Passing loop counter as argument in Node.js

Question

I was trying this as a tutorial from nodeschool, and I'm new to Node.js. The code is below, and I know the problem in it, but I am not able to resolve it. The problem is that j has a value of 3 for every iteration of the loop inside the bl function, but Why is that happening?

var bl = require('bl');
var http = require('http');
var urls = process.argv.slice(2);



var result = [];

for (var i = 0; i < urls.length; i++) {
    result.push(null);
}

for(j = 0 ; j < urls.length; ++j){
    http.get(urls[j],function(response){
        response.pipe(bl(function(err,data){
            //console.log(result[i]);
            //console.log(data.toString());
            result[j] = data.toString();
            console.log('J : ' + j);
            console.log(data.toString());
            var flag = 0;
            for (var i = 0; i < result.length; i++) {
                console.log('here1');
                console.log(result[i]);
                if(result[i] == null){
                    flag = 1;
                    console.log('here');
                    break;
                }
            }
            if(flag == 0){
                for (var i = 0; i < result.length; i++) {
                    console.log(result[i]);
                }
            }
        }));
    });
}

@GiuseppePes Its a part of tutorial from nodeschool, learnyounode. This is one of the core workshop and is run into command line. — phraniiac
– phraniiac, Commented Jun 28, 2015 at 12:33

Jose Mato · Accepted Answer · 2015-06-28 09:34:42Z

http.get is an async request but the for is sync so for is "most fast" and when http.get finish to download the url data then variable "j" takes last value.

You have another mistake I think, on your for loop you increase variable "j" as "++j" and it will be "j++".

To fix the first issue (variable "j" value) you can use an anonymous function and pass the value "j" like:

for(j = 0 ; j < urls.length; j++) {
    (function(j) {
        http.get(urls[j],function(response){
            response.pipe(bl(function(err,data){
                //console.log(result[i]);
                //console.log(data.toString());
                result[j] = data.toString();
                console.log('J : ' + j);
                console.log(data.toString());
                var flag = 0;
                for (var i = 0; i < result.length; i++) {
                    console.log('here1');
                    console.log(result[i]);
                    if(result[i] == null){
                        flag = 1;
                        console.log('here');
                        break;
                    }
                }
                if(flag == 0){
                    for (var i = 0; i < result.length; i++) {
                        console.log(result[i]);
                    }
                }
            }));
        });
    }(j));
}

There are a lot of code but in resume i did this:

for(j = 0 ; j < urls.length; j++) {
    (function(j) {
        /* your code inside this function will have the correct 
        value to variable "j" if you use async methods */
    } (j));
}

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