[Seaside] Periodical and running a script

[Squeaker]

Hello,

I have a SUPeriodical set up as follows:

html
   script:
     ((html periodical)
	id: 'compositeGraph';
	frequency: 5 seconds;
	on: #renderNextImageOn: of: self)


What I want is to run some javascript when the periodical is triggered. 
  For example, I would like the following javascript to be executed each 
time the periodical is triggered:

html
  script: ('document.getElementById("testDiv").src="new text";').


I cannot figure our how to run the javascript code when the periodical 
is triggered.

How do I do this?

Thanks,
Frank