Select last day of the month for a specific date

by Génesis García Morilla

HTML

<div>
  <label>In <input type="date"></label>
  <label>Out <input type="date"></label>
</div>

CSS

div {
  display: flex;
  justify-content: space-around;
}

JavaScript

const $origin_day = document.querySelector('label:first-of-type > input')
const $last_day = document.querySelector('label:last-of-type > input')

$origin_day.onchange = () => {
  if (!$origin_day.value) return;
  
  const date = new Date($origin_day.value)

  $last_day.value = new Date(date.getFullYear(), date.getMonth() + 1, 1)
    .toISOString().slice(0, 10)
}