smoothScroll explained

smoothScroll.js

function (destination, duration) {
    // destination (relative to window top) in px
    // duration in ms
    /* Variables */
    var startPosition = "pageYOffset" in window ? window.pageYOffset : document.documentElement.scrollTop,
        startTime = +new Date(),
        endTime = startTime + duration,
    // if top of element is outside the document, then define end point to document limit
    // elsewhere, go the the top of the element
        destinationSafe = document.height < (destination + window.innerHeight) ? document.height - window.innerHeight : destination,
    // check if scroll direction is up or down
        direction = destinationSafe - startPosition < 0 ? true : false,
    // animate
        interval = setInterval(function () {
            // get current time every 10ms
            var currentTime = +new Date();
            // if time is still before end (var e)
            if (currentTime <= endTime) {
                // then scroll every 10ms to the logical position
                scrollTo(0, direction ? startPosition - (currentTime - startTime) / (endTime - startTime) * startPosition + destinationSafe : startPosition + (currentTime - startTime) / (endTime - startTime) * (destinationSafe - startPosition))
            } else {
                // if time is over, scroll to the destination (10ms can't be always precise)
                scrollTo(0, destinationSafe);
                // stop setInverval mechanism
                clearInterval(interval)
            }
        }, 10);
}