You gave the answer yourself, the WebSocket is the API that you would be using to allow the server to communicate to the client without having the client to send a new request to the server.
It would be a
full-duplex communication mode because server and client both can communicate without having other party to ask for data from either side and can send the data to each other as they wish to. You can read more on this from
Mozilla Developer Network[
^] to understand the actual concept of this.