app.js

const hamburger = document.querySelector('.header .nav-bar .nav-list .hamburger');
const mobile_menu = document.querySelector('.header .nav-bar .nav-list ul');
const menu_item = document.querySelectorAll('.header .nav-bar .nav-list ul li a');
const header = document.querySelector('.header.container');

hamburger.addEventListener('click', () => {
	hamburger.classList.toggle('active');
	mobile_menu.classList.toggle('active');
});

document.addEventListener('scroll', () => {
	var scroll_position = window.scrollY;
	if (scroll_position > 250) {
		header.style.backgroundColor = '#29323c';
	} else {
		header.style.backgroundColor = 'transparent';
	}
});

menu_item.forEach((item) => {
	item.addEventListener('click', () => {
		hamburger.classList.toggle('active');
		mobile_menu.classList.toggle('active');
	});
});

var mybutton = document.getElementById("back2Top");


window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    mybutton.style.display = "block";
  } else {
    mybutton.style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
}