[scroll snip] snip para identificar la posición en la página.

scroll_snip.js

<script>
/*!
 * @preserve
 * jquery.scrolldepth.js | v0.4.1
 * Copyright (c) 2014 Rob Flaherty (@robflaherty)
 * Licensed under the MIT and GPL licenses.
 */
;(function ( $, window, document, undefined ) {

  "use strict";

  var defaults = {
    elements: [],
    minHeight: 0,
    percentage: true,
    testing: false
  },

  $window = $(window),
  cache = [];

  /*
   * Plugin
   */

  $.scrollDepth = function(options) {

    var startTime = +new Date;

    options = $.extend({}, defaults, options);

    // Return early if document height is too small
    if ( $(document).height() < options.minHeight ) {
      return;
    }

    // Establish baseline (0% scroll)
    sendEvent('Percentage', '0%');

    /*
     * Functions
     */

    function sendEvent(action, label, timing) {
      if (!options.testing) {

        if (typeof(dataLayer) !== "undefined") {
          dataLayer.push({'event':'ScrollDistance', 'eventCategory':'Scroll Depth', 'eventAction': action, 'eventLabel': label, 'eventValue': 1, 'eventNonInteraction': true});

          if (arguments.length > 2) {
            dataLayer.push({'event':'ScrollTiming', 'eventCategory':'Scroll Depth', 'eventAction': action, 'eventLabel': label, 'eventTiming': timing});
          }
        } else {

          if (typeof(ga) !== "undefined") {
            ga('send', 'event', 'Scroll Depth', action, label, 1, {'nonInteraction': 1});

            if (arguments.length > 2) {
              ga('send', 'timing', 'Scroll Depth', action, timing, label);
            }
          }

          if (typeof(_gaq) !== "undefined") {
            _gaq.push(['_trackEvent', 'Scroll Depth', action, label, 1, true]);

            if (arguments.length > 2) {
              _gaq.push(['_trackTiming', 'Scroll Depth', action, timing, label, 100]);
            }
          }
        }

      } else {
        $('#console').html(action + ': ' + label);
      }
    }

    function calculateMarks(docHeight) {
      return {
        '25%' : parseInt(docHeight * 0.25, 10),
        '50%' : parseInt(docHeight * 0.50, 10),
        '75%' : parseInt(docHeight * 0.75, 10),
        // 1px cushion to trigger 100% event in iOS
        '100%': docHeight - 1
      };
    }

    function checkMarks(marks, scrollDistance, timing) {
      // Check each active mark
      $.each(marks, function(key, val) {
        if ( $.inArray(key, cache) === -1 && scrollDistance >= val ) {
          sendEvent('Percentage', key, timing);
          cache.push(key);
        }
      });
    }

    function checkElements(elements, scrollDistance, timing) {
      $.each(elements, function(index, elem) {
        if ( $.inArray(elem, cache) === -1 && $(elem).length ) {
          if ( scrollDistance >= $(elem).offset().top ) {
            sendEvent('Elements', elem, timing);
            cache.push(elem);
          }
        }
      });
    }

    /*
     * Throttle function borrowed from:
     * Underscore.js 1.5.2
     * http://underscorejs.org
     * (c) 2009-2013 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
     * Underscore may be freely distributed under the MIT license.
     */

    function throttle(func, wait) {
      var context, args, result;
      var timeout = null;
      var previous = 0;
      var later = function() {
        previous = new Date;
        timeout = null;
        result = func.apply(context, args);
      };
      return function() {
        var now = new Date;
        if (!previous) previous = now;
        var remaining = wait - (now - previous);
        context = this;
        args = arguments;
        if (remaining <= 0) {
          clearTimeout(timeout);
          timeout = null;
          previous = now;
          result = func.apply(context, args);
        } else if (!timeout) {
          timeout = setTimeout(later, remaining);
        }
        return result;
      };
    }

    /*
     * Scroll Event
     */

    $window.on('scroll.scrollDepth', throttle(function() {
      /*
       * We calculate document and window height on each scroll event to
       * account for dynamic DOM changes.
       */

      var docHeight = $(document).height(),
        winHeight = window.innerHeight ? window.innerHeight : $window.height(),
        scrollDistance = $window.scrollTop() + winHeight,

        // Recalculate percentage marks
        marks = calculateMarks(docHeight),

        // Timing
        timing = +new Date - startTime;

      // If all marks already hit, unbind scroll event
      if (cache.length >= 4 + options.elements.length) {
        $window.off('scroll.scrollDepth');
        return;
      }

      // Check specified DOM elements
      if (options.elements) {
        checkElements(options.elements, scrollDistance, timing);
      }

      // Check standard marks
      if (options.percentage) {
        checkMarks(marks, scrollDistance, timing);
      }
    }, 500));

  };

})( jQuery, window, document );
jQuery.scrollDepth();
</script>