对象数组：计算 JavaScript 中唯一属性的出现次数 [重复]

Question

给定一个示例 JSON 对象数组，像这样......

[
  {
    "firstName": "Bob",
    "lastName": "Smith",
    "city": "Preston",
    "st": "Florida",
    "age": 15
  },
  {
    "firstName": "Tom",
    "lastName": "Jones",
    "city": "Springfield",
    "st": "Illinois",
    "age": 34
  },
  {
    "firstName": "Mary",
    "lastName": "Hart",
    "city": "Miami",
    "st": "Florida",
    "age": 22
  },
  {
    "firstName": "Jenny",
    "lastName": "Dixon",
    "city": "Palm City",
    "st": "Florida",
    "age": 26
  }
]

根据特定属性的分组获取出现次数的最佳方法是什么？所以，假设我想生成一个 JSON 对象，它具有每个唯一状态（“st”）和出现次数......

[
  {
    "st": "Illinois",
    "count": 1
  },
  {
    "st": "Florida",
    "count": 3
  }
]

我可以使用 for-let 手动完成，循环遍历数组，在循环时跟踪值等。但我确信使用 ES6 有更有效的方法。你能帮帮我吗？谢谢。

Answer 1

您可以使用Array#reduce 进行计数

let counts = json.reduce((b, a) => {
  let index = b.findIndex(j => j.st === a.st);
  if (index > -1) b[index].count++;
  else b.push({st: a.st, count: 1});
  return b;
}, [])

#UPDATE：正如@epascarello 所提到的，有一种更有效的方法来解决这个问题，删除findIndex 循环并使用Object.values

const results = Object.values(json.reduce((obj, item) => {
   obj[item.st] = obj[item.st] || { st: item.st, count: 0 };
   obj[item.st].count++;       
   return obj;}, {}))

let json = [{
    "firstName": "Bob",
    "lastName": "Smith",
    "city": "Preston",
    "st": "Florida",
    "age": 15
  },
  {
    "firstName": "Tom",
    "lastName": "Jones",
    "city": "Springfield",
    "st": "Illinois",
    "age": 34
  },
  {
    "firstName": "Mary",
    "lastName": "Hart",
    "city": "Miami",
    "st": "Florida",
    "age": 22
  },
  {
    "firstName": "Jenny",
    "lastName": "Dixon",
    "city": "Palm City",
    "st": "Florida",
    "age": 26
  }
]

let counts = json.reduce((b, a) => {
  let index = b.findIndex(j => j.st === a.st);
  if (index > -1) b[index].count++;
  else b.push({st: a.st, count: 1});
  return b;
}, [])

console.log(counts)

const results = Object.values(json.reduce((obj, item) => {
obj[item.st] = obj[item.st] || { st: item.st, count: 0 };
obj[item.st].count++;       
return obj;}, {}))

console.log(results)