CODEWITHSUNDEEP
javascriptcallback
xShirase
xShirase

Reputation: 12399

Passing variable into named callback

OK, let's keep it simple, I have the following :

function loopDaLoop(){
    for (var i = 0; i < tempItemsLength; i++) {
        var product = tempItems[i];
        dust.render('product', product, addProductOrFinish);
    }
}

and i'd like to get the current value of i inside my callback function

function addProductOrFinish(err, out) {
   console.log(i); // undefined
}

I do know it's simple, really I do... Help?

Edit : I know I'm supposed to use a closure so I tried and failed with this :

(function(i){
    dust.render('product', product, addProductOrFinish);
};(i)

Upvotes: 0

Views: 75

Answers (1)

jfriend00
jfriend00

Reputation: 707376

There are several different structures that could be used to solve this problem. The simplest is to use .bind() to add the desired parameter to the function call.

function loopDaLoop(){
    for (var i = 0; i < tempItemsLength; i++) {
        var product = tempItems[i];
        dust.render('product', product, addProductOrFinish.bind(null, i));
    }
}

function addProductOrFinish(i, err, out) {
   console.log(i);
}

This will cause the value of this to change in addProductOrFinish if that was important. If so, you could work around that too, but it's not as simple.

Here's another approach using a closure that returns a function and preserves the value of this in case dust.render() is setting that:

function loopDaLoop(){
    for (var i = 0; i < tempItemsLength; i++) {
        var product = tempItems[i];
        dust.render('product', product, getAddProductOrFinish(i));
    }
}

function getAddProductOrFinish(loopArg) {
     return function(err, out) {
         return addProductOrFinish.call(this, loopArg, err, out);
     }
}

function addProductOrFinish(i, err, out) {
   console.log(i);
}

Or, if addProductOrFinish can be an inline function, then it can use the IIFE-type structure you tried like this:

function loopDaLoop(){
    for (var i = 0; i < tempItemsLength; i++) {
        var product = tempItems[i];
        (function(i) {
            dust.render('product', product, function(err, out) {
                console.log(i);
                // rest of your addProductOrFinish logic here
            });
        )(i);
    }
}

Upvotes: 2

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