JSFiddle - React, Tailwind, and code Playground

HTML

<div>
<p class="example" id="firstOne">
More stuff
</p>
<p class="example" id="secondOne">
Stuff
</p>
<span class="example">A span element</span>
</div>

JavaScript

var elementIds = [];
var elements = document.getElementsByClassName("example");
for (var i = 0, len = elements.length; i < len; i++) {
    if (elements[i].id != '') {
        elementIds.push(elements[i].id);
    }
}
alert(elementIds);