I don't know Javascript but from reading your other post I think I know what's going on.
the string document.ipForm.ipList.selectedIndex includes the actual names of the form and control your working with. That is "ipForm" corresponds to the form name "ipForm" and "ipList" corresponds to the listbox name "ipList"
You can change the input parameter but make sure the form and control you put in the parameter exist. Your example says this.form.ipList.selectedIndex, which leads me to ask is there a form with the name "form" in your page and does that form contain a control with the name "ipList"?
EDIT: Assuming you're working with the function in the other post, you'd have to change all the 'hard coded' references to the control inside the function to re-use it. So even if the controls exist as I explained above, the function is coded to work with the control "document.ipForm.ipList". If you want it to be more generic, then make the names of the form and control parameters to the script and change all references to "document.ipForm.ipList" to whatver form and control you passed into the function. This would be beyond my passing knowledge of Javascript, but if you understand what I'm saying you should be able to modify that function with a little research.
Someone who knows Javascript PLEASE say loud and clear if I'm off base, these days I'm much more familiar with VBscript than anything else
