Javascript, outside variable scope in callback function?

Question

everyone.

I have a problem about the 'callback function - scope of variable',

I wanna use the 'i in for loop' to 'the callback function User_UidSearch',

but I cannot use it.

(I hope the solution don't use the global variable.)

Task_TidSearchExecution = function(tid, callback) {
    var sql = "SELECT * FROM execution WHERE task = '" + tid + "'";
    dbclient.query(sql, function (err, results) {
        if (err || results.length <= 0)
            callback(false);
        else {
            console.log(results.length);
            for (var i = 0 ; i < results.length ; i++) {
                User_UidSearch(results[i].employee, function (user) {
                    console.log(i);
                    // results[i]['email'] = user.email;
                });
            }

            callback(results);
        }
    });
}

the "console.log(i);"

Recheck, this is wrong. -> Outputs are "undefined" of all.

undefined is "console.log(result[i]);"

But the "i" is keep '2' console twice, if results.length is 2.

I know becuz the for loop ending then execute the User_UidSearch,

but how can I slove the it "i" is 0 and 1.

No, it's not. It's defined in outer function local scope. But it's used in closure, which needs special logic to work. @Salmon - are you sure console.log(i) writes undefined and not the value of results.length? — Ilya Luzyanin
– Ilya Luzyanin, Commented Aug 13, 2014 at 7:55
"Outputs are 'undefined' of all" doesn't really fit with the snippet provided. i should at least be a number, just maybe not the particular values expected. If each log shows the value of results.length, then see: JavaScript closure inside loops – simple practical example. — Jonathan Lonowski
– Jonathan Lonowski, Commented Aug 13, 2014 at 7:56
wrap the user_UidSearch call in a closure, pass in i and results — MartinWebb
– MartinWebb, Commented Aug 13, 2014 at 7:57
@IlyaLuzyanin: rigt.my bad i didnt see it through clearly.removed my comment — Prabhu Murthy
– Prabhu Murthy, Commented Aug 13, 2014 at 7:58

Community · Accepted Answer · 2017-05-23 10:29:50Z

3

Your problem may be solved, but let me still add these points so others know the correct approach.

Firstly, its not a good practice to call an asynchronous function inside a loop. This would lead to unexpected results.

Using closures is fine for some cases, but not this one. Read here for Practical uses of closures.

There are several problems in the approach you've taken,

If the results array is too long, there'll be a too many open requests.
callback(results) will be called before even a single callback of User_UidSearch is called.

In your case, you should be using recursive function like this:

var len = results.length;
var count = 0;
function my_func() {
    User_UidSearch(results[count].employee, function (user) {
        console.log(count);
        // results[count]['email'] = user.email;

        count += 1;
        if (count < len) {
            my_func();
        } else {
            callback(results);
        }
    });
}

Read this if interested detailed explaination of different control flows.

edited May 23, 2017 at 10:29

CommunityBot

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answered Aug 13, 2014 at 9:10

Salman

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1 Comment

Salmon Over a year ago

Yes, you're right. I had solved your saying. But your solution is better than mine, thank you very much :)

Ilya Luzyanin · Accepted Answer · 2014-08-13 08:41:03Z

1

You're dealing with closures, rewrite your code as follows:

...
(function(id) {
    User_UidSearch(results[id].employee, function (user) {
        console.log(id);
        // results[i]['email'] = user.email;
    });
})(i);

I.e. wrap your function to unbind i.

answered Aug 13, 2014 at 8:41

Ilya Luzyanin

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2 Comments

Salmon Over a year ago

Thank you very much, I follow this way solving.

Ilya Luzyanin Over a year ago

You're welcome! Closures are great stuff if you know how to use them right!

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