Creating a sticky menu

sticky-menu.js

var menu = document.querySelector('.menu');
var menuPosition = menu.getBoundingClientRect();
var placeholder = document.createElement('div');
placeholder.style.width = menuPosition.width + 'px';
placeholder.style.height = menuPosition.height + 'px';
var isAdded = false;
 
window.addEventListener('scroll', function() {
    if (window.pageYOffset >= menuPosition.top && !isAdded) {
        menu.classList.add('sticky');
        menu.parentNode.insertBefore(placeholder, menu);
        isAdded = true;
    } else if (window.pageYOffset < menuPosition.top && isAdded) {
        menu.classList.remove('sticky');
        menu.parentNode.removeChild(placeholder);
        isAdded = false;
    }
});

/* A sticky class
.sticky {
    top: 0;
    position: fixed;
}
*/