SharepPoint 2010 ECMAScript Client Object Model : how to get selected metadata for listitem

Often, it is requried to retrieve information relevant to what we need, especially in the case of client side programming to reduce bandwidth and improve response time. Following are sample code to fetch only selective metadata for a listitem

var context = SP.ClientContext.get_current();
this.web = context.get_web();
var listId = SP.ListOperation.Selection.getSelectedList();
var list = this.web.get_lists().getById(listId);
var items = SP.ListOperation.Selection.getSelectedItems(context);
var itemId = items[0];
this.listItem = list.getItemById(itemId.id);

context.load(this.listItem, '_dlc_DocIdUrl', 'FileLeafRef');

var context = SP.ClientContext.get_current();
this.web = context.get_web();
this.listCollection = this.web.get_lists();

context.load(this.listCollection, 'Include(Title, Id)');