You're asking for an api that tells a user if his message was sent or not to the other user, right? I checked all of the messaging apis that the JSCSDK has, but in this case the answer is that the JSCSDK does not support it :(
For now, the JSCSDK does not handle any cases where the user A can see what is the status of his or any messages on the user B side.
And yes, isRead() is a method only for received messages.
Charly - JSCSDK Support Team
This message was edited 1 time. Last update was at 11/03/2020 15:32:57