Кнопка наверх

<div class="to-up">Наверх</div>

.to-up {
    position: fixed;
    bottom: 28px;
    right: 28px;
    display: none;
    z-index: 22;
    cursor: pointer;
}

jQuery

jQuery(($) => {
    $(window).scroll(function(){
        if ($(this).scrollTop() > 780) $('.to-up').fadeIn();
        else $('.to-up').fadeOut();
    });
    $('.to-up').click(function(){
        $("html, body").animate({ scrollTop: 0 }, 566);
        return false;
    });
});

или на JavaScript

document.addEventListener('DOMContentLoaded', () => {

    let toTopBtn = document.querySelector('.to-up');

    window.onscroll = function () {
        if (window.pageYOffset > 580) {
            toTopBtn.style.display = 'block'
        } else {
            toTopBtn.style.display = 'none'
        }
    }

    // плавный скролл наверх 
    toTopBtn.addEventListener('click', function () {
        window.scrollBy({
            top: -document.documentElement.scrollHeight,
            behavior: 'smooth'
        });
    });
});

Ещё

<span>space!</span>
<a href="#" onclick="animateToTop(event)">to the top</a>
<span>more space!</span>
<a href="#" onclick="animateToTop(event)">to the top</a>
<span>even more space!</span>
<a href="#" onclick="animateToTop(event)">to the top</a>

function animateToTop(e) {
    e.preventDefault();
    var scrollToTop = window.setInterval(function() {
        var pos = window.pageYOffset;
        if ( pos > 0 ) {
            window.scrollTo( 0, pos - 20 ); // how far to scroll on each step
        } else {
            window.clearInterval( scrollToTop );
        }
    }, 16); // how fast to scroll (this equals roughly 60 fps)
}

span {
display: block;
margin: 250px 0;
}

Источник Источник

Самое простое:

// this changes the scrolling behavior to "smooth"
window.scrollTo({ top: 0, behavior: 'smooth' });