如何减少 Javascript 中多个变量的对象数组？

Question

我想知道如何根据多个属性减少大量对象。该数组看起来像：

[{count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:6, district:12, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:14, district:19, to_timestamp:"2015-10-01T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"VANDALISM"},
...
{count:4, district:19, to_timestamp:"2016-03-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:7, district:10, to_timestamp:"2016-03-24T00:00:00.000Z", type:"ASSAULT"}]

生成的减少的对象数组需要包含在任何给定日期 (to_timestamp) 的每个 district 给定犯罪 type 的聚合 count；日期是每周一次。类似于：

[{key: "MOTOR VEHICLE THEFT", value: 57, date:"2015/09/23"},
...
  {key: "ASSAULT", value: 77, date:"2016/03/23"}]

我已经在使用 Moment 进行日期转换。

Answer 1

您可以使用一个对象作为所需组（type 和 to_timestamp）的散列 table，并使用 Array#forEach 迭代数组。

var data = [{ count: 4, district: 19, to_timestamp: "2015-09-24T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 6, district: 12, to_timestamp: "2015-09-24T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 14, district: 19, to_timestamp: "2015-10-01T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 4, district: 19, to_timestamp: "2015-09-24T00:00:00.000Z", type: "VANDALISM" }, { count: 4, district: 19, to_timestamp: "2016-03-24T00:00:00.000Z", type: "MOTOR VEHICLE THEFT" }, { count: 7, district: 10, to_timestamp: "2016-03-24T00:00:00.000Z", type: "ASSAULT" }],
    groupBy = ['type', 'to_timestamp'],
    grouped = [];

data.forEach(function (a) {
    var key = groupBy.map(function (k) { return a[k]; }).join('|');
    if (!this[key]) {
        this[key] = { key: a.type, value: 0, date: a.to_timestamp.slice(0, 10).replace(/-/g, '/') };
        grouped.push(this[key]);
    }
    this[key].value += a.count;
}, Object.create(null));

console.log(grouped);

Answer 2

我想你可以用一个简单的 Array.prototype.forEach 循环来做到这一点：

var crimes = [
 {count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
 {count:6, district:12, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
 {count:14, district:19, to_timestamp:"2015-10-01T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
 {count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"VANDALISM"},
 {count:4, district:19, to_timestamp:"2016-03-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
 {count:7, district:10, to_timestamp:"2016-03-24T00:00:00.000Z", type:"ASSAULT"}
];

var dict = {},
 result = [];
crimes.forEach(crime => {
 var date = new Date(crime.to_timestamp);
 date.setDate(date.getDate() - date.getDay());
 var hash = crime.type + date.toDateString()
 if (!dict[hash]) {
  dict[hash] = {
   count: crime.count,
   key: crime.type,
   date: date
  };
  result.push(dict[hash])
 } else
  dict[hash].count += crime.count;
});

console.log(result);

Answer 3

但我认为你减少一个对象比一个数组更容易。得到对象后，可以将其转换为数组

此代码只是将 data 和 type 组合到对象键。没有加district.

const object = [{count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:6, district:12, to_timestamp:"2015-09-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:14, district:19, to_timestamp:"2015-10-01T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:4, district:19, to_timestamp:"2015-09-24T00:00:00.000Z", type:"VANDALISM"},
{count:4, district:19, to_timestamp:"2016-03-24T00:00:00.000Z", type:"MOTOR VEHICLE THEFT"},
{count:7, district:10, to_timestamp:"2016-03-24T00:00:00.000Z", type:"ASSAULT"}]

// reduce a object 
.reduce( ( res, item) => {
  
  const date = moment(item.to_timestamp).format('YYYY/MM/DD')
  const key = item.type + date
  
  if ( !res[key] ) {
    res[key] = {key: item.type, date: date, value: item.count}
  } else {
    res[key]['value'] += item.count
  }
  return res
}, {} )

//get a object, and then the object to array
//console.log(object) 
var result = []
for ( var key in object ) {
  result.push(object[key])
}
console.log(result)

<script src="http://momentjs.com/downloads/moment.min.js"></script>

如何减少 Javascript 中多个变量的对象数组？

How to reduce an array of objects on multiple variables in Javascript?

javascript

arrays

reduce