JSFiddle - React, Tailwind, and code Playground

HTML

<button onclick="fin()">Click me</button>
<div id="div1">A</div>

CSS

#div1{opacity: 0}

JavaScript

function fin(){
var i = 0;
var el = document.getElementById("div1");
fadeIn(el,i);
}

function fadeIn(el,i) {

    i = i + 0.01;
    seto(el,i);
    if (i<1){setTimeout(function(){fadeIn(el,i);}, 10);}
}

function seto(el,i)
{
    el.style.opacity = i;
}