You’ve got many valid answers in here but none of them really tells you why it works as it does.
In JavaScript commands are executed one at a time, synchronously in the order they come, unless you explicitly tell them to be asynchronous by using a timeout or interval.
This means that your .append method will be executed and nothing else (disregarding any potential timeouts or intervals that may exist) will execute until that method have finished its job.
To summarize, there’s no need for a callback since .append will be run synchronously.