scroll-version2.js

// if target is 0 (back to top), 
// the position is: 
// current pos + (current pos * percentage of duration)

// if target > 0 (not back to top), 
// the positon is current pos + (target pos * percentage of duration)
pos = (target === 0)? (firstPos - (firstPos * progress)) : (firstPos + (target * progress));
    
window.scrollTo(0, pos);
    
if(target !== 0 && pos >= (firstPos + target) || target === 0 && pos <= 0) {
    cancelAnimationFrame(start);
    if(element) { element.focus(); }
    pos = 0;
} else {
    window.requestAnimationFrame(showAnimation);
}