How to determine whether an asychronous call is the last one or not

Question

How to determine whether an asychronous call is the last one or not

Working with an asynchronous language (node.js in my case) I often end up with a two dimensional loop and an response at the very end:

firstArray.forEach(function(first){
     sencondArray[first].forEach(function(second){

         doStuff(function callback(resultSum){
             if(isLastCallback(this))
                 response.send(resultSum);
         });

     });
});

My first approach to get the proper result of isLastCallback, was using the provided counter i of both forEach iterations, but that ended up in a huge mess.

What is the common way to detect if the current run of callback is the last one?

node.js
design-patterns
asynchronous

Answer 1

I think what you're looking for is to do several asynchronous calls and then combine the results. You should look at the async module. You probably want async#parallel. It will run your async functions in parallel and then call the final callback with an array of all the results.