JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box">
    hello
</div>

CSS

#box {
    width:100px;
    height:100px;
    background:red;
    opacity:1;
}

JavaScript

var box = document.querySelector('#box');

function addCssTransition(elem, speed) {
                elem.style.cssText +=
                    ';transition: all ' + speed + 's ease-in-out; ' +
                    '-moz-transition: all ' + speed + 's ease-in-out;' +
                    '-webkit-transition: all ' + speed + 's ease-in-out';
}

function fadeOut(elem, speed){
        addCssTransition(elem, speed);
        elem.style.opacity = 0;
        elem.style.display = 'none';
}

fadeOut(box, 0.75);