Can't connect to WebSocket changes feed from Javascript client

I have a React application for browser where I currently request data updates via the changes feed by calling the GET /{db}/_changes endpoint every 30 seconds. I want to upgrade to an asynchronous WebSocket connection.

However, when I try to establish with vanilla JS a WebSocket connection to the sgw server, I receive an error “WebSocket connection to ‘ws://’ failed”. No logs are written for the failed connection.

JS code for opening the WebSocket connection:

const client = new WebSocket(
client.onopen = () => {
    console.log('WebSocket Client Connected');
        "limit": 10,
        "include_docs": true,
        "active_only": true,
        "since": 0,
        "heartbeat": 60000,

On the other side, establishing the WebSocket connection from my Python backend works. The only difference that I see is that I am setting a cookie for authorisation in the Python code. However, it isn’t possible in JS to set cookies or auth headers in vanilla JS WebSockets.
A similar problem has already been adressed here: Authentication issue with WebSocket using SyncGateway Public REST API 2.8

What is the problem?
Do I have to pass authorisation parameters?
Is the WebSocket changes feed even supported for JS clients?

Thanks a lot for any help!

Any request against the Sync Gateway public REST API, including changes with feed=websocket, will require authentication. Sync Gateway’s supported authentication mechanisms are basic auth, session token as cookie, and bearer token (JWT) for OIDC-based authentication - it doesn’t support any custom websocket authentication options for clients that don’t support any of these three.

As suggested on the issue you linked, you might evaluate continuous HTTP streaming (feed=continuous) for your use case.