Same value keeps appearing in Javascript callback

Question

I have the following bit of code in Node.js.

function homeCallback(reply, twid) {
    var c = reply.length;

    for (var i = c - 1; i >= 0; i--) {
        var isRT;
        var tweet_id;

        if (reply[i].hasOwnProperty('retweeted_status')) {
            tweet_id = reply[i].retweeted_status.id_str;
            isRT = true;
        } else {
            tweet_id = reply[i].id_str;
            isRT = false;
        }

        console.log(tweet_id);
        var existsQ = "SELECT * FROM tweets WHERE tweet_id=" + connection.escape(tweet_id);

        connection.query(existsQ, function (err, rows) {
            console.log(tweet_id);
            //need to use tweet_id here
        });
    }
}

reply is a json response from a call to stauses/home_timeline of Twitter's API, connection is a mysql connection

If there are a couple of tweets in reply with ids of 11 and 12 I get an output like this:

Although I expect an output like this:

Are you able to create a [jsFiddle](http://jsfiddle.net/) using this code to demonstrate? — Xotic750, Feb 17 '14 at 17:21
[This](http://pastebin.com/9aZEtXCD) is an example of what reply can be. — Jasjeev, Feb 17 '14 at 17:31
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/47684/discussion-between-jasjeev-singh-and-xotic750) — Jasjeev, Feb 17 '14 at 17:33
Sorry, can't. So here is a starting [jsFiddle](http://jsfiddle.net/Xotic750/hT8Z3/) with your function and the `canned` data you provided. Try filling in the rest of the details for us. — Xotic750, Feb 17 '14 at 17:43
Except connection is an object of [this](https://github.com/felixge/node-mysql) Node js module. I don't know how to get that working in jsFiddle — Jasjeev, Feb 17 '14 at 17:46
So [here you go](http://jsfiddle.net/Xotic750/hT8Z3/2/) Is that the problem that you are having? — Xotic750, Feb 17 '14 at 18:01
I am actually using twid in my actual function. I have posted a simplified version of my actual function. — Jasjeev, Feb 17 '14 at 18:06
Ok, but in your example it is not used. The question is, did my example demonstrate the issue that you are having? — Xotic750, Feb 17 '14 at 18:08
Then you need to capture the variable in a closure. Lots of information here on SO. Here is one particular example that relates to your problem. http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example — Xotic750, Feb 17 '14 at 18:15
And now, does this [jsFiddle](http://jsfiddle.net/Xotic750/hT8Z3/3/) solve the problem? Is the output as you expect? — Xotic750, Feb 17 '14 at 18:15

score 0 · Answer 1 · answered Feb 17 '14 at 19:12

The callback on connection.query() would execute asynchronously while local variable tweet_id scoped in the for-loop might get overwritten by next iteration.

Try to duplicate tweet_id inside a closure/function call.

(function(tid) {
        console.log(tid);
        var existsQ = "SELECT * FROM tweets WHERE tweet_id=" + connection.escape(tid);

        connection.query(existsQ, function (err, rows) {
            console.log(tid);
            //need to use tweet_id here
        });
})(tweet_id);

This is not ideal, but would give you some idea.

Same value keeps appearing in Javascript callback

1 Answers1