Javascript - waiting for a number of asynchronous callbacks to return?

Question

What's the best way/library for handling multiple asynchronous callbacks? Right now, I have something like this:

_.each(stuff, function(thing){
   async(thing, callback);
});

I need to execute some code after the callback has been fired for each element in stuff.

What's the cleanest way to do this? I'm open to using libraries.

Answer 1

Since you're already using Underscore you might look at _.after. It does exactly what you're asking for. From the docs:

after    _.after(count, function)

Creates a version of the function that will only be run after first being called count times. Useful for grouping asynchronous responses, where you want to be sure that all the async calls have finished, before proceeding.

Answer 2

There is a great library called Async.js that helps solve problems like this with many async & flow control helpers. It provides several forEach functions that can help you run callbacks for every item in a an array/object.

Check out: https://github.com/caolan/async#forEach

// will print 1,2,3,4,5,6,7,all done

var arr = [1,2,3,4,5,6,7];

function doSomething(item, done) {
  setTimeout(function() {
    console.log(item);
    done(); // call this when you're done with whatever you're doing
  }, 50);
}

async.forEach(arr, doSomething, function(err) {
    console.log("all done");
});

Answer 3

I recommend https://github.com/caolan/async for this. You can use async.parallel to do this.

function stuffDoer(thing) {
    return function (callback) {
        //Do stuff here with thing
        callback(null, thing);
    }
}

var work = _.map(stuff, stuffDoer)
async.parallel(work, function (error, results) {
    //error will be defined if anything passed an error to the callback
    //results will be an unordered array of whatever return value if any
    //the worker functions passed to the callback
}

Answer 4

async.parallel() / async.series should suit your requirement. You can provide with a final callback that gets executed when all the REST calls succeed.

async.parallel([
    function(){ ... },
    function(){ ... }
], callback);
async.series([
    function(){ ... },
    function(){ ... }
], callback);