: value});
  return result;
}, []);

console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.21/lodash.min.js"></script>

Answer 2

万岁！！！

经过这几天的研究，我终于找到了这样的解决方法groupBy。

const arr = [ {name: 'Server 1', country: 'DE'}, {name: 'Server 2', country: 'PL'}, {name: 'Server 3', country: 'US'}, {name: 'Server 4', country: 'DE'}, {name: 'Server 5', country: 'US'}];
const result = _(arr).groupBy(x => x.country)
                     .map((value, key) => ({country: key, count: value.length})); 
console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.21/lodash.min.js"></script>

说明：

第一步：根据country属性
第 2 步：使用具有 2 个参数值的 .map：key 是组的名称（国家/地区），value 是对象数组。

参考资源：

using lodash .groupBy. how to add your own keys for grouped output?

对数组中的项目进行分组和计数的简洁方法

Concise way to Group by and Count item in array

javascript

group-by

count

lodash