如何从nodejs中的其他文件回调函数？

Question

我开始使用异步编程，我做了一个nodejs应用程序，我在一些文件中分割了代码：index.js，ctlUser.js，DAO.js等等...... Index.js是主要文件它需要ctlUser和ctlUser需要DAO.js ... DAO连接到数据库和执行查询...

抽象，我的结构是这样的......

Index.js

var ctlUser = require('./ctlUser.js'); 
var username = ctlUser.getUserName('1'); 
console.log("Return from ctlUser" + username); 

ctlUser.js

var DAO = require('./DAO.js'); 
var getUserName = function(id){ 
    var userName = DAO.executeQuery("SELECT username FROM tbUsers WHERE id = " + id); 
    console.log(Return from DAO = userName); 
    return username; 
} 

个

DAO.js这里是样样精...

var mysql  = require('mysql'); 
var executeQuery = function(query) {  
     var connection = mysql.createConnection({ 
      host  : SERVER, 
      user  : USER, 
      password : PASSWORD, 
      database : DATABASE 
     }); 
     connection.connect(); 
     connection.query(query, function(err, rows, fields) { 
      if (err) throw err; 
      connection.end(); 
      console.log("Here in DAO: " + rows[0].username); 
      return rows[0].username; 
     }); 
    }; 

的$ node index.js输出是：

Return of ctlUser: undefined 
Return of DAO: undefined 
Here in DAO: Filipe Tagliacozzi 

林抽象所有模块的出口everithing正常工作与固定变量，但数据库的响应不要..如何在此结构中实现回调以将userName引用到index.js？

Answer 1

更改DAO.js模块中的executeQuery函数以使用回调：

var executeQuery = function(query,callback) {  
     var connection = mysql.createConnection({ 
      host  : SERVER, 
      user  : USER, 
      password : PASSWORD, 
      database : DATABASE 
     }); 
     connection.connect(); 
     connection.query(query, function(err, rows, fields) { 
      if (err) throw err; 
      connection.end(); 
      console.log("Here in DAO: " + rows[0].username); 
      callback(rows[0].username); 
     }); 
    }; 

链通过您getUserName函数的回调中ctlUser.js：

var getUserName = function(id,callback){ 
     DAO.executeQuery("SELECT username FROM tbUsers WHERE id = " + id ,function(username){ 
      console.log("Return from DAO =" ,userName); 
      callback(username); 
     });  
    } 

然后使用它在index.js像这样：

var ctlUser = require('./ctlUser.js'); 
ctlUser.getUserName('1',function(username){ 
    console.log("Return from ctlUser" + username); 
}); 

Answer 2

它们未定义，因为您尚未导出任何内容。

节点文件在您需要它们时执行，但返回的内容是导出的内容。

what-is-require

在ctlUser.js需要getUserName导出为这样： -

exports.getUserName = getUserName; 

并在您需要的executeQuery导出为这样DAO.js： -

exports.executeQuery = executeQuery;