Tuesday, May 28, 2013

Closures...

I'm sorry for the long lack of updates. Recently I've been very busy. I travelled a lot and I had a lot of work to do, so not really much time to work on my project and discover new interesting things.
Today I want to post something that bothered me for a while. I was calling an asynchronous function, m_cli.get(), inside a for loop and wanted to keep the index variable as it was going to be used in the callback.
The first approach was the following, and of course was not working:

for(var i = 0; i < list.length; i++){
    mc_cli.get(list[i], function(err, response) {
        do_something(i);
    });
}


With this approach, the callback would always execute using the last value of i. So, for example, in the list was long 10, it would always call do_something(9). To fix the problem I tried with a closure:

do_something((function(x){return x})(i)) 


The idea is to "keep" somehow the variable so that it could call the function only when the callback returns and keeps the right index. Unfortunately, also this approach wont work. Also by creating the closure outside the for-loop and calling it in the callback would not lead to a satisfying result.
Later on, I managed somehow to fix the problem like this:

for(var i = 0; i < list.length; i++){
    (function(i){
        mc_cli.get( parsed_result.list[i], function(err, response) {
            do_something(i);
        });
    }(i));
}


The idea is that the parameter now is the input parameter of an anonymous function which will then call the asynchronous function and when the callback fires, call the do_something(i) which the local input parameter, which is the correct index (and not the last element of the array). Basically it will treat the index as an input parameter, thus "remembering" it through the execution of the asynchronous function.
Hope I helped somebody!
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