Session management and inmemory

I am able to write a program uisng expressjs. I have written the username and password ina json file and accessing when I get a request.

req.body.password == users[req.body.username].password)

I want to copy the sessionID (SID) created and send to the client. once I get a new request I want to check if it is there in local(inmemmory) if it is not there I want to send a different Error code to the user.

if(req.session.id=

My question is when not using redis (or) any DB, how can I store these session ID inmemory ? I understand if it is inmemory on Restart all the data is gone.

How can I do that ?