With Ably, you can set up presence on an existing channel, using the JavaScript SDK. Within client code, you can send enter, leave and update messages. These may include custom data payloads as a string or JavaScript object, so long as is can be serialized as a JSON string. For the client to receive updates on the channel, you need to subscribe them. Typically, you include a callback function in the subscription. That callback function might have filters over the presence updates and logic for pushing the latest presence data to the user interface.