“нагрузка на мероприятие в JS” Ответ

javaScript страница загрузка

//two ways of executing JS code after page is loaded, use "DOMContentLoaded" when able

document.addEventListener("DOMContentLoaded", function(){
    //dom is fully loaded, but maybe waiting on images & css files
});

window.addEventListener("load", function(){
    //everything is fully loaded, don't use me if you can use DOMContentLoaded
});