NodeJS callback in a for-loop value is the same

Question

I was cleaning up my code and ran into a little trouble with callbacks, specifically getting the correct values to output when a callback is called. Could some explain to me why the following code spits out something that I'm not expecting and a possible solution without having to put in another parameter of i to the run() function, or is passing in i to know my index upon calling the callback the only way to do this?

for (var i in dls) {
    run(dls[i][0], dls[i][1], function(isTrue){
        if (isTrue) {
            // Do true stuff here
        } else {
            console.log("Value is: " + dls[i][3])
        }
    });
}

Calling run() actually has the correct inputs inside, but upon that function calling the callback and going into the else statement, dls[i][3] spits out the same value i times.

I've tried putting different scopes around (run()) and such but to no avail and can't seem to wrap my head around this.

Thanks

EDIT:

If I wanted to split it up into a separate function, how would I do it?

var run = function(cb){
    setTimeout(function() {
        cb(false)
    }, 3000);
}

for (var i in dls) {
    run(dls[i][0], dls[i][1], (function(index) {
        return extraction
    })(i));
}

function extraction(isTrue){
    if (isTrue) {
        // stuff
    } else {
        console.log("Nothing changed in " + dls[i][3])
    }
}

Here dls[i][3] is still incorrect and prints the same value 3 times.

Answer 1

You have fallen into the traditional "loop trap"

When it comes time for your callback to run i is now a different value.

What you can do is cache that value in another wrapper function:

for (var i in dls) {
    run(dls[i][0], dls[i][1], (function (currentIndex) { 
        return function(isTrue){
            if (isTrue) {
                // Do true stuff here
            } else {
                console.log("Value is: " + dls[currentIndex][3])
            }
        };
    })(i));
}

Answer 2

In regards to the edit / second question, assuming this is what you wanted to do:

// note that I changed the function signature of `run`
var run = function(val1, val2, cb) {
    setTimeout(function() {
        cb(false);
    }, 3000);
};

// note the `wrapper` here
for (var i in dls) {
    run(dls[i][0], dls[i][1], wrapper(i));
}

// this is the same as what the IIFE is doing,
// just with an external function instead
function wrapper(scopedIndex) {

    // return a function to be used as the callback for `run`
    return function extraction(isTrue) {
        if (isTrue) {
            // stuff
        }
        else {
            // use the scoped index here
            console.log("Nothing changed in " + dls[scopedIndex][3]);
        }
    }
}

Take a look at function makeExitCallback(i) in the other linked question , as well. It directly relates to what's going on here.

You should post what's in dls as well, just to make it easier to run your snippets locally.