How to use the node-opcua-transport/dist/test_helpers.DirectTransport function in node-opcua-transport

constructor(expectedReplies: any) {

        super();

        this._replies = expectedReplies;
        this._counter = 0;

        this._mockTransport = new DirectTransport();
        this._mockTransport.initialize(() => {
            console.log("initialized");
        });

        this._mockTransport.server.on("data", (data: Buffer) => {

            let reply = this._replies[this._counter];
            this._counter++;
            if (reply) {

                if (_.isFunction(reply)) {
                    reply = reply.call(this);
                    // console.log(" interpreting reply as a function" + reply);
                    if (!reply) {
                        return;
                    }