The problem is that the records field of the parameter object is always an empty array:
app.merge({records:[], in:app.databases.byId(6).parents.byId(2147483645)})
--> null
is what Script Editor says, regardless of what I put into the records array (I even tried to pass an NSArray unsuccessfully, but at least that was not empty 
). We’ve talked about that before in the context of summarizeHighlightsOf, where the first parameter is also empty (Script Summarize Highlights from Javascript - #2 by chrillek).
This thread petered out inconclusively. Since there are apps out there which can work with arrays of data in JXA and AppleScript lists map just fine to JavaScript Arrays in other situations, I still assume that there’s a subtle glitch in DT’s approach here. @cgrunenberg?