@types/sockjs-client

const SockJS: {
    (url: string, _reserved?: any, options?: SockJS.Options): WebSocket;
    new (url: string, _reserved?: any, options?: SockJS.Options): WebSocket;
    prototype: WebSocket;
    CONNECTING: SockJS.CONNECTING;
    OPEN: SockJS.OPEN;
    CLOSING: SockJS.CLOSING;
    CLOSED: SockJS.CLOSED;
};

type CLOSED = 3;

type CLOSING = 2;

type CONNECTING = 0;

type OPEN = 1;

type OpenEvent = BaseEvent;

type SessionGenerator = () => string;

type State = CONNECTING | OPEN | CLOSING | CLOSED;