In cases while our all routes need a DB connection, and we would not want the server running, if the database connection isn’t working. Then, in such a case, we can call app.listen in the DB connection callback.
We have 365 days in a year. And when we see a year as it has got 12 months and every month on average has 30 days, it seems like a lot of time. On the other hand, we have got 24 hours on a given day and even if you sleep for 7 hours and spend 2 hours for things that are absolutely necessary for the survival, you have got 15 hours on your hand daily. Multiply it by 365, we have got 5475 hours a year. This might seem like a lot of time, but this is the time span within which you do all your schooling, or job, parties, travel and everything you do in a year. Time management is an absolutely necessary skill.