How mongodb connection works on concurrent requests in NodeJS express server?

I have answered for both of your question separately.

1. How will the mongodb connection in the express server work?

Once a connection is created to the mongodb database.(using mongoose or any other framework) It will create a pool of connections with that. (Mongoose default pool size is 5, 100 in python) The created connection pool is maintained by the driver therefore those connections can be re-used when connections to the database are required.

The best practice is to create a new connection only once for the whole application. Once connection is created the connection object will be used as a singleton. When you connect to the database using mongoose models, separate connections are allocated from the created connection pool.

If you are going to create a new connection each time then It will cause to a connection churn.

2. How can I close the connection object in mongodb efficiently after the operation ?

I am not sure 100% about this answer. My suggestion is to disconnect the connection when the express application exits.

var db = mongoose.connect('mongodb://localhost:27017/database');    db.disconnect();

According to my knowledge what you have don in the code is correct. You have created a new connection only once. Since the connection pool is created with that you don't need to create more connections.

Please go through this link to get a clear understanding on connection pools and their usage.https://dzone.com/articles/deep-dive-connection-pooling