How do I access my .plist with JXA (Javascript for Automation)?
I just had to figure out how to open and read a plist with JXA myself, and it was not intuitive at all. The documentation on JXA is frankly terrible. I searched the dictionaries in Script Editor, but also made liberal use of the properties() method in Script Editor, which will show you all the properties of many objects.

I finally did figure it out for myself, and I think this will work for you:

let app = Application.currentApplication()
app.includeStandardAdditions = true

let systemEvents = Application("System Events")
let cats = systemEvents.propertyListFiles.byName("/Users/iain/testing.plist").contents.value()['catsList'];


Results in the log:

/* Blog,Devops,Drupal,Formal Methods,Parish Councils,Scripting */