dom.js

var dom = (function(window) {
     
    'use strict';
 
    var docElem = window.document.documentElement;
 
    function getViewportSize() {
         return {
           w: Math.max(document.documentElement.clientWidth, window.innerWidth || 0),
           h: Math.max(document.documentElement.clientHeight, window.innerHeight || 0)
         }
    }
 
    function scrollY() {
        return window.pageYOffset || docElem.scrollTop;
    }
    
        // http://stackoverflow.com/a/5598797/989439
        //finding element's position relative to the document
    function getOffset(el) {
        var offsetTop = 0, offsetLeft = 0;
        do {
            if ( !isNaN( el.offsetTop ) ) {
                offsetTop += el.offsetTop;
            }
            if ( !isNaN( el.offsetLeft ) ) {
                offsetLeft += el.offsetLeft;
            }
        } while( el = el.offsetParent )
 
        return {
            top : offsetTop,
            left : offsetLeft
        }
    }
    
     function inViewport(el, h) {
        var elH = el.offsetHeight,
            scrolled = scrollY(),
            viewed = scrolled + getViewportSize().h,
            elTop = getOffset(el).top,
            elBottom = elTop + elH,
            // if 0, the element is considered in the viewport as soon as it enters.
            // if 1, the element is considered in the viewport only when it's fully inside
            // value in percentage (1 >= h >= 0)
            h = h || 0;
 
        return (elTop + elH * h) <= viewed && (elBottom) >= scrolled;
    }
    
    return {
      viewportSize: getViewportSize,
      scrollY: scrollY,
      offset: getOffset,
      isInViewport: inViewport
    }
    
})(window);