Brr_io.MessageSourceMessage events, ports, channels and broadcast channels.
The type for objects to transfer.
transfer v indicates valule v should be transfered, not just cloned, meaning they are no longer usable on the sending side.
The type for messaging options.
opts ~target_origin ~transfer () are messaging options. See here for the semantics of target_origin and transfer.
Broadcast channels.
window_post w v ~opts posts value v to window w with options opts.