JAVASCRIPT

window is not define Nextjs

if (typeof window !== 'undefined') {
  // You now have access to `window`
}

window is not defined Next Js

if (typeof window !== 'undefined') {
  // Boom
}

Next js window is not defined solution

var currentUrl = '';
  if (typeof window !== 'undefined') {
    currentUrl = window.location.href;
  }
  if (typeof window !== 'undefined') {
    window.onload = function afterWebPageLoad() {

      setLoading(false);
    }
  }
  setTimeout(function () {
    setLoading(false);
  }, 30000);