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# Async.js [](https://travis-ci.org/caolan/async) [](https://www.npmjs.org/package/async) [](https://coveralls.io/r/caolan/async?branch=master) Async is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript. Although originally designed for use with [Node.js](http://nodejs.org) and installable via `npm install async`, it can also be used directly in the browser. Async is also installable via: - [bower](http://bower.io/): `bower install async` - [component](https://github.com/component/component): `component install caolan/async` - [jam](http://jamjs.org/): `jam install async` - [spm](http://spmjs.io/): `spm install async` Async provides around 20 functions that include the usual 'functional' suspects (`map`, `reduce`, `filter`, `each`…) as well as some common patterns for asynchronous control flow (`parallel`, `series`, `waterfall`…). All these functions assume you follow the Node.js convention of providing a single callback as the last argument of your `async` function. ## Quick Examples ```javascript async.map(['file1','file2','file3'], fs.stat, function(err, results){ // results is now an array of stats for each file }); async.filter(['file1','file2','file3'], fs.exists, function(results){ // results now equals an array of the existing files }); async.parallel([ function(){ ... }, function(){ ... } ], callback); async.series([ function(){ ... }, function(){ ... } ]); ``` There are many more functions available so take a look at the docs below for a full list. This module aims to be comprehensive, so if you feel anything is missing please create a GitHub issue for it. ## Common Pitfalls <a name="stack-overflow"> ### Synchronous iteration functions If you get an error like `RangeError: Maximum call stack size exceeded.` or other stack overflow issues when using async, you are likely using a synchronous iterator. By *synchronous* we mean a function that calls its callback on the same tick in the javascript event loop, without doing any I/O or using any timers. Calling many callbacks iteratively will quickly overflow the stack. If you run into this issue, just defer your callback with `async.nextTick` to start a new call stack on the next tick of the event loop. This can also arise by accident if you callback early in certain cases: ```js async.eachSeries(hugeArray, function iterator(item, callback) { if (inCache(item)) { callback(null, cache[item]); // if many items are cached, you'll overflow } else { doSomeIO(item, callback); } }, function done() { //... }); ``` Just change it to: ```js async.eachSeries(hugeArray, function iterator(item, callback) { if (inCache(item)) { async.setImmediate(function () { callback(null, cache[item]); }); } else { doSomeIO(item, callback); //... ``` Async guards against synchronous functions in some, but not all, cases. If you are still running into stack overflows, you can defer as suggested above, or wrap functions with [`async.ensureAsync`](#ensureAsync) Functions that are asynchronous by their nature do not have this problem and don't need the extra callback deferral. If javascript's event loop is still a bit nebulous, check out [this article](http://blog.carbonfive.com/2013/10/27/the-javascript-event-loop-explained/) or [this talk](http://2014.jsconf.eu/speakers/philip-roberts-what-the-heck-is-the-event-loop-anyway.html) for more detailed information about how it works. ### Binding a context to an iterator This section is really about `bind`, not about `async`. If you are wondering how to make `async` execute your iterators in a given context, or are confused as to why a method of another library isn't working as an iterator, study this example: ```js // Here is a simple object with an (unnecessarily roundabout) squaring method var AsyncSquaringLibrary = { squareExponent: 2, square: function(number, callback){ var result = Math.pow(number, this.squareExponent); setTimeout(function(){ callback(null, result); }, 200); } }; async.map([1, 2, 3], AsyncSquaringLibrary.square, function(err, result){ // result is [NaN, NaN, NaN] // This fails because the `this.squareExponent` expression in the square // function is not evaluated in the context of AsyncSquaringLibrary, and is // therefore undefined. }); async.map([1, 2, 3], AsyncSquaringLibrary.square.bind(AsyncSquaringLibrary), function(err, result){ // result is [1, 4, 9] // With the help of bind we can attach a context to the iterator before // passing it to async. Now the square function will be executed in its // 'home' AsyncSquaringLibrary context and the value of `this.squareExponent` // will be as expected. }); ``` ## Download The source is available for download from [GitHub](https://github.com/caolan/async/blob/master/lib/async.js). Alternatively, you can install using Node Package Manager (`npm`): npm install async As well as using Bower: bower install async __Development:__ [async.js](https://github.com/caolan/async/raw/master/lib/async.js) - 29.6kb Uncompressed ## In the Browser So far it's been tested in IE6, IE7, IE8, FF3.6 and Chrome 5. Usage: ```html <script type="text/javascript" src="async.js"></script> <script type="text/javascript"> async.map(data, asyncProcess, function(err, results){ alert(results); }); </script> ``` ## Documentation ### Collections * [`each`](#each) * [`eachSeries`](#eachSeries) * [`eachLimit`](#eachLimit) * [`forEachOf`](#forEachOf) * [`forEachOfSeries`](#forEachOfSeries) * [`forEachOfLimit`](#forEachOfLimit) * [`map`](#map) * [`mapSeries`](#mapSeries) * [`mapLimit`](#mapLimit) * [`filter`](#filter) * [`filterSeries`](#filterSeries) * [`reject`](#reject) * [`rejectSeries`](#rejectSeries) * [`reduce`](#reduce) * [`reduceRight`](#reduceRight) * [`detect`](#detect) * [`detectSeries`](#detectSeries) * [`sortBy`](#sortBy) * [`some`](#some) * [`every`](#every) * [`concat`](#concat) * [`concatSeries`](#concatSeries) ### Control Flow * [`series`](#seriestasks-callback) * [`parallel`](#parallel) * [`parallelLimit`](#parallellimittasks-limit-callback) * [`whilst`](#whilst) * [`doWhilst`](#doWhilst) * [`until`](#until) * [`doUntil`](#doUntil) * [`forever`](#forever) * [`waterfall`](#waterfall) * [`compose`](#compose) * [`seq`](#seq) * [`applyEach`](#applyEach) * [`applyEachSeries`](#applyEachSeries) * [`queue`](#queue) * [`priorityQueue`](#priorityQueue) * [`cargo`](#cargo) * [`auto`](#auto) * [`retry`](#retry) * [`iterator`](#iterator) * [`apply`](#apply) * [`nextTick`](#nextTick) * [`times`](#times) * [`timesSeries`](#timesSeries) * [`timesLimit`](#timesLimit) ### Utils * [`memoize`](#memoize) * [`unmemoize`](#unmemoize) * [`ensureAsync`](#ensureAsync) * [`log`](#log) * [`dir`](#dir) * [`noConflict`](#noConflict) ## Collections <a name="forEach" /> <a name="each" /> ### each(arr, iterator, [callback]) Applies the function `iterator` to each item in `arr`, in parallel. The `iterator` is called with an item from the list, and a callback for when it has finished. If the `iterator` passes an error to its `callback`, the main `callback` (for the `each` function) is immediately called with the error. Note, that since this function applies `iterator` to each item in parallel, there is no guarantee that the iterator functions will complete in order. __Arguments__ * `arr` - An array to iterate over. * `iterator(item, callback)` - A function to apply to each item in `arr`. The iterator is passed a `callback(err)` which must be called once it has completed. If no error has occurred, the `callback` should be run without arguments or with an explicit `null` argument. The array index