window.onload = () =>{
    this.sessionStorage.setItem("username", "admin");
    this.sessionStorage.setItem("password", "qwerty");
}
var input = document.getElementsByTagName('input');
var login = document.getElementById('log-in');
var form = document.querySelector('form');
form.onsubmit = ()=>{return false}

login.onclick = ()=>{
    if ((input[0].value != "") && (input[1].value != "")) 
    {
        if ((input[0].value == sessionStorage.getItem("username")) && (input[1].value == sessionStorage.getItem("password"))) 
        {
            form.onsubmit = ()=>{return 1}
        }
        else
        {
            if ((input[0].value != sessionStorage.getItem("username"))) 
            {
                input[0].nextElementSibling.textContent = "Имя не правильно";
                 setTimeout(()=>{
                     input[0].nextElementSibling.textContent = "";
                 }, 2000);
            }
            if ((input[1].value != sessionStorage.getItem("password"))) 
            {
                input[1].nextElementSibling.textContent = "Пароль не правильно";
                 setTimeout(()=>{
                     input[1].nextElementSibling.textContent = "";
                 }, 2000);
            }
        }
    }
    else
    {
        if (input[0].value == "")
        {
            input[0].nextElementSibling.textContent = "Username is empty";
            setTimeout(()=>{
                input[0].nextElementSibling.textContent = "";
            }, 2000);
        }
        if (input[1].value == "")
        {
            input[1].nextElementSibling.textContent = "Password is empty";
            setTimeout(()=>{
                input[1].nextElementSibling.textContent = "";
            }, 2000);
        }
    }
}