Node.js: Best way to perform multiple async operations, then do something else?

HACK-FREE SOLUTION\n\nPromises to be included in next JavaScript version\n\nThe popular Promise libraries give you an .all() method for this exact use case (waiting for a bunch of async calls to complete, then doing something else). It's the perfect match for your scenario\n\nBluebird also has .map(), which can take an array of values and use it to start a Promise chain. \n\nHere is an example using Bluebird .map(): \n\nvar Promise = require('bluebird');\nvar request = Promise.promisifyAll(require('request'));\n\nfunction processAllFeeds(feedsToFetch) { \n return Promise.map(feedsToFetch, function(feed){ \n // I renamed your 'feed' fn to 'processFeed'\n return processFeed(feed) \n })\n .then(function(articles){\n // 'articles' is now an array w/ results of all 'processFeed' calls\n // do something with all the results...\n })\n .catch(function(e){\n // feed server was down, etc\n })\n}\n\nfunction processFeed(feed) { \n // use the promisified version of 'get'\n return request.getAsync(feed.url)... \n}\n\n\nNotice also that you don't need to use closure here to accumulate the results.\n\nThe Bluebird API Docs are really well written too, with lots of examples, so it makes it easier to pick up. \n\nOnce I learned Promise pattern, it made life so much easier. I can't recommend it enough. \n\nAlso, here is a great article about different approaches to dealing with async functions using promises, the async module, and others\n\nHope this helps! ",

No hacks necessary\n\nI would recommend using the async module as it makes these kinds of things a lot easier.\n\nasync provides async.eachSeries as an async replacement for arr.forEach and allows you to pass a done callback function when it's complete. It will process each items in a series, just as forEach does. Also, it will conveniently bubble errors to your callback so that you don't have to have handler logic inside the loop. If you want/require parallel processing, you can use async.each.\n\nThere will be no blocking between the async.eachSeries call and the callback.\n\nasync.eachSeries(feedsToFetch, function(feedUri, done) {\n\n // call your async function\n feed(feedUri, function(err, feedArticles) {\n\n // if there's an error, \"bubble\" it to the callback\n if (err) return done(err);\n\n // your operation here;\n articles = articles.concat(feedArticles);\n\n // this task is done\n done();\n });\n}, function(err) {\n\n // errors generated in the loop above will be accessible here\n if (err) throw err;\n\n // we're all done!\n console.log(\"all done!\");\n});\n\n\nAlternatively, you could build an array of async operations and pass them to async.series. Series will process your results in a series (not parallel) and call the callback when each function is complete. The only reason to use this over async.eachSeries would be if you preferred the familiar arr.forEach syntax.\n\n// create an array of async tasks\nvar tasks = [];\n\nfeedsToFetch.forEach(function (feedUri) {\n\n // add each task to the task array\n tasks.push(function() {\n\n // your operations\n feed(feedUri, function(err, feedArticles) {\n if (err) throw err;\n articles = articles.concat(feedArticles);\n });\n });\n});\n\n// call async.series with the task array and callback\nasync.series(tasks, function() {\n console.log(\"done !\");\n});\n\n\n\n\nOr you can Roll Your Own™\n\nPerhaps you're feeling extra ambitious or maybe you don't want to rely upon the async dependency. Maybe you're just bored like I was. Anyway, I purposely copied the API of async.eachSeries to make it easy to understand how this works.\n\nOnce we remove the comments here, we have just 9 lines of code that can be reused for any array we want to process asynchronously! It will not modify the original array, errors can be sent to \"short circuit\" the iteration, and a separate callback can be used. It will also work on empty arrays. Quite a bit of functionality for just 9 lines :)\n\n// void asyncForEach(Array arr, Function iterator, Function callback)\n// * iterator(item, done) - done can be called with an err to shortcut to callback\n// * callback(done) - done recieves error if an iterator sent one\nfunction asyncForEach(arr, iterator, callback) {\n\n // create a cloned queue of arr\n var queue = arr.slice(0);\n\n // create a recursive iterator\n function next(err) {\n\n // if there's an error, bubble to callback\n if (err) return callback(err);\n\n // if the queue is empty, call the callback with no error\n if (queue.length === 0) return callback(null);\n\n // call the callback with our task\n // we pass `next` here so the task can let us know when to move on to the next task\n iterator(queue.shift(), next);\n }\n\n // start the loop;\n next();\n}\n\n\nNow let's create a sample async function to use with it. We'll fake the delay with a setTimeout of 500 ms here.\n\n// void sampleAsync(String uri, Function done)\n// * done receives message string after 500 ms\nfunction sampleAsync(uri, done) {\n\n // fake delay of 500 ms\n setTimeout(function() {\n\n // our operation\n // <= \"foo\"\n // => \"async foo !\"\n var message = [\"async\", uri, \"!\"].join(\" \");\n\n // call done with our result\n done(message);\n }, 500);\n}\n\n\nOk, let's see how they work !\n\ntasks = [\"cat\", \"hat\", \"wat\"];\n\nasyncForEach(tasks, function(uri, done) {\n sampleAsync(uri, function(message) {\n console.log(message);\n done();\n });\n}, function() {\n console.log(\"done\");\n});\n\n\nOutput (500 ms delay before each output)\n\nasync cat !\nasync hat !\nasync wat !\ndone\n",