Pikaday date formatting without Moment.js

I'm using Pikaday, which requires Moment.js to format dates. This allows easy date formatting:

var picker = new Pikaday({
  format: 'YYYY-MM-DD'
});

However, when I include the Pikaday npm package, Moment.js is over 40kb. Literally all I need it for is to change the date format to YYYY-MM-DD from the practically unusable default Pikaday format.

Can I do this without having to include a 40kb library?

If you want to use only format YYYY-MM-DD, you can build the date string using native Date methods:

const picker = new Pikaday({
  field: document.getElementById('datepicker')
 ,onSelect: date => {
    const year = date.getFullYear()
         ,month = date.getMonth() + 1
         ,day = date.getDate()
         ,formattedDate = [
            year
           ,month < 10 ? '0' + month : month
           ,day < 10 ? '0' + day : day
          ].join('-')
    document.getElementById('datepicker').value = formattedDate
  }
})

Demo

const picker = new Pikaday({
  field: document.getElementById('datepicker')
 ,onSelect: date => {
    const year = date.getFullYear()
         ,month = date.getMonth() + 1
         ,day = date.getDate()
         ,formattedDate = [
            year
           ,month < 10 ? '0' + month : month
           ,day < 10 ? '0' + day : day
          ].join('-')
    document.getElementById('datepicker').value = formattedDate
  }
})

@import url("https://rawgit.com/dbushell/Pikaday/master/css/pikaday.css");

<script src="https://rawgit.com/dbushell/Pikaday/master/pikaday.js"></script>
<input type="text" id="datepicker">