Javascript Node.js从回调函数中提取值

我有一个服务器文件，带有一个使用URL显示适当内容的开关。其中一个例子是/users，它应该显示某个表的JSON字符串。这是从mysql文件返回的

server.js

var http = require('http')
var url = require('url')
var port = 8080

function onRequest(request, response) {
    var pathname = url.parse(request.url).pathname
    console.log('Request for ' + pathname + ' received.')

    response.writeHead(200, {'Content-Type': 'text/html'})
    response.write(run(pathname))
    response.end()
}

function run(pathname) {
    switch(pathname) {
        case '/':
            response = 'Welcome to my little test'
            break
        case '/time':
            response = 'The time is ' + new Date().toLocaleTimeString()
            break
        case '/users':
            var response
            require('./mysql').getUsers(function(users) {
                console.log(users)
                response = users
            })
            return response
            break
        default:
            response = 'Unable to locate the requested page'
    }
    return response
}

http.createServer(onRequest).listen(port)
console.log('Server started on port ' + port + '.')

var mysql = require('mysql')

var connection = mysql.createConnection({ 
    user: "root", 
    password: "password", 
    database: "main"
})

exports.getUsers = function(callback) {
    connection.query('SELECT * FROM users;', function (error, rows, fields) {
        callback(JSON.stringify(rows));
    });
};

mysql.js

var http = require('http')
var url = require('url')
var port = 8080

function onRequest(request, response) {
    var pathname = url.parse(request.url).pathname
    console.log('Request for ' + pathname + ' received.')

    response.writeHead(200, {'Content-Type': 'text/html'})
    response.write(run(pathname))
    response.end()
}

function run(pathname) {
    switch(pathname) {
        case '/':
            response = 'Welcome to my little test'
            break
        case '/time':
            response = 'The time is ' + new Date().toLocaleTimeString()
            break
        case '/users':
            var response
            require('./mysql').getUsers(function(users) {
                console.log(users)
                response = users
            })
            return response
            break
        default:
            response = 'Unable to locate the requested page'
    }
    return response
}

http.createServer(onRequest).listen(port)
console.log('Server started on port ' + port + '.')

var mysql = require('mysql')

var connection = mysql.createConnection({ 
    user: "root", 
    password: "password", 
    database: "main"
})

exports.getUsers = function(callback) {
    connection.query('SELECT * FROM users;', function (error, rows, fields) {
        callback(JSON.stringify(rows));
    });
};

server.js中的

console.log（users）

可以很好地显示JSON字符串，但我不知道如何从回调中获取值并将其放入响应变量中

---

如果您有任何想法，我们将不胜感激。

从回调中提取值的方法是将该值分配给回调范围之外的变量，但我不建议您这样做，因为这样会产生大量全局变量，而且您不知道何时将分配该变量。尝试此操作，看看会发生什么，以便了解回调和node.js的工作原理：

function run(pathname) {
    switch(pathname) {
        case '/':
            response = 'Welcome to my little test'
            break
        case '/time':
            response = 'The time is ' + new Date().toLocaleTimeString()
            break
        case '/users':
            var response
            var out_of_callback_users
            require('./mysql').getUsers(function(users) {
                out_of_callback_users = users
                console.log("In the callback")
                console.log(users)
                response = users
            })
            console.log("After require");
            console.log(out_of_callback_users) //Users have not been assigned yet
            setTimeout(function(){
              console.log("In the timeout") 
              console.log(out_of_callback_users)
            },5000) //After 5 secs the query has been completed and users have been assigned.
            return response
            break
        default:
            response = 'Unable to locate the requested page'
    }
    return response
}

我要走的路是这样的：

function onRequest(request, response) {
    var pathname = url.parse(request.url).pathname
    console.log('Request for ' + pathname + ' received.')

    response.writeHead(200, {'Content-Type': 'text/html'})
    run(pathname, function(response){
      response.write(response)
      response.end()
    })
}

function run(pathname,cb) {
    switch(pathname) {
        case '/':
            cb('Welcome to my little test');
            break;
        case '/time':
            cb('The time is ' + new Date().toLocaleTimeString());
            break;
        case '/users':
            require('./mysql').getUsers(function(users) {
                console.log(users);
                cb(users);
            })
            break;
        default:
            cb('Unable to locate the requested page');
    }
    return;
}

http.createServer(onRequest).listen(port)
console.log('Server started on port ' + port + '.')

 case '/users':
   var response
   require('./mysql').getUsers(function(users) {
     console.log(users)
     response = users
   })
   return response
   break

---

您不需要序列化mysql返回的

行

即可使用它。您可以在

getUsers

中处理它，也可以将它返回控制器。如果返回，请将代码更改为：

exports.getUsers = function(callback) {
    connection.query('SELECT * FROM users;', function (error, rows, fields) {
        callback(rows);
    });
};

现在，在

server.js

文件中，您可以处理返回的行，如：

case '/users':
  var response = ''
  require('./mysql').getUsers(function(users) {
    for (var i in users) {
      var user = users[i];
      var userId = user.id;
      var userName = user.user_name;
      response += "User - ID: "+userId+" Name: "+userName+"\n";
    }
  })
  return response;
break;

---

你可以处理

你不能这样做。问题很简单。让我们来谈谈： 函数

getUsers

是一个异步函数。因此，代码如下所示：

function onRequest(request, response) {
    var pathname = url.parse(request.url).pathname
    console.log('Request for ' + pathname + ' received.')

    response.writeHead(200, {'Content-Type': 'text/html'})
    run(pathname, function(response){
      response.write(response)
      response.end()
    })
}

function run(pathname,cb) {
    switch(pathname) {
        case '/':
            cb('Welcome to my little test');
            break;
        case '/time':
            cb('The time is ' + new Date().toLocaleTimeString());
            break;
        case '/users':
            require('./mysql').getUsers(function(users) {
                console.log(users);
                cb(users);
            })
            break;
        default:
            cb('Unable to locate the requested page');
    }
    return;
}

http.createServer(onRequest).listen(port)
console.log('Server started on port ' + port + '.')

 case '/users':
   var response
   require('./mysql').getUsers(function(users) {
     console.log(users)
     response = users
   })
   return response
   break

首先，运行

require（'./mysql'）.getUser（）

，然后直接执行

返回响应

，然后执行

中断

。当

getUser

函数完成时，它将运行

 function(users) {
     console.log(users)
     response = users
   })

因此，您需要遵循一条规则：一旦使用异步，另一个函数必须是异步的。 我想知道你可以修改如下：

function onRequest(request, response) {
  var pathname = url.parse(request.url).pathname
  console.log('Request for ' + pathname + ' received.')

  response.writeHead(200, {'Content-Type': 'text/html'})
  run(pathname, function(res){ response.write(res)})  //changed
  response.end()
}

function run(pathname, callback) {
  switch(pathname) {
      case '/':
        callback('Welcome to my little test')
        break
      case '/time':
        callback('The time is ' + new Date().toLocaleTimeString())
        break
      case '/users':
        var response
        require('./mysql').getUsers(function(users) {
            console.log(users)
            callback(users) # changed
        })
        break
      default:
        callback('Unable to locate the requested page')
  }
}

http.createServer(onRequest).listen(port)
console.log('Server started on port ' + port + '.')

---

尝试在以下语句之后打印

响应

：

response=users

。在我看来，它应该工作得很好。回调的值位于

users

变量中，您正在将其分配给

响应

变量。但是，我不确定如果

return response

语句等待

getUsers

函数完成，或者在它不等待之前触发，这就是问题所在。我发布了一个答案，希望能对您有所帮助，或者他可以在

console.log（users）

@RaulRene之后调用

return

语句，如果他在

console.log（users）

之后调用

return

，该语句将从传递给

getUsers

函数的回调返回，不是从

run

function这不会尝试修复原始问题。很抱歉误解，我只是想帮助您。我很高兴你现在有办法了。