JSFiddle - React, Tailwind, and code Playground

HTML

<input id="username"/>

JavaScript

$(document).ready(function(){

        $('#username').on('keyup paste',username_check);
});

function username_check(){ 
    setTimeout( function() {
var username = $('#username').val();
    },100);
if(username == "" || username.length < 4){
alert("error");
}
}