scrollOCD.js

function scrollOCD (settings) {
        "use strict";
        settings = (typeof settings !== "undefined" ) ? settings : {};
        var defaults = {
            'classname':  'ocd',
            'diff':       200,
            'speed':      1,
            'timeout':    1500,
            'minWidth':   1280,
            'exclude':    ''
        };
        for(var i in defaults) {
            if(typeof settings[i] === "undefined") {
                settings[i] = defaults[i];
            }
        }
        if(String(settings.diff).indexOf('%') > -1){
            settings.diff = window.innerHeight*parseInt(settings.diff,10)/100;
        }

        function smoothScroll(elemTop,speedInt) {
            clearInterval(smoothInt);
            var start = window.pageYOffset,
                    stop = elemTop,
                    distance = stop - start,
                    step = Math.round(distance / 20 * speedInt);
            smoothInt = setInterval(function(){
                scrollWindow(step,stop);
            },10);
        }

        function scrollWindow(incr,dest) {
            var yPos = window.pageYOffset;
            window.scrollTo(0,yPos + incr);
            if((incr > 0 && yPos + incr >= dest) || (incr < 0 && yPos + incr <= dest)){
                window.scrollTo(0,dest);
                clearInterval(smoothInt);
            }
        }

        function excludeElem(elem){
            if(typeof elem[0] === "undefined"){
                return false;
            }
            else{
                elem = elem.split(',');
                var exclude = null,
                        isVisible = false;
                for (var i = elem.length - 1; i >= 0; i -= 1) {
                    if(elem[i].substring(0,1) === '.'){
                        exclude = document.getElementsByClassName(elem[i].substring(1))[0];
                    }
                    else if(elem[i].substring(0,1) === '#'){
                        exclude = document.getElementById(elem[i].substring(1));
                    }
                    else{
                        exclude = document.getElementsByTagName(elem[i])[0];
                    }
                    var excludeTop = exclude.offsetTop,
                            excludeBottom = excludeTop + exclude.offsetHeight,
                            visibleTop = window.pageYOffset,
                            visibleBottom = visibleTop + window.innerHeight,
                            offsetTop = (excludeTop + exclude.offsetHeight) - visibleTop,
                            offsetBottom = visibleBottom - excludeTop,
                            magicDiff = settings.diff + 101;
                    isVisible = (excludeTop >= visibleTop && excludeBottom <= visibleBottom) ? true : false;
                    if(!isVisible){
                        isVisible = (offsetBottom <= magicDiff && offsetBottom > 0) ? true : false;
                    }
                    
                    if(!isVisible){
                        isVisible = (offsetTop <= magicDiff && offsetTop > 0) ? true : false;
                    }
                    if(isVisible){
                        return true;
                    }
                }
                if(!isVisible){
                    return false;
                }
            }
        }

        function execute(settings){
            window.clearTimeout(scrollInt);
            scrollInt = window.setTimeout(function(){
                if(!excludeElem(settings.exclude)){
                    var ocd = document.getElementsByClassName(settings.classname),
                            scrollPos = window.pageYOffset;
                    for (var i = ocd.length - 1; i >= 0; i -= 1) {
                        var elemTop = ocd[i].offsetTop;
                        if(Math.abs(scrollPos - elemTop) <= settings.diff){
                            smoothScroll(elemTop,settings.speed);
                        }
                    }
                }
            },settings.timeout);
        }

        var scrollEvent = 'mousewheel',
                isFireFox = (navigator.userAgent.indexOf('Firefox') !== -1);
        if(isFireFox){
            scrollEvent = 'DOMMouseScroll';
        }
        if(!!('ontouchstart' in window)){
            scrollEvent = 'touchmove';
        }

        var scrollInt = null,
                smoothInt = null;
        if(!settings.minWidth || window.innerWidth >= settings.minWidth){
            document.addEventListener(scrollEvent,function(){
                execute(settings);
            },false);

            document.addEventListener('keydown',function(e){
                if(e.keyCode === 38 || e.keyCode === 40){
                    execute(settings);
                }
            },false);
        }
}