node-opcua client in typescript.js

import {
    BrowseResult,
    BrowseDescription,
    BrowseDirection,
    OPCUAClient,
    MessageSecurityMode,
    SecurityPolicy,
    ClientSession
} from "node-opcua";
import async = require("async");


const client = new OPCUAClient({
    securityMode: MessageSecurityMode.None,
    securityPolicy: SecurityPolicy.NONE,
    connectionStrategy: {
        maxRetry:0
    }
});

type Callback = (err?: Error) => void;


let session: ClientSession= null;

async.series([

    function (callback: Callback) {
        client.connect("opc.tcp://localhost:48010", function (err?:Error) {
            callback(err);
        });
    },
    function (callback: Callback) {
        client.createSession(function(err: Error|null,result?: ClientSession){
            if (err) return callback(err);
            session = result;
            callback();
        })
    },

    function (callback: Callback) {

        session.browse("RootFolder",function(err: Error|null,result: BrowseResult) {
            if(err) return callback(err);
            console.log(result.toString());
            callback();
        });
    },

    function (callback: Callback) {

        session.browse(["RootFolder"],function(err: Error|null,result: Array<BrowseResult>) {
            if(err) return callback(err);
            console.log(result[0].toString());
            callback();
        });
    },
    function (callback: Callback) {

        let browseDescription: BrowseDescription={
            browseDirection: BrowseDirection.Forward,
            nodeId: "RootFolder"
        };
        session.browse(browseDescription,function(err: Error|null,result: Array<BrowseResult>) {
            if(err) return callback(err);
            console.log(result[0].toString());
            callback();
        });
    },


    function (callback: Callback) {
        client.closeSession(session,false,callback);
    },
    function (callback: Callback) {
        client.disconnect(function (err?:Error|null) {
            callback(err);
        });
    },


], function (err:Error) {
    console.log("done",err);
});