JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">
    <div id="b" class="my_class"></div>
    <div id="c" class="my_class"></div>
    <div id="d" class="my_class"></div>
    <div id="e" class="my_class"></div>
    <div id="f" class="my_class"></div>
</div>

CSS

.my_class {
    width: 100px;
    height: 20px;
    margin-bottom: 7px;
    background-color: blue;
}

.last {
    background-color: red;
}

JavaScript

$(function() {
    $("#a div:last").addClass("last");
});