In HTML5 publications you can create javascript actions directly from within your Opus publication and thereby attach the script to any Opus object. Simply choose the trigger as usual and then select the JavaScript Action from the Programming tab.
You can then enter your javascript into the edit box provided and Opus will automatically include it in your published file.
Note:
Opus does not check the JavaScript in any way so you must ensure that any script entered here works as required. If your HTML5 publication fails to run or you get a black screen then this may be because the javascript has errors.