Smart Updating of Collections with Backbone

gistfile1.js

Backbone.Collection.prototype.update = function(col_in){
  var that = this,
      ids = [];
      
  var cur_ids   = that.pluck('id'),
      new_ids  = _(col_in).pluck('id'),
      to_remove = _(cur_ids).difference(new_ids);

  this.remove(to_remove);

  _(col_in).each(function(mod_in){
    if (that.get(mod_in.id)){
      that.get(mod_in.id).set(mod_in);
    } else {
      that.add(mod_in);
    }
  });

  this.trigger('update');
};

what is the use ids = []