I'm facing a flickering issue on click of my login and logout button. I'm using the javascript below for my buttons:

window.onload=function(){ 

var div = document.getElementById('login_box'); 
var divss = document.getElementById('logout_box'); 
var addBtn = document.getElementById('login_ID'); 
var removeBtn = document.getElementById('logout_ID'); 
var sessionToken = localStorage.getItem('SessionToken'); 
var userName = localStorage.getItem('UserName'); 
var userID = localStorage.getItem('UserID'); 
var grade = localStorage.getItem('Grade'); 
var userDiv = document.getElementById('User_ID'); 
var register = document.getElementById('register_box'); 

if (sessionToken===null){ 
  divss.classList.add('shadow'); 
  div.classList.remove('shadow'); 
  register.classList.remove('shadow'); 
} else{ 
  div.classList.add('shadow'); 
  divss.classList.remove('shadow'); 
  register.classList.add('shadow'); 
  userDiv.innerHTML = userName; 
} 
removeBtn.addEventListener('click', function () { 
  localStorage.clear(); 
  location.reload(); 
}, false); 

userDiv.addEventListener('click', function () { 
  window.location = "https://wpimei.edotteam.com/profilescreen/"; 
}); }