如何从类中访问变量？

Question

我有一个名为client的变量，它在类的函数中定义。我只想将其导出到该类之外，以便可以在控制器中使用它。 这是我的代码

const path = require('path');
const bodyParser = require('body-parser');
const mongoose = require('mongoose');
const flash = require('connect-flash');
var methodOverride = require('method-override');
const cookieParser = require('cookie-parser');
const session = require('express-session');
const http = require('http');
const socketio = require('socket.io');
const passport = require('passport');
const rememberLogin = require('./http/middlewares/rememberLogin');
const Helpers = require('./helpers');
const app = express();

module.exports = class Application{
    constructor() {
        this.setExpress();
        this.setupMongo();
        this.setConfig();
        this.setRoutes();
        // this.updateDataBase();
    }

    setExpress() {    
        const server = http.createServer(app);
        const PORT = process.env.PORT || 3000;
        server.listen(PORT, () => console.log(`Server running on port ${PORT}`));
        let client = socketio(server);    
    }

    setupMongo() {
        mongoose.connect(config.database.url, {
            useUnifiedTopology: true,
            useCreateIndex: true,
            useNewUrlParser: true,
            useFindAndModify: false
        }).then(() => console.log('Connected to mongo....'));        
    }

    setConfig() {
        require('./passport/passport-local');

        // Set ejs config
        app.set('view engine', 'ejs');
        app.set('views', path.join(__dirname, '../resource/views'));
        app.use(config.layout.ejs.expressLayouts);
        app.set("layout extractScripts", config.layout.ejs.extractScripts);
        app.set("layout extractStyles", config.layout.ejs.extractStyles);
        app.set("layout" , config.layout.ejs.master);

        app.use(express.static(path.join(__dirname, '../public')));
        app.use(bodyParser.json());
        app.use(methodOverride('_method'));
        app.use(bodyParser.urlencoded({ extended : true }));
        app.use(session({...config.session}));
        app.use(cookieParser('Thisismycookiesecretekey'));
        app.use(flash());
        app.use(passport.initialize());
        app.use(passport.session());
        app.use(rememberLogin.handle);
        app.use((req, res, next) => {
            app.locals = new Helpers(req, res).getObjects();
            next();
        })
    }

    setRoutes() {
        app.use(require('./routes/index'));
    }
}

module.exports.client = client;

我只是通过从类中删除setupExpress函数来弄清楚的，然后我可以访问 client变量。 但是 我认为这不是正确的方法。 有什么办法可以以不同的方式将其导出？！