Looks like it\'s easy to add custom HTTP headers to your websocket client with any HTTP header client which supports this, but I can\'t find how to do it with the JSON API.
Totally hacked it like this, thanks to kanaka's answer.
Client:
var ws = new WebSocket(
'ws://localhost:8080/connect/' + this.state.room.id,
store('token') || cookie('token')
);
Server (using Koa2 in this example, but should be similar wherever):
var url = ctx.websocket.upgradeReq.url; // can use to get url/query params
var authToken = ctx.websocket.upgradeReq.headers['sec-websocket-protocol'];
// Can then decode the auth token and do any session/user stuff...