Как правильно закрывать соединение с MySQL в express.js?

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');

var mysql = require('mysql');

var indexRouter = require('./routes/index');
var usersLoginRouter = require('./routes/usersLogin');
var checkTokenRouter = require('./routes/checkToken');
var usersRegRouter = require('./routes/usersReg');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'twig');

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use(function (req, res, next) {
    res.setHeader('Content-Type', 'application/json');

    next();
});

app.use(function (req, res, next) {
    var connection = mysql.createConnection({
        host     : 'localhost',
        user     : 'loto_users',
        password : 'loto_users',
        database : 'loto_users'
    });

    connection.connect();

    connection.on('error', function (error) {

    });

    res.mysql = connection;

    next();
});

app.use('/', indexRouter);
app.use('/users/login', usersLoginRouter);
app.use('/users/check/token', checkTokenRouter);
app.use('/users/reg', usersRegRouter);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

var express = require('express');
var router = express.Router();
var crypto = require('crypto');

router.get('/', function(req, res, next) {
    const token = req.query.hasOwnProperty('token') ? req.query['token'] : null;
    
    res.mysql.query('SELECT * FROM user_tokens WHERE token = ?', [token], function (error, results, fields) {
        if (error || results.length === 0) {
            return res.send(JSON.stringify({
                ok: false,
                message: 'Wrong token'
            }));
        }

        const user_token = results[0];

        if (user_token['end_at'] < Math.floor(new Date() / 1000)) {
            res.mysql.query('DELETE FROM user_tokens WHERE token = ?', [token]);

            return res.send(JSON.stringify({
                ok: false,
                message: 'Token expire'
            }));
        }

        res.send(JSON.stringify({
            ok: true,
            user_id: user_token['user_id'],
            end_at: user_token['end_at'],
            message: 'OK'
        }));
    });

});

module.exports = router;