从对象数组中获取本月的第一天

Question

如果今天是 2018-11-28，我有一个这样的对象数组：

  const dataset = [
  {
    "timestamp": "2018-11-28T16:38:07.610Z",
    "value": 751.998557581834
  },
  {
    "timestamp": "2018-11-27T16:38:07.610Z",
    "value": 644.9987628195244
  },
  {
    "timestamp": "2018-11-26T16:38:07.610Z",
    "value": 766.9985288101943
  },
  {
    "timestamp": "2018-11-25T16:38:07.610Z",
    "value": 953.9981701237627
  },
  {
    "timestamp": "2018-11-24T16:38:07.610Z",
    "value": 912.9982487662423
  },
  {
    "timestamp": "2018-11-23T16:38:07.610Z",
    "value": 402
  },
  {
    "timestamp": "2018-11-22T16:38:07.610Z",
    "value": 914.9982449300243
  },
  {
    "timestamp": "2018-11-21T16:38:07.610Z",
    "value": 769.9985230558668
  },
  {
    "timestamp": "2018-11-20T16:38:07.610Z",
    "value": 772.9985173015398
  },
  {
    "timestamp": "2018-11-19T16:38:07.610Z",
    "value": 176
  },
  {
    "timestamp": "2018-11-18T16:38:07.610Z",
    "value": 978.9981221710306
  },
  {
    "timestamp": "2018-11-17T16:38:07.611Z",
    "value": 342
  },
  {
    "timestamp": "2018-11-16T16:38:07.611Z",
    "value": 498.9990428634777
  },
  {
    "timestamp": "2018-11-15T16:38:07.611Z",
    "value": 326
  },
  {
    "timestamp": "2018-11-14T16:38:07.612Z",
    "value": 649.9987532289786
  },
  {
    "timestamp": "2018-11-13T16:38:07.612Z",
    "value": 70
  },
  {
    "timestamp": "2018-11-12T16:38:07.612Z",
    "value": 349
  },
  {
    "timestamp": "2018-11-11T16:38:07.612Z",
    "value": 191
  },
  {
    "timestamp": "2018-11-10T16:38:07.612Z",
    "value": 154
  },
  {
    "timestamp": "2018-11-09T16:38:07.613Z",
    "value": 109
  },
  {
    "timestamp": "2018-11-08T16:38:07.613Z",
    "value": 237
  },
  {
    "timestamp": "2018-11-07T16:38:07.613Z",
    "value": 398
  },
  {
    "timestamp": "2018-11-06T16:38:07.613Z",
    "value": 606.9988357076774
  },
  {
    "timestamp": "2018-11-05T16:38:07.614Z",
    "value": 131
  },
  {
    "timestamp": "2018-11-04T16:38:07.614Z",
    "value": 397
  },
  {
    "timestamp": "2018-11-03T16:38:07.614Z",
    "value": 583.9988798241893
  },
  {
    "timestamp": "2018-11-02T16:38:07.614Z",
    "value": 362
  },
  {
    "timestamp": "2018-11-01T16:38:07.614Z",
    "value": 686.998682258936
  },
  {
    "timestamp": "2018-10-31T16:38:07.614Z",
    "value": 131
  },
  {
    "timestamp": "2018-10-30T16:38:07.614Z",
    "value": 212
  }
]

对象是使用此代码创建的：

  import { DateTime } from 'luxon'

  const timestamp = startDate.minus({ days: i }).toJSDate()
  return { timestamp: timestamp, value: randomValue }

我想要包含本月第一天的对象，所以在这个例子中，我想要：

  {
    "timestamp": "2018-11-01T16:38:07.614Z",
    "value": 686.998682258936
  }

这是我试过的：

const date = new Date()
const firstDayOfThisMonth = new Date(date.getFullYear(), date.getMonth(), 1)
const firstDayOfThisMonthSub = firstDayOfThisMonth.toString().substring(0, 15)
const bo = dataset.map((d, i) => {
  const sub = d.toString().substring(0, 15)
  if (sub === firstDayOfThisMonthSub) return d
})

它不起作用（我得到一个 undefined 的数组），我希望有更聪明的方法来做到这一点。我可以使用 Javascript Date 对象或 Luxon 库。

谢谢

Answer 1

map() 不是正确的工具。它将为原始数组中的每个项目提供一个值，即使它是 undefined。要获取数组的子集，请使用 filter().

因为这些都是很好的 ISO 8601 日期字符串。您可以构建一个像“2018-11-01”这样的日期字符串，并根据 dat 是否以它开头进行过滤：

let a = [  {"timestamp": "2018-11-28T16:38:07.610Z","value": 751.998557581834},{"timestamp": "2018-11-27T16:38:07.610Z","value": 644.9987628195244},{"timestamp": "2018-11-26T16:38:07.610Z","value": 766.9985288101943},{"timestamp": "2018-11-25T16:38:07.610Z","value": 953.9981701237627},{"timestamp": "2018-11-24T16:38:07.610Z","value": 912.9982487662423},{"timestamp": "2018-11-23T16:38:07.610Z","value": 402},{"timestamp": "2018-11-22T16:38:07.610Z","value": 914.9982449300243},{"timestamp": "2018-11-21T16:38:07.610Z","value": 769.9985230558668},{"timestamp": "2018-11-20T16:38:07.610Z","value": 772.9985173015398},{"timestamp": "2018-11-19T16:38:07.610Z","value": 176},{"timestamp": "2018-11-18T16:38:07.610Z","value": 978.9981221710306},{"timestamp": "2018-11-17T16:38:07.611Z","value": 342},{"timestamp": "2018-11-16T16:38:07.611Z","value": 498.9990428634777},{"timestamp": "2018-11-15T16:38:07.611Z","value": 326},{"timestamp": "2018-11-14T16:38:07.612Z","value": 649.9987532289786},{"timestamp": "2018-11-13T16:38:07.612Z","value": 70},{"timestamp": "2018-11-12T16:38:07.612Z","value": 349},{"timestamp": "2018-11-11T16:38:07.612Z","value": 191},{"timestamp": "2018-11-10T16:38:07.612Z","value": 154},{"timestamp": "2018-11-09T16:38:07.613Z","value": 109},{"timestamp": "2018-11-08T16:38:07.613Z","value": 237},{"timestamp": "2018-11-07T16:38:07.613Z","value": 398},{"timestamp": "2018-11-06T16:38:07.613Z","value": 606.9988357076774},{"timestamp": "2018-11-05T16:38:07.614Z","value": 131},{"timestamp": "2018-11-04T16:38:07.614Z","value": 397},{"timestamp": "2018-11-03T16:38:07.614Z","value": 583.9988798241893},{"timestamp": "2018-11-02T16:38:07.614Z","value": 362},{"timestamp": "2018-11-01T16:38:07.614Z","value": 686.998682258936},{"timestamp": "2018-10-31T16:38:07.614Z","value": 131},{"timestamp": "2018-10-30T16:38:07.614Z","value": 212}]

let now =  new Date()
 // start of month is always 01
 // month is zero indexed
let pattern = `${now.getUTCFullYear()}-${now.getUTCMonth()+1}-01`   
let filtered = a.filter(item => item.timestamp.startsWith(pattern))
console.log(filtered)

Answer 2

与 luxon :

const firstDayOfThisMonth = DateTime.local().startOf('month')
const firstDayRecord = dataset.find(record => {
 return DateTime.fromISO(record.timestamp).hasSame(firstDayOfThisMonth, 'day')
})

这应该可以解决问题

Answer 3

您有 UTC 时间戳，但我不清楚您是想要当地的本月第一天还是 UTC 本月的第一天。如果你想要 UTC，获取月份的第一天作为 ISO 8601 日期可以简化为：

let date = new Date().toISOString().slice(0,8) + '01';

请注意，对于主机时区在月初或月底的偏移量，UTC 月份将与本地月份不同，具体取决于它分别位于格林威治东部还是西部。然后，您可以使用 filter 来获取匹配的元素，例如

var data = 
  [{"timestamp": "2018-11-02T16:38:07.614Z","value": 362},
   {"timestamp": "2018-11-01T16:38:07.614Z","value": 686.998},
   {"timestamp": "2018-10-31T16:38:07.614Z","value": 131},
   {"timestamp": "2018-10-30T16:38:07.614Z","value": 212}];
   
var s = new Date().toISOString().slice(0,8) + '01';
var d = data.filter(o => o.timestamp.slice(0,10) == s);
console.log(d);

但是，如果要查找本地当月第一天的时间戳，则应将时间戳转换为日期，并与本地当月第一天的开始和结束进行比较，例如

var data = 
  [{"timestamp": "2018-11-02T16:38:07.614Z","value": 362},
   {"timestamp": "2018-11-01T16:38:07.614Z","value": 686.998},
   {"timestamp": "2018-10-31T16:38:07.614Z","value": 131},
   {"timestamp": "2018-10-30T16:38:07.614Z","value": 212}];
    
var d = new Date();
d.setDate(1);
let firstOfMonthStart = d.setHours(0,0,0,0);
let firstOfMonthEnd   = d.setHours(23,59,59,999);
let t = data.filter(o => {
  let d = new Date(o.timestamp);
  return d >= firstOfMonthStart && d <= firstOfMonthEnd;
});
console.log(t);

请注意，firstOfMonthStart 和 firstOfMonthEnd 将是时间值，而不是日期，但比较有效，因为 < 和 > 将值强制转换为数字，因此比较工作就像它们是日期一样。

对于本地时区为 +10:00 的人，2018 年 11 月返回的数组为：

[{timestamp: "2018-10-31T16:38:07.614Z", value: 131}]

因为他们当地的月初是 2018-10-31T14:00:00Z。

从对象数组中获取本月的第一天

Get the first day of this month from an array of objects

javascript

date

luxon