回调函数 - NodeJS(callback function - NodeJS)

 我创建了一个简短的例子，我有一个dubt： 
 
var request = require("request");
var url = "http://api.openweathermap.org/data/2.5/weather?q=turin&APPID=xxxxxxxxxxxxxxxxxxxxxx";
module.exports = function (callback) {

    request(
    {
        url: url,
        json: true
    }, function (error, response, body) {
        if (error) {
            callback("Unable to fetch weather"); // callback function
        } else {
            callback("It is " + body.main.temp + " in " + body.name);
        }
    });

    console.log("After request");
};
 
 从外部文件，我需要这个模块： 
 
var weather = require("./weather.js");

weather(function (currentWeather) {
    console.log(currentWeather);
});
 
 在这种情况下，我调用一个weather模块，我得到一个callback函数（它是天气模块的参数），用于在都灵打印到命令行的天气。 但它是如何工作的？ 

i create an short example and i have a dubt:
 
var request = require("request");
var url = "http://api.openweathermap.org/data/2.5/weather?q=turin&APPID=xxxxxxxxxxxxxxxxxxxxxx";
module.exports = function (callback) {

    request(
    {
        url: url,
        json: true
    }, function (error, response, body) {
        if (error) {
            callback("Unable to fetch weather"); // callback function
        } else {
            callback("It is " + body.main.temp + " in " + body.name);
        }
    });

    console.log("After request");
};
 
And from external file, i required this module:
 
var weather = require("./weather.js");

weather(function (currentWeather) {
    console.log(currentWeather);
});
 
In this case, i call a weather module and i get a callback function ( it is an argument of weather module ) for print into command line the weather in Turin. But how it's work? 

原文：https://stackoverflow.com/questions/40113342