JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="txt"></textarea>
<div id="block">&nbsp;</div>

CSS

#block {
    border: 1px solid red;
    width: 200px;
    background: #008800;
    display: none;
}

JavaScript

document.getElementById('txt').addEventListener('keyup', function(){
    var b = document.getElementById('block');
    if (this.value==0 || this.value=='') {
        b.style.display='none';
    } else {
        b.style.display='block';
    }
});