The Get Utility provides a mechanism for attaching script and css resources — including cross-domain resources — to the DOM after the page has loaded.

There are two common use cases for the Get Utility:

1. Cross-site data retrieval: Because XMLHttpRequest (and YUI IO Utility, which uses XMLHttpRequest) adheres to a strict same-origin policy, the retrieval of third-party data via XHR requires a server-side proxy. Where you control or absolutely trust a cross-domain source, you can eliminate the server-side proxy by loading a script file directly from the external domain; that script file, which would typically contain JSON-formatted data, is executed immediately upon load. It is crucial to understand that if there is malicious code present in the loaded script there is no safe way to innoculate your users from that malicious code...the browser will execute the code with full privileges. Never expose your users to JavaScript whose source is not absolutely trustworthy.
2. Progressive loading of functionality: In rich web applications, it's often useful to load some script and CSS resources only when they become necessary (based on user action). The Get Utility provides a flexible mechanism for bringing additional resources on-demand. (Note: If you're loading YUI resources specifically, use the YUI Loader Utility; the YUI Loader employs the Get Utility under the hood to bring in YUI components and has an intrinsic understanding of the YUI dependency tree.)

What Is the Difference Between the Get Utility and the IO Utility?

The Get Utility inserts new content into a window by creating new nodes and supplying a src attribute. Files inserted into the window in this manner are processed (and, in the case of scripts, executed) immediately upon load. While querystring parameters can be passed in the src attribute URL, no data can be sent to the server via HTTP POST using this method. The Get Utility can only make HTTP GET requests. It can, however, interact with disparate domains. As noted above, the Get Utility is ideal for loading your own scripts or CSS progressively (lazy-loading) or for retrieving cross-domain JSON data from sources in which you have total trust.

The YUI IO Utility, by contrast, uses the XMLHttpRequest object to interact with the server. XMLHttpRequest is limited by a strict same origin policy, but it supports a greater range of HTTP methods (including POST). Moreover, script data retrieved via XMLHttpRequest can be validated on the server side or in JavaScript (using, for example, the JSON Utility) prior to being executed. As a result, IO Utility is a more appropriate choice for rich two-way communication between browser and server and gives you more control over data before it's processed within the browser.

Version 3.0 of the IO Utility now supports cross domain requests. There are specific trust requirements as described in documentation for the IO Utility. This may be a better choice if the service you are accessing can be configured to trust the server that is hosting your application.