undefined on this string to array

Question

ok am starting on nodeJS and am stuck at this code, if i output inside the loop i get the array as i want, but if i do outside the loop it returns undefined.

question is: What is wrong with this?, thx in advance;

var array = [];
var temp = "";

var http = require("http");

for (var i = 2; i < 5; i++) {
    http.get(process.argv[i],function(res){
        res.setEncoding("utf8");
        res.on("data",function(data){
            temp += data;
        });
        res.on("end", function(){

            array.push(temp);
            console.log(array[0]);//this returns the array[0] OK NP;
            temp = "";
        });}

    );

}
console.log(array[0]);//this returns undefined;

In the console, the one that shows undefined happens first, right? that should clue you in on what's going on. — Kevin B, Apr 24 '15 at 19:29
`.get` is asynchronous. Your array hasn't been populated yet. — Matt Burland, Apr 24 '15 at 19:31
yes, you are right, the undefined is printed first, but that makes sense since its an asynchronous request — Tymofek, Apr 24 '15 at 19:33
And if you understand that, it should clear why your array hasn't been populated yet. — idbehold, Apr 24 '15 at 19:40
your approach for multiple ajax through for loop is wrong, you need to use promise/deffered then function for simultaneous multiple ajax calls. — vinayakj, Apr 24 '15 at 19:42
@vinayakj it isn't wrong, it just isn't a good way of doing it in your opinion. — Kevin B, Apr 24 '15 at 19:48

score 2 · Accepted Answer · answered Apr 24 '15 at 19:31

2

It's because the code is asynchronous and when you call console.log outside of the http request not yet finshed so the array[0] is undefined.

answered Apr 24 '15 at 19:31

jcubic

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score 1 · Answer 2 · answered Apr 24 '15 at 19:34

It's because "console.log" is being run before "array.push". This is because array.push happens at the end of the response, and since node.js follows event based programming, the array.push only happens when the end of the response is triggered. Console.log happens before res.on "end" gets triggered.

score 0 · Answer 3 · answered Apr 24 '15 at 19:48

In your code you are basically launching 3 calls to 3 destinations and then printing the contents of the array before getting any response.

If you want to print the resoults after all three calls has been completed you may use a counter, init it at 0 before the loop and increase it by one before any http call. Then, in the end event decrease the counter by 1 and if the counter has reached 0 print the array or do any other stuff you want.

Hope this helps

undefined on this string to array

3 Answers3