JSFiddle - React, Tailwind, and code Playground

HTML

<script>

function fadeIn(elem, speed) {

    if(!elem.style.opacity)
    {
        elem.style.opacity = 0;
    }
   setInterval(function(){
      elem.style.opacity = parseFloat(elem.style.opacity)  + 0.03;
    }, speed /100);
}

</script>
<div id="hey">

</div>

CSS

#hey {
  background-color: red;
  width: 100px;
  height: 100px;
}

JavaScript

ele = document.getElementById('hey');
fadeIn(ele, 4000);