variable scope in module asynchronous function

Question

This is my first week in node so I'm sorry if this is a no brainier.

The code works and does what it should. But I can't figure out how to match the name (url) that starts http.get whit the result it gets from the website.

I found this witch is almost like my problem, except this is a premade function so I can't edit the function and add a callback.

variable scope in asynchronous function

If I could run this code synchronous or make a callback in the http.get function it would all be good. But I don't have the skills and don't know if you even can do it.

Thanks - Robin.

http = require('http');
function download(name) {
//name is an array whit csgo items names.
for (var i = 0; i < name.length; i++) {

    var marketHashName = getGoodName(name[i]);
    var url = 'http://steamcommunity.com/market/priceoverview/?currency=1&appid=730&market_hash_name=' + marketHashName;

    http.get(url, function (res) {
        var data = "";
        res.on('data', function (chunk) {
            data += chunk;
        });
        res.on("end", function () {

            data = JSON.parse(data);
            var value= 0;
            //get the value in the json array
            if(data.median_price) {
                value = data.median_price;
            }else{
                value = data.lowest_price;
            }

            value = value.substr(5);
            console.log("WEAPON",value);
            //callback whit name/link and value?
            //callback(name,value);
        });
    }).on("error", function () {

    });
}

}

jfriend00 · Accepted Answer · 2015-03-21 00:37:43Z

1

You can just add a callback argument and then call it with the final data. And, if you want to pass to the callback the particular marketHashName that was being processed, then you can create a closure to capture that uniquely for each time through the for loop:

http = require('http');

function download(name, callback) {
    //name is an array whit csgo items names.
    for (var i = 0; i < name.length; i++) {

        var marketHashName = getGoodName(name[i]);
        // create closure to capture marketHashName uniquely for each 
        // iteration of the for loop
        (function(theName) {
            var url = 'http://steamcommunity.com/market/priceoverview/?currency=1&appid=730&market_hash_name=' + marketHashName;

            http.get(url, function (res) {
                var data = "";
                res.on('data', function (chunk) {
                    data += chunk;
                });
                res.on("end", function () {

                    data = JSON.parse(data);
                    var value= 0;
                    //get the value in the json array
                    if(data.median_price) {
                        value = data.median_price;
                    }else{
                        value = data.lowest_price;
                    }

                    value = value.substr(5);
                    console.log("WEAPON",value);

                    // now that the async function is done, call the callback
                    // and pass it our results
                    callback(theName, value, data);
                });
            }).on("error", function () {

            });
        })(marketHasName);
    }
}

// sample usage:
download("whatever", function(name, value, data) {
   // put your code here to use the results
});

FYI, you may find that the request module which is a higher level set of functionality on top of the http module will save you some work.

edited Mar 21, 2015 at 0:37

answered Mar 21, 2015 at 0:32

jfriend00

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3 Comments

zexuz Over a year ago

Thanks! Whats is this method/way of doing it called if i wan't to learn more about it. Or is it callbacks 101?

jfriend00 Over a year ago

@user2034253 - it's just asynchronous callbacks.

jfriend00 Over a year ago

@user2034253 - FYI, if you want to read more about handling asynchronous operations, you may want to read about promises. I use the Bluebird promise library for my node.js development and find it much easier than just callbacks, particular when trying to coordinate multiple async operations.

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