Access same instance of db connection in different files in express js

I have 4 files:

1) db.js - mongodb connection
2) socket.js - for all socket events
3) route.js - route for /
4) app.js - main start point for express app

I want to share same db instance from db.js in socket.js and route.js so that the connection to db is made only once.

I have tried 2 ways for this:

1) require db.js in both files, but this creates new connection every time require is called.
2) pass the collections in request object in db.js, but then how would I access the request object in socket.js in 

io.on("connection",function(socket){
 // request ??
})

How can it be solved?

via Sahil