code: function shouldUnloadPage(index, rangeLength) {
  
          // within page load range?
          if (indexInRange(index, rangeLength)) {
              return false;
          }
  
          // is it visible?
          if (pageIsVisible(index)) {
              return false;
          }
  
          return true;
      }
docs: Returns true if the given page index should be unloaded, and false otherwise
  @param   {int} index The page index
  @param   {int} rangeLength The range length
  @returns {bool}      Whether the page should be unloaded
  @private