So this is what I’m trying to achieve:
One part of my javascript client side code joins a topic "data:data" with a payload of data to sync to the backend.

def join("data:data", %{"query" => q} = params, socket) when is_binary(q) do
    send(self(), {:sync_data, params})
    {:ok, socket}
 end

def handle_info({:sync_data, params}, socket) do
    response = response(params)
    broadcast(socket, "data-synced", response)
    {:noreply, socket}
 end

This part of the client side code subscribes to the "data-synced" event and does something with the response. So far this part is working:

this.dataChannel = this.socket.channel("data:data", params);
this.dataChannel.on("data-synced", this.dataSyncedCb);

Now other parts of the code using the same socket (I’m using Phoenix.js) will join another topic "data:client:" <> id and also subscribe to the "data-synced" event and do different things with the response.

def join("data:client:" <> _id, _params, socket), do: {:ok, socket}

this.dataSyncChannel = args.socket.channel("data:client:289", {});
this.dataSyncChannel.on("data-synced", this.dataClientSyncedCb);

But these other parts of the code are not receiving the data-synced event.
Please note that these are all the same clients but they all have different responsibilities for the event response.

So I’m thinking may be Phoenix.Channel.broadcast/3 is not the way to go.