How can I handle disconnect event using graphql-subscriptions

I am using graphql-subscriptions to manage session (rooms).

I have simple application like "chat".

I can send message to chat using:

import { PubSub } from 'graphql-subscriptions';
...

pubSub.publish(triggerName, payload)

I can listen to subscription events..

But I don't know how to properly handle when user left to room (for example closing full browser)...

Is there any "disconnect" event?

I am using nestjs server and react with apollo-link-ws

Thank you all.