node.js : Variables outside of functions

Question

I'm new to NodeJS and I was trying to figure out stuff but unfortunately I couldn't find information on it with my knowledge.

Basicly, I want to use a variable from a function inside another function wich is a child of the main function.

Here is my code :

http.get(url, function(res) {
var body = '';

res.on('data', function(chunk) {
    body += chunk;
});

res.on('end', function() {
    var jsonResult = JSON.parse(body);
    for (var i=0;i<5;i++)
    {
        gameId = jsonResult.gameList[i].gameId;
        url = 'http://somesite.com/' + gameId + '/0/token';
        http.get(url, function(res) {
            var body = '';

            res.on('data', function(chunk) {
                body += chunk;
            });

            res.on('end', function() {
                jsonRes = JSON.parse(body);
                switch(i)
                {
                    case 0:
                        var elo0 = jsonRes.interestScore;
                        module.exports.elo0 = elo0;
                        break;
                    case 1:
                        var elo1 = jsonRes.interestScore;
                        module.exports.elo1 = elo1;
                        break;
                    case 2:
                        var elo2 = jsonRes.interestScore;
                        module.exports.elo2 = elo2;
                        break;
                    case 3:
                        var elo3 = jsonRes.interestScore;
                        module.exports.elo3 = elo3;
                        break;
                    case 4:
                        var elo4 = jsonRes.interestScore;
                        module.exports.elo4 = elo4;
                        break;

                }


        });
        }).on('error', function(e) {
            console.log("Got error: ", e);
        });

    }

});
}).on('error', function(e) {
  console.log("Got error: ", e);
});

Note that I didn't include everything, just the problematic part. I want to use the variable i from the loop inside the switch but it doesn't work.

Move the inner part of the loop into its own function and call it passing in i (and anything else you need). This is a classic js problem - referencing a loop counter inside an asynch — dc5
– dc5, Commented Aug 8, 2013 at 4:14
Could you please give me an example on that? I'm not quite sure I understand where to move the loop. — user2663041
– user2663041, Commented Aug 8, 2013 at 4:21
For future reference. Perhaps, the answer is easy, but I think you can scrape the code (omit unnecesary part to show the structure) to present what exactly you need. 'use the variable i from the loop inside the switch. is not that clear. — user1028880
– user1028880, Commented Aug 8, 2013 at 4:22
Here is a better explanation: jslinterrors.com/dont-make-functions-within-a-loop I will post an example a bit later unless someone else beats me to it. — dc5
– dc5, Commented Aug 8, 2013 at 4:29
I tought I needed to make the function within a loop because I would actually need it at this part of the code : gameId = jsonResult.gameList[i].gameId; — user2663041
– user2663041, Commented Aug 8, 2013 at 4:36

dc5 · Accepted Answer · 2013-08-08 05:16:31Z

The problem here is that you are referencing i in your switch statement within an asynchronous callback. When you do this, you won't get the value of i at the time the function is created, but the final value of i at the end of the loop iteration.

There are a couple of ways to fix this - both involve trapping the current loop value of i in a closure for later reference by the callback.

for example:

for (var i=0;i<5;i++)
{
    (function(idx) {
        gameId = jsonResult.gameList[idx].gameId;
        url = 'http://somesite.com/' + gameId + '/0/token';
        http.get(url, function(res) {
            ...

            res.on('end', function() {
                jsonRes = JSON.parse(body);
                switch(idx)
                {
                    case 0:
                      break;

                    ...
                }
            });
            ...
        });
    })(i);
}

Here, an anonymous function is created for each pass through the loop and called immediately passing the current value of the loop counter i as the incoming parameter idx.

Another approach (as mentioned in the comments above) is to refactor the inner part of your loop into a separate function and call it passing all necessary context:

function scoreHandler(jsonResult, idx) {
    var gameId = jsonResult.gameList[idx].gameId;
    var url = 'http://somesite.com/' + gameId + '/0/token';
    http.get(url, function(res) {
        ...

        res.on('end', function() {
            jsonRes = JSON.parse(body);
            switch (idx) {
               ...
            }
        });
    })
    .on('error', function(e) {
        console.log("Got error: ", e);
    });
}

Your refactored loop would look something like:

res.on('end', function() {
    var jsonResult = JSON.parse(body);
    for (var i = 0; i < 5; i++) {
        scoreHandler(jsonResult, i);
    }
});

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