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) {

        for ( i = 0 ; i<=1 ; i = i + 0.01){
        setTimeout( function(){ seto(el,i); } , i * 1000);}
    }

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