JSFiddle - React, Tailwind, and code Playground

HTML

<div data-attr="sel" data-num="1"></div>
<div data-attr="notSel" data-num="2"></div>
<div data-attr="sel" data-num="3"></div>

JavaScript

var getSelected = document.querySelectorAll('[data-attr~="sel"]');
var selectedNums = [];
for (var i = 0; i < getSelected.length; i++) {
    selectedNums.push(getSelected[i].dataset.num);
}

alert(selectedNums.join(','));