JSFiddle - React, Tailwind, and code Playground

HTML

<body onLoad="checkcooki()">
<input type="button" onclick="f()" value="klick">

JavaScript

function setCookie(cname,cvalue,exdays)
{
var d=new Date();
d.setTime(d.getTime()+(exdays*24*60*60*1000));
var expires="expires="+d.toUTCString();
document.cookie=cname +"="+cvalue+"; "+expires;
}
function f()
{
var user=prompt("What is your name?","");
if(user!="" && user!=null){
setCookie("username",user,30);}
}
function getC(cname)
{
var name=cname+"=";
var ca=document.cookie.split(";");
    for(var i=0;i<ca.length;i++){
        var c=ca[i];
        while(c.charAt(0)==" ")c=c.substring(1);
        if(c.indexOf(name)==0) return c.substring(name.length,c.length);
    }
    return "";
}
function checkcooki()
{
var user=getC("username");alert(user);
    if(user!=""){
    alert("Welcome back "+user);
    }
}