Available since OmniFaces 1.8

The <o:deferredScript> is a component based on the standard <h:outputScript> which defers the loading of the given script resource to the window load event. In other words, the given script resource is only loaded when the window is really finished with loading. So, the enduser can start working with the webpage without waiting for the additional scripts to be loaded. Usually, it are those kind of scripts which are just for progressive enhancement and thus not essential for the functioning of the webpage.

This will give bonus points with among others the Google PageSpeed tool, on the contrary to placing the script at bottom of body, or using defer="true" or even async="true".

Since 2.4 this will render the crossorigin attribute with a value of anonymous. Since 3.13 this will also render the integrity attribute with a base64 encoded sha384 hash as SRI, see also MDN.


Just use it the same way as a <h:outputScript>, with a library and name.

<o:deferredScript library="yourlibrary" name="scripts/filename.js" />

You can use the optional onbegin, onsuccess and onerror attributes to declare JavaScript code which needs to be executed respectively right before the script is loaded, right after the script is successfully loaded, and/or when the script loading failed.


This component is also used on this showcase web application on the two JS files prettify.js and onload.js. Rightclick the page and View Source and find the OmniFaces.DeferredScript.add call at the bottom of <body>. Note that this showcase application is also using the CombinedResourceHandler which transparently recognizes and combines the deferred script resources into a single resource as well.

Using <o:deferredScript> on PrimeFaces JavaScript resources

Refer to the following Stack Overflow Question&Answer for a detailed guide: Defer loading and parsing of PrimeFaces JavaScript files.