如何在javascript中的回调函数中获取结果？

Question

如果我想将结果写入回调函数内部进行响应，但我无法访问函数中的 res 变量，也无法访问函数外部的结果。

那么如何在内部和外部之间传递值呢？

var http = require('http');
var url = require('url');

http.createServer
(
    function (req, res)
    {
        var output='';
        res.writeHead(200, {'Content-Type': 'text/plain'});


        //The url we want is: 'www.random.org/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        var options = {
          host: 'www.random.org',
          path: '/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        };

        callback = function(response) {
          var str = '';

          //another chunk of data has been recieved, so append it to `str`
          response.on('data', function (chunk) {
            str += chunk;
          });

          //the whole response has been recieved, so we just print it out here
          response.on('end', function () {
            console.log(str);

            //output+=str; //get result
          });
        }

        http.request(options, callback).end();


        //output+=str; //get result
        res.end(output);
    }
).listen(80, '127.0.0.1');

---

ver2

var http = require('http');
var url = require('url');

http.createServer
(
    function (req, res)
    {
        var output='';
        res.writeHead(200, {'Content-Type': 'text/plain'});


        //The url we want is: 'www.random.org/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        var options = {
          host: 'www.random.org',
          path: '/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        };

        callback = function(response) {
          var str = '';

          //another chunk of data has been recieved, so append it to `str`
          response.on('data', function (chunk) {
            str += chunk;
          });

          //the whole response has been recieved, so we just print it out here
          response.on('end', function () {
            res.write('inside');
          });
        }

        http.request(options, callback).end();

        res.write('outside');
        res.end(output);
    }
).listen(80, '127.0.0.1');

---

ver3

var http = require('http');
var url = require('url');

http.createServer
(
    function (req, res)
    {
        res.writeHead(200, {'Content-Type': 'text/plain'});


        //The url we want is: 'www.random.org/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        var options = {
          host: 'www.random.org',
          path: '/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        };

        callback = function(res) {
          var str = '';

          //another chunk of data has been recieved, so append it to `str`
          res.on('data', function (chunk) {
            str += chunk;
          });

          //the whole response has been recieved, so we just print it out here
          res.on('end', function () {
            res.write('inside');
            //or
            this.write('inside');
          });
        }

        http.request(options, callback).end();


        res.write('outside');
        res.end();
    }
).listen(80, '127.0.0.1');

---

ver4

var http = require('http');
var url = require('url');

http.createServer
(
    function (req, res)
    {
        res.writeHead(200, {'Content-Type': 'text/plain'});


        //The url we want is: 'www.random.org/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        var options = {
          host: 'www.random.org',
          path: '/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new'
        };

        callback = function(response) {
          var str = '';

          //another chunk of data has been recieved, so append it to `str`
          response.on('data', function (chunk) {
            str += chunk;
          });

          //the whole response has been recieved, so we just print it out here
          response.on('end', function () {
            res.write('inside');
          });
        }

        http.request(options, callback);


        res.write('outside');
        res.end();
    }
).listen(80, '127.0.0.1');

Answer 1

你不能。设置回调的函数将在回调执行之前完成并返回。如果没有，则不需要回调。

在回调本身中做任何你需要做的工作，而不是在父函数中。

我无法访问函数中的 res 变量

你应该可以的。它仍在范围内。

（虽然如果你在回调运行之前调用end()，那么应该会损坏）。