To select messages in Mac Mail using AppleScript you can use the following syntax:
tell application "Mail"
set selectedMessages to selected messages of the first message viewer
set selected messages of the first message viewer to item 1 of selectedMessages
end tell
However a solution for setting the selection using JXA evades me and the Library documentation it not much help either. I have tried:
var app = Application('Mail')
var selectedMessages = app.messageViewer[0].selectedMessages()
app.messageViewer[0].selectedMessages(selectedMessages[0])
and
var app = Application('Mail')
var selectedMessages = app.messageViewer[0].selectedMessages()
app.messageViewer[0].selectedMessages([selectedMessages[0]])
and
var app = Application('Mail')
var selectedMessages = app.messageViewer[0].selectedMessages()
app.messageViewer[0].selectedMessages = selectedMessages
None of these work. Any ideas?