If this is guaranteed by design, don't assume but assert

This is convoluted for no reason, and completely wrong, \n is a single char. You're not writing \n in the messages.

Note that there is an off-by-one error here because the join() method doesn't add an \n at the end of each line.
E.g. there is a single \n in the output here:

process.stdout.write(['Two', 'lines'].join('\n'));

This is an horrible anti-pattern, just do this after the loop.
This can also potentially send an empty message, not sure what's the outcome of that is.

That doesn't tell what the output of this function is

Can msgSuccess be false ? Is it guaranteed that the bot raises an exception if the message failed to send ?
The Telegram bot API doesn't work like this but I couldn't find the node driver API documentation

That's brittle, if the branch is false for whatever reason you don't return anything.
Right now this shouldn't happen, but you don't know who will edit this function in the future.

2 solutions:

• have a default return value
• assert the length is what is expected

has this been generated ? If yes an @generated marker would be great

As per the code, it's <=
It's also a good use case for console.assert()

Please comment that it's a telegram bot undocumented behavior that causes the \n to be accounted as 2 chars

Another good use for console.assert()

I was thinking the opposite, if an error occurs it doesn't raise but return an error messsage

This is duplicated, either assert and return msgSuccessArray unconditionally or log unconditionally then return false