how to check the client is available?

[古格沙]

my first plan is like bellow.

mongodb.MongoClient.connect( uri, function( err, client )
{
     let db = client.db( ... );
     // do something with db
     client.close();
} );

but client.close() failed. i read some article which suggests to remain the client in the whole life of process. then, how can i check the client is working or broken by some accident?

[Jack Macdonald]

Should be able to check the result of 

client.isConnected()

See http://mongodb.github.io/node-mongodb-native/3.1/api/MongoClient.html#isConnected

Also remember to handle `err` on the callback