从一系列对象中获取本月的第一天

时间：2018-11-28 18:20:27

标签： javascript date luxon

如果今天是2018-11-28，我有一个像这样的对象数组：

  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
  }
]

使用以下代码创建对象：

  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，因此，在此示例中，我想要：

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

这是我尝试过的：

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（我得到了undefined的数组），我希望有一种更聪明的方法可以做到这一点。 我可以使用Javascript Date对象或Luxon库。

谢谢

3 个答案:

答案 0 :(得分：1)

使用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

答案 1 :(得分：1)

您有UTC时间戳，但是我不清楚您想要本地的第一天还是UTC的第一天。如果要使用UTC，则可以将每月的第一天作为ISO 8601日期简化为：

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

请注意，在当地时区的UTC月在月初或月末偏移时，它与本地月会有所不同，具体取决于它是格林威治的东边还是西边。然后，您可以使用 filter 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，如果要查找本月本地第一天的时间戳，则应将时间戳转换为日期，并与本月本地第一天的开始和结束进行比较。

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，但是比较有效，因为<和>将值强制为数字，因此比较的工作方式就好像它们是日期一样。

对于本地时区为+10：00的用户，2018年11月返回的数组为：

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

因为他们的本地月开始时间是2018-10-31T14：00：00Z。

答案 2 :(得分：0)

map()不是正确的工具。即使原始数组为undefined，它也会为您提供每个数组的值。要获取数组的子集，请使用filter()。

因为这些都是不错的ISO 8601日期字符串。您可以构建日期字符串（例如“ 2018-11-01”），并根据数据是否以日期开头进行过滤：

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)

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