You do not get a qFieldList when you fetch a bookmarklist but a qBookmarkList. Try that instead. You could also simply log the reply to the console and inspect it, or set a breakpoint.
Thank you Eric. This worked for listing bookmarks.
Could you also point me to where I could save my bookmark with sheet selections, please.
I am using the below, but this does not give any option for such details.
app.bookmark.create('Test bookmark via JS - 1','Test bookmark via JS - 1 - using ids for both app and sheet names','662202cd-c708-494a-9690-3508d4f39fdb');