HTML -коллекция элементов для массива
const boxes = Array.from(document.getElementsByClassName('box'));
Lucky Locust
const boxes = Array.from(document.getElementsByClassName('box'));const boxArray = [...document.getElementsByClassName('box')];var boxes = Array.prototype.slice.call(document.getElementsByClassName('box'));
var boxes = [].slice.call(document.getElementsByClassName('box'));<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<script>
const boxes = document.getElementsByClassName('box');
</script>