Function onupgrade

packages/socket.io/client-dist/socket.io.js:2263–2267

(to)

      }
      // When the socket is upgraded while we're probing
      function onupgrade(to) {
        if (transport && to.name !== transport.name) {
          freezeTransport();
        }
      }
      // Remove all listeners on the transport and on self
      var cleanup = function cleanup() {
        transport.removeListener("open", onTransportOpen);