handler(...data, [ackFn])

Socket event listener handler expects one or more data arguments and an optional ACK callback.

// Client:
socket.on( "news", function handler( data, ackCb ) {
    console.log( "received some news", data );
    ackCb( "Acknowledged", "thank you" );
} );

// Server:
server.emit( "news", { some: "news here" }, function ackFn( ...data ) {
    console.log( "Client acknowledged data receiving" );
} );