JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <span id="span1" myflag>
    <span id="span2" myflag="true">
    <span id="span3">
</div>
        <div id="output"></div>

JavaScript

var count=$('span[myflag]').length;
$('#output').text('Found ' +count + ' attributes. Should be 2.');