JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a" id="div0">
    <div>
        <div class="a" id="div1"></div>
    </div>
    <div class="b" id="div2"></div>
    <div id="element"></div>
    <div class="a" id="div3"></div>
    <div class="b" id="div4"></div>
</div>

JavaScript

var $collection = $(".a, #element");

var eleIndex = $collection.index($("#element"));

var prevEl = $collection.eq(eleIndex - 1);

alert(prevEl.attr("id"));