How can I pass an incrementing for-loop variable to a function in node.js?

Question

How can I pass an incrementing for-loop variable to a function in node.js?

Possible Duplicate:
How to pass values to a callback function at the time of the request call?

I know this is a general problem and not only in node.js. I have to pass a variable from the for loop to a function which I want to generate. The functions are stored in an array and have to remember these variables.

Here is my example:

for(var i = 0; i < administratorIds.length; i ++){
  adminCallbacks[adminCallbacks.length] = function(callback){
    model_user.getById(administratorIds[i], function(user){
      administrators[administrators.length] = {name: user.getName()};
      callback(null, null);
    });
  };
}

javascript
node.js
for-loop

Answer 1

You need to create a new variable scope for each iteration of the loop.

Only way to do that will be by invoking a function.

for(var i = 0; i < administratorIds.length; i++){
  adminCallbacks[adminCallbacks.length] = createHandler(i); // pass i
}

 // receive i as j-----v
function createHandler(j) {
     // return a function...
    return function(callback){
          // ...that uses j-----------------v
        model_user.getById(administratorIds[j], function(user){
          administrators[administrators.length] = {name: user.getName()};
          callback(null, null);
        });
    };
}

Answer 2

a closure perhaps?

for(var i = 0; i < administratorIds.length; i ++){
    adminCallbacks.push(
        (function(i){
            return function(callback){
                model_user.getById(
                    administratorIds[i],
                    function(user){
                        administrators[administrators.length] = {name: user.getName()};
                        callback(null, null);
                    }
                );
            };
        })(i)
    );
}

Yikes... look at that madness. Anyway, this part:

(function(i){
    // stuff!
})(i);

captures the current value for i in a closure and thus ensures that it doesn't change once the callback is fired.

An example:

var i = 10;
var lockedI = (function(i){
    return function(){ console.log('i is --> ' + i); };
})(i);
var unlockedI = function(){ console.log('i is --> ' + i); };

lockedI();
// i is --> 10

unlockedI();
// i is --> 10

var i = 666;

lockedI();
// i is --> 10;

unlockedI();
// i is --> 666

Victory?