ES6 scrollTo

scrollTo.js

// Linear scrollTo

// used more than once, so better off in function
const getNowTimestamp = () => (
    'now' in window.performance ? performance.now() : +new Date
);
const scrollTo = (dest, duration = 300, callback) => {
    const body = document.body,
        docElement = document.documentElement;

    const titleOffset = 48,
        startY = window.pageYOffset,
        startTime = getNowTimestamp();
    const docHeight = Math.max(body.scrollHeight, body.offsetHeight, docElement.clientHeight, docElement.scrollHeight, docElement.offsetHeight);
    const winHeight = window.innerHeight || docElement.clientHeight || body.clientHeight;
    const destOffset = (typeof dest === 'number' ? dest : dest.offsetTop) - titleOffset;
    const destOffsetToScroll = Math.round(docHeight - destOffset < winHeight ? docHeight - winHeight : destOffset);
    const scroll = () => {
        const now = getNowTimestamp();
        const time = Math.min(1, ((now - startTime) / duration));
        window.scroll(0, Math.ceil((time * (destOffsetToScroll - startY)) + startY));
        if (window.pageYOffset === destOffsetToScroll) {
            if (callback) {
                callback();
            }
            return;
        }
        requestAnimationFrame(scroll);
    }
    scroll();
};