简化了编写异步JavaScript

``` //Every asynchronous call requires coding callbacks. // Sample Code: // // The code below first retrieves the list of discussion groups from the rpcService in a callback // Next retrieves the list of discussions from the rpcService in another callback // Finally retrieves and processes the list of messages from the rpcService from a callback (function() { rpcService.getDiscussionGroups(); rpcService.onComplete = function(groups) { for each (var group in groups) { rpcService.getDiscusionsForGroup(group); rpcService.onComplete = function(discussions) { for each (var discussion in discussions) { rpcService.getMessages(discussion); rpcService.onComplete = function(messages) { //do something } } }; } } })(); //Compare the sample above