函数调用在有机会设置重要数据之前被传递 [英] function call is passed over before having the chance to set important data

问题描述

所以我有这个功能在node.js获取外部IP从 https://icanhazip.com/ （方便的网站，检查出来） ，但是当调用该请求时，不允许在调用 return 之前完成该请求。这里是代码：

So I have this function in node.js that gets the external IP by grabbing it from https://icanhazip.com/ (handy website, check it out), but when the request is called it isn't allowed to finish before return is called. Here is the code:

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：如何让这个脚本等待回调函数在返回值之前完成

So I guess, the question here is: how do I make this script wait for that callback function to finish before returning a value?

推荐答案

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

That's because of the asynchronous nature of I/O requests in Node: the function get_ext_ip returns before the request has finished.

处理此问题的一种方法是传递一个回调函数将在请求完成时被调用：

One of the methods of handling this is to pass a callback function which will get called when the request is done:

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);
});

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