不能在 NodeJs 中要求 class 吗？

Question

我有两个文件：

mysqlDAOFactory.js:

var mysql = require('mysql');
var mysqlUserDAO = require('./mysqlUserDAO');
var mysqlSessionDAO = require('./mysqlSessionDAO');


class mysqlDAOFactory {

    static createConnection() {
        var connection = mysql.createConnection({
            host:'localhost',
            user:'root',
            password:'',
            database:'QASys'
        });
        return connection;
    }

    static getDbInstance() {
        return mysql;
    }

    getUserDAO() {
        return new mysqlUserDAO();
    }
    getSessionDAO() {
        return new mysqlSessionDAO();
    }
}

module.exports = mysqlDAOFactory;

mysqlUserDAO.js:

var mysqlDAOFactory = require('./mysqlDAOFactory.js');
var bcrypt = require('bcryptjs');
var preparedStatements = require('./preparedStatements');

class mysqlUserDAO {
    constructor() {
        this.connection = mysqlDAOFactory.createConnection();
        this.mysql = mysqlDAOFactory.getDbInstance();
    }
}

module.exports = mysqlUserDAO;

当我运行文件test.js:

var mysqlDAOFactory = require('./mysqlDAOFactory.js');
var UserDAO = mysqlDAOFactory.getUserDAO();

程序无法识别mysqlUserDAO.js中class的构造函数中的变量mysqlDAOFactory，虽然我之前需要这个变量。谁能向我解释为什么会这样？

results in chrome debugger tool

Answer 1

我查看了你的 cod，你犯了循环依赖的错误，因为你没有运行一个代码。

因为你没有post MySQL session 文件，所以我临时注释了mysqlSession文件。

请查看运行ning 示例，然后也请查看 link。 https://repl.it/@DipakC/SO-50542396cannot-require-a-class-in-nodejs

test.js:

//var mysqlDAOFactory = require('./mysqlDAOFactory');
const mysqlUserDAO = require("./mysqlUserDAO");
let objMysqlUserDAO = new mysqlUserDAO();
console.log("---- ---- ----");
console.log(objMysqlUserDAO);
console.log("---- ---- ----");

mysqlUserDAO.js

var mysqlDAOFactory = require('./mysqlDAOFactory');
var bcrypt = require('bcryptjs');    
class mysqlUserDAO {
    constructor() {
        this.connection = mysqlDAOFactory.createConnection();
        this.mysql = mysqlDAOFactory.getDbInstance();
    }
}   
module.exports = mysqlUserDAO;

mysqlDAOFactory.js

var mysql = require('mysql');
var mysqlUserDAO = require('./mysqlUserDAO');
//var mysqlSessionDAO = require('./mysqlSessionDAO');
class mysqlDAOFactory {
    static createConnection() {
        var connection = mysql.createConnection({
            host:'localhost',
            user:'root',
            password:'',
            database:'QASys'
        });
        return connection;
    }

    static getDbInstance() {
        return mysql;
    }

    static getUserDAO() {
        return new mysqlUserDAO();
    }
    /* getSessionDAO() {
        return new mysqlSessionDAO();
    } */
}
module.exports = mysqlDAOFactory;

不能在 NodeJs 中要求 class 吗？

Cannot require a class in NodeJs?

node.js

es6-class