How to detect when a user leaves web-game lobby

Question

I've a basic multiplayer web-game where players get matched in a lobby of 2 when they enter the website. I'm using nodejs and express for the server side.

I want the server to detect when a player "leaves" the lobby so that i can then match the remaining player in the lobby with someone else. Leaving should include closing the tab, the browser or refreshing the page.

I tried sending an ajax request from the client side when the "unload" or "beforeunload" events fire. However i found this to be pretty inconsistent and not reliable. I've seen people use websockets for managing such client-server communications but never used them. I thought of using a timer on the server side to see if a player stops replying which might do the trick. How should i go about implementing this and how do other multiplayer web-games solve this problem?

Details about my server-side if it helps: I'm currently using express-session to decide ID's of players. Once the players are in a lobby of 2, they send continuous (every 0.5 seconds) ajax requests to the server to get information about the other player.