Question

139

Calling an asynchronous function within a for loop in JavaScript

rated 0 times [ 140] [ 1] / answers: 1 / hits: 79761 / 12 Years ago, mon, november 12, 2012, 12:00:00

I have the following code:

for(var i = 0; i < list.length; i++){

    mc_cli.get(list[i], function(err, response) {

        do_something(i);

    });

}

mc_cli is a connection to a memcached database. As you can imagine, the callback function is asynchronous, thus it may be executed when the for loop already ended. Also, when calling in this way do_something(i) it always uses the last value of the for loop.

I tried with a closure in this way

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

but apparently this is again using always the last value of the index of the for loop.

I also tried declaring a function before the for loop like so:

var create_closure = function(i) {

    return function() {

        return i;

    }

}

and then calling

do_something(create_closure(i)())

but again without success, with the return value always being the last value of the for loop.

Can anybody tell me what am I doing wrong with closures? I thought I understood them but I can't figure why this is not working.

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

marinal

Add To Favorites

Follow

Total Points: 655

Total Questions: 99

Total Answers: 99

Location: Svalbard and Jan Mayen

Member since Sun, Sep 25, 2022

2 Years ago

answered 12 Years ago janettejordynm · Accepted Answer

Since you're running through an array, you can simply use forEach which provides the list item, and the index in the callback. Iteration will have its own scope.

list.forEach(function(listItem, index){

  mc_cli.get(listItem, function(err, response) {

    do_something(index);

  });

});