JSFiddle - React, Tailwind, and code Playground

HTML

<div class='leftbar'>THIS IS LEFT BAR</div>

CSS

.myclass {
    background:#999;
}
.leftbar {
    width: 200px;
    height: 500px;    
    border: 1px solid black;
}

JavaScript

$(document).ready(function () {
    $(".leftbar").hover(function () {
        $(this).delay(300).queue(function (next) {
            $(this).addClass("myclass");
            next();
        });
    }, function () {
        $(this).delay(300).queue(function (next) {
            $(this).removeClass("myclass");
            next();
        });
    });
});