Asynchronous Process inside a javascript for loop

I am running an event loop of the following form:

var i;
var j = 10;
for (i = 0; i < j; i++) {

    asynchronousProcess(callbackFunction() {

        alert(i);
    });
}

I am trying to display a series of alerts showing the numbers 0 through 10. The problem is that by the time the callback function is triggered, the loop has already gone through a few iterations and it displays a higher value of i. Any recommendations on how to fix this?

Answer

The for loop runs immediately to completion while all your asynchronous operations are started. When they complete some time in the future and call their callbacks, the value of your loop index variable i will be at its last value for all the callbacks.

This is because the for loop does not wait for an asynchronous operation to complete before continuing on to the next iteration of the loop and because the async callbacks are called some time in the future. Thus, the loop completes its iterations and THEN the callbacks get called when those async operations finish. As such, the loop index is "done" and sitting at its final value for all the callbacks.

To work around this, you have to uniquely save the loop index separately for each callback. In Javascript, the way to do that is to capture it in a function closure. That can either be done be creating an inline function closure specifically for this purpose (first example shown below) or you can create an external function that you pass the index to and let it maintain the index uniquely for you (second example shown below).

As of 2016, if you have a fully up-to-spec ES6 implementation of Javascript, you can also use let to define the for loop variable and it will be uniquely defined for each iteration of the for loop (third implementation below). But, note this is a late implementation feature in ES6 implementations so you have to make sure your execution environment supports that option.

Use .forEach() to iterate since it creates its own function closure

someArray.forEach(function(item, i) {
    asynchronousProcess(function(item) {
        console.log(i);

    });
});

Create Your Own Function Closure Using an IIFE

var j = 10;
for (var i = 0; i < j; i++) {
    (function(cntr) {
        // here the value of i was passed into as the argument cntr

        // and will be captured in this function closure so each
        // iteration of the loop can have it's own value
        asynchronousProcess(function() {
            console.log(cntr);
        });
    })(i);
}

Create or Modify External Function and Pass it the Variable

If you can modify the asynchronousProcess() function, then you could just pass the value in there and have the asynchronousProcess() function the cntr back to the callback like this:

var j = 10;
for (var i = 0; i < j; i++) {
    asynchronousProcess(i, function(cntr) {
        console.log(cntr);
    });
}

Use ES6 let

If you have a Javascript execution environment that fully supports ES6, you can use let in your for loop like this:

const j = 10;
for (let i = 0; i < j; i++) {
    asynchronousProcess(function() {
        console.log(i);
    });

}

let declared in a for loop declaration like this will create a unique value of i for each invocation of the loop (which is what you want).

Serializing with promises and async/await

If your async function returns a promise, and you want to serialize your async operations to run one after another instead of in parallel and you're running in a modern environment that supports async and await, then you have more options.

async function someFunction() {

    const j = 10;
    for (let i = 0; i < j; i++) {
        // wait for the promise to resolve before advancing the for loop
        await asynchronousProcess();
        console.log(i);
    }
}

This will make sure that only one call to asynchronousProcess() is in flight at a time and the for loop won't even advance until each one is done. This is different than the previous schemes that all ran your asynchronous operations in parallel so it depends entirely upon which design you want. Note: await works with a promise so your function has to return a promise that is resolved/rejected when the asynchronous operation is complete. Also, note that in order to use await, the containing function must be declared async.

Run asynchronous operations in parallel and use Promise.all() to collect results in order

 function someFunction() {
     let promises = [];
     for (let i = 0; i < 10; i++) {
          promises.push(asynchonousProcessThatReturnsPromise());
     }
     return Promise.all(promises);
 }


 someFunction().then(results => {
     // array of results in order here
     console.log(results);
 }).catch(err => {
     console.log(err);
 });