Orbeon Forms: Populate Hidden Field at xform-ready

I know we can populate a hidden control at the xforms-ready event:

<xf:action event="xforms-ready" type="javascript"> 
   var vProtocol = window.location.protocol;
   var vSystem;
   if (vProtocol == 'file:') { vSystem = 'Yes' } else  { vSystem = 'No' };      
   ORBEON.xforms.Document.setValue(ORBEON.fr.API.findControlsByName("SystemType")[0],vSystem); 
</xf:action>

When we use xPath to within a control to query the value in SystemType, then we get no value. However, if I make the SystemType control visible then it all works as expected.

So how can I have a hidden control, and still retrieve the value?

Many thanks in advance

PeteA

You can't change the value of a readonly or hidden control with JavaScript. This is because, on the web, JavaScript running in the browser can't be "trusted" in the sense that what can be done in JavaScript, a malicious user can also do. In general, you wouldn't want users to be able to change the value of readonly or hidden controls.

If in your case this isn't a concern, you can use a regular field and hide it with CSS, for instance by adding xforms-hidden in Custom CSS classes. This way, the control will be visually hidden but still present on the page, which will allow you to set its value with JavaScript.