JSFiddle - React, Tailwind, and code Playground
HTML
Username: <input type="text" id="username">
<span id="val"></span>
<input type="button" value="click" onclick="check();"/>
JavaScript
function check() {
var username = document.getElementById("username").value;
if(username == ""){
document.getElementById("val").innerHTML = "Empty";
} else {
document.getElementById("val").innerHTML = "Full";
}
}