scrollin-up.js adds .su--scrollinUp to body when (and only) scrolling up http://soqr.fr/scrollin-up Pascal Cauhépé @eQRoeil

scrollin-up.js

/*! scrollin-up.js 
* adds .su--scrollinUp to body 
* when (and only) scrolling up
* http://soqr.fr/scrollin-up
* @author Pascal Cauhépé @eQRoeil
*/

//do not execute if addEventListener is not supported
if (window.addEventListener) { 
    //from underscore.js
    var debounce = function (func, wait, immediate) {
            var timeout;
            return function () {
                var context = this, args = arguments,
                    later = function () {
                        timeout = null;
                        if (!immediate) {func.apply(context, args); }
                    },
                    callNow = immediate && !timeout;
                clearTimeout(timeout);
                timeout = setTimeout(later, wait);
                if (callNow) {func.apply(context, args); }
            };
        },
        b = document.body,
        h = window.innerHeight,
        origOffsetY = window.pageYOffset,
        lastPos = origOffsetY;
    var scrollinUpAddClass = function () {
        var scrolY = window.pageYOffset;
            if (scrolY >= origOffsetY && scrolY < lastPos && scrolY >= 0.5 * h) {
                if (b.className.indexOf('su--ready') !== -1) {b.className = b.className.replace('su--ready', 'su--scrollinUp'); }
            } else {
                if (b.className.indexOf('su--scrollinUp') !== -1) {b.className = b.className.replace('su--scrollinUp', 'su--ready'); }
            }
            lastPos = scrolY;
        };
    var scrollinUp = function() {
        if (b.className.indexOf('su--ready') === -1 && b.className.indexOf('su--scrollinUp') === -1 ) { b.className += ' su--ready'; }
        h = window.innerHeight;
        scrollinUpAddClass();
    }
    window.addEventListener('scroll', debounce(scrollinUp, 150, false), false);
    window.addEventListener('resize', debounce(scrollinUp, 150, false), false);
}

scrollin-up.min.js

/*! scrollin-up.js 
* add class su--scrollinUp to body
* when (and only) scrolling up
* http://soqr.fr/scrollin-up
* @author Pascal Cauhépé @eQRoeil
*/
if(window.addEventListener){var debounce=function(a,b,c){var d;return function(){var e=this,f=arguments,g=function(){d=null,c||a.apply(e,f)},h=c&&!d;clearTimeout(d),d=setTimeout(g,b),h&&a.apply(e,f)}},b=document.body,h=window.innerHeight,origOffsetY=window.pageYOffset,lastPos=origOffsetY,scrollinUpAddClass=function(){var a=window.pageYOffset;a>=origOffsetY&&lastPos>a&&a>=.5*h?-1!==b.className.indexOf("su--ready")&&(b.className=b.className.replace("su--ready","su--scrollinUp")):-1!==b.className.indexOf("su--scrollinUp")&&(b.className=b.className.replace("su--scrollinUp","su--ready")),lastPos=a},scrollinUp=function(){-1===b.className.indexOf("su--ready")&&-1===b.className.indexOf("su--scrollinUp")&&(b.className+=" su--ready"),h=window.innerHeight,scrollinUpAddClass()};window.addEventListener("scroll",debounce(scrollinUp,150,!1),!1),window.addEventListener("resize",debounce(scrollinUp,150,!1),!1)}