jQuery extension to reduce the set of matched elements to those being (vertically) in the viewport

visible.js

$.fn.visible = function() {
    'use strict';

    var $window = $(window),
        windowScrollTop = $window.scrollTop(),
        windowHeight = $window.height();

    return this.filter(function() {
        var $this = $(this),
            thisOffsetTop = $this.offset().top,
            thisHeight = $this.height();

        return thisOffsetTop < 
                windowScrollTop + 
                windowHeight &&
            thisOffsetTop + 
                thisHeight > 
                windowScrollTop;

    });
};