title: Get the first day of this month from an array of objects
body: <p> If today is 2018-11-28, I have an array of object like that:</p>
  
  <pre><code>  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
    }
  ]
  </code></pre>
  
  <p>The objects are created using this code:</p>
  
  <pre><code>  import { DateTime } from 'luxon'
  
    const timestamp = startDate.minus({ days: i }).toJSDate()
    return { timestamp: timestamp, value: randomValue }
  </code></pre>
  
  <p>I want the object containing the first day of this month so, in this example, I want:</p>
  
  <pre><code>  {
      "timestamp": "2018-11-01T16:38:07.614Z",
      "value": 686.998682258936
    }
  </code></pre>
  
  <p>This is what I tried:</p>
  
  <pre><code>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) =&gt; {
    const sub = d.toString().substring(0, 15)
    if (sub === firstDayOfThisMonthSub) return d
  })
  </code></pre>
  
  <p>It doesn't work (I get an array of <code>undefined</code>) and I hope there is a smarter way to do that.
  I can use Javascript <code>Date</code> object or <a href="https://moment.github.io/luxon/index.html" rel="nofollow noreferrer">Luxon</a> library.</p>
  
  <p>Thanks</p>