如何按单个属性汇总JavaScript数组并生成统计结果？

参考大量示例后仍无法实现需求，当前结果会保留汇总时应剔除的属性（如period）。希望输出一个按location汇总的新数组，仅包含location、totalCountIn和totalCountOut，不保留时段等细节。推测问题出在Object.assign({}, o)这行代码——它会创建对象的完整副本，但不知道如何仅创建含location的对象来累加统计值。以下是简化示例，实际场景中需要汇总多列并剔除多列：

示例代码：

let helper = {};
let locationPeriodCounts = [
  {"period": "2023-10-21", "location": "228", "countIn": 6, "countOut": 1},
  {"period": "2023-10-22", "location": "228", "countIn": 8, "countOut": 2},
  {"period": "2023-10-23", "location": "228", "countIn": 3, "countOut": 3},
  {"period": "2023-10-24", "location": "228", "countIn": 1, "countOut": 4},
  {"period": "2023-10-21", "location": "229", "countIn": 5, "countOut": 6},
  {"period": "2023-10-22", "location": "229", "countIn": 18, "countOut": 6},
  {"period": "2023-10-23", "location": "229", "countIn": 8, "countOut": 6},
  {"period": "2023-10-24", "location": "230", "countIn": 3, "countOut": 6},
  {"period": "2023-10-25", "location": "230", "countIn": 4, "countOut": 6}
];

let locationCounts = locationPeriodCounts.reduce(function(r, o) {
  let key = o.location;
  if (!helper[key]) {
    helper[key] = Object.assign({}, o); // create a copy of o
    helper[key].totalCountIn = 0;
    helper[key].totalCountOut = 0;
    r.push(helper[key]);
  } else {
    helper[key].totalCountIn += o.countIn;
    helper[key].totalCountOut += o.countOut;
  }
  return r;
}, []);

console.log(locationCounts);

期望输出：

[
  {"location": "228", "totalCountIn": 18, "totalCountOut": 10},
  {"location": "229", "totalCountIn": 31, "totalCountOut": 18},
  {"location": "230", "totalCountIn": 7, "totalCountOut": 12}
]

你猜的没错，问题确实出在Object.assign({}, o)——它会把原对象的所有属性（包括period）都复制过来。要解决这个问题，我们不需要复制整个原对象，而是手动构建只包含目标属性的初始对象，具体修改如下：

let helper = {};
let locationPeriodCounts = [
  {"period": "2023-10-21", "location": "228", "countIn": 6, "countOut": 1},
  {"period": "2023-10-22", "location": "228", "countIn": 8, "countOut": 2},
  {"period": "2023-10-23", "location": "228", "countIn": 3, "countOut": 3},
  {"period": "2023-10-24", "location": "228", "countIn": 1, "countOut": 4},
  {"period": "2023-10-21", "location": "229", "countIn": 5, "countOut": 6},
  {"period": "2023-10-22", "location": "229", "countIn": 18, "countOut": 6},
  {"period": "2023-10-23", "location": "229", "countIn": 8, "countOut": 6},
  {"period": "2023-10-24", "location": "230", "countIn": 3, "countOut": 6},
  {"period": "2023-10-25", "location": "230", "countIn": 4, "countOut": 6}
];

let locationCounts = locationPeriodCounts.reduce(function(r, o) {
  let key = o.location;
  if (!helper[key]) {
    // 只初始化需要的属性，完全避免多余字段
    helper[key] = {
      location: o.location,
      totalCountIn: 0,
      totalCountOut: 0
    };
    r.push(helper[key]);
  }
  // 统一放在判断外部累加，减少重复代码
  helper[key].totalCountIn += o.countIn;
  helper[key].totalCountOut += o.countOut;
  return r;
}, []);

console.log(locationCounts);

关键优化点 #

1. 手动构建初始对象：不再复制原对象的所有属性，只创建包含location和统计字段的对象，从根源上避免带入period等多余属性。
2. 统一累加逻辑：把累加代码移到判断外部，首次创建对象后直接累加当前的数值，无需在else分支重复编写累加逻辑，代码更简洁。

适配多列汇总场景 #

如果实际需要汇总更多字段，只需要在初始对象中添加对应的统计字段，再补充累加逻辑即可：

// 示例：新增countTransfer字段的汇总
helper[key] = {
  location: o.location,
  totalCountIn: 0,
  totalCountOut: 0,
  totalCountTransfer: 0 // 新增统计字段
};
// 新增累加逻辑
helper[key].totalCountTransfer += o.countTransfer;

内容的提问来源于stack exchange，提问作者Chris