How to get the session expiration event in express using ioredis?

Question

I am using ioRedis coupled with express-session in my NodeJS app and I'd like to fire an event when a session expires. I wrote this:

let Redis = require('ioredis');
let clientRedis = new Redis();
let RedisStore = require('connect-redis')(session);

const sessionMiddleware = session({
  store: new RedisStore({
    client: clientRedis,
  }),
  secret: SESSIONSECRET,
  resave: true,
  saveUninitialized: true,
  cookie: {
    maxAge: 5000,
    secure: false,
    httpOnly: true,
    domain: config.get('app.domainecookie')
   }
});

app.use(sessionMiddleware);

I read the doc and I have no clue on how capture the "expire" event... There must be a ioredis function, but I don't see which one. I read a lot of questions talking about key expiration but this is not a particular key I want to get the notifications of, I want the session of the user to get a notification (I'm handling the messages with a socket which works like a charm, I just don't know how to capture the event).

For the moment I created an express middleware firing timed out functions in a custom "activeSessions" object, but this is using memory of my NodeJS.app, there must be a better way to do this.

Thanks in advance :)