How does the scroll works?

Question

i like yo know how can i do a Button to scroll up

Answer 1

Try this:

var scrollUp = document.getElementsByClassName('back-to-top')[0];

window.onscroll = function(){
if(window.pageYOffset >= 400){
scrollUp.style.display = 'block';
}else{
scrollUp.style.display = 'none';
}
}

scrollUp.onclick = function(){
window.scrollTo(0,0);
}