callback function - NodeJS

Question

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?

It's not clear what you're asking. The export of weather.js is a function. You call that function with a callback function parameter. — Dave Newton
– Dave Newton, Commented Oct 18, 2016 at 16:26
"But how is possible? " I think his real question is "how it's work?".. — Marwen Trabelsi
– Marwen Trabelsi, Commented Oct 18, 2016 at 16:35

Community · Accepted Answer · 2017-05-23 10:27:30Z

1

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 is possible?

Functions in Javascript are a first class object means that you can store a function into a variable and pass it into another function. This pattern is very common on Node.js and in Javasript in general, this is called the Continuation passing style ( CPS )

Hope it helps.

edited May 23, 2017 at 10:27

CommunityBot

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answered Oct 18, 2016 at 16:31

Marwen Trabelsi

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1 Comment

Morris Over a year ago

Clear! Thank you so much :)

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