javascript - 在有机会设置重要数据之前就忽略了函数调用

Question

所以我在node.js中有这个函数，它通过从https://icanhazip.com/(方便的网站，检查一下)获取外部IP，但是当请求是调用它不允许在调用 return 之前完成。这是代码:

var request = require('request');
var get_ext_ip = function(){
    var _ip; //Initialize a return variable

    //Launch an HTTPS request to a simple service that shows IP of client
    //The callback function isn't allowed to finish.
    request('https://icanhazip.com', function(error, response, body){
        _ip = body; //when finished store the response in the variable that is going to be returned.
    });

    //Return
    return _ip;
}

var ip = get_ext_ip();
console.log(ip); //undefined

所以我想，这里的问题是:如何让这个脚本在返回值之前等待回调函数完成？

Answer 1

这是因为 Node 中 I/O 请求的异步特性:函数 get_ext_ip 在请求完成之前返回。

处理此问题的方法之一是传递一个回调函数，该函数将在请求完成时被调用:

var request = require('request');
var get_ext_ip = function(callback) {
  request('https://icanhazip.com', function(error, response, body) {
    callback(error, body);
  });
}

get_ext_ip(function(err, ip) {
  if (err)
    console.log('an error occurred:', error);
  else
    console.log(ip);
});