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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) [](https://gitter.im/caolan/async?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) *For Async v1.5.x documentation, go [HERE](https://github.com/caolan/async/blob/v1.5.2/README.md)* 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 --save 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 70 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 asynchronous function -- a callback which expects an Error as its first argument -- and calling the callback once. ## Quick Examples ```js 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'], function(filePath, callback) { fs.access(filePath, function(err) { callback(null, !err) }); }, function(err, 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 _{[(StackOverflow)](http://stackoverflow.com/questions/tagged/async.js)} ### 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 iteratee. 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.setImmediate` 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 iteratee(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 iteratee(item, callback) { if (inCache(item)) { async.setImmediate(function () { callback(null, cache[item]); }); } else { doSomeIO(item, callback); //... } }); ``` Async does not guard against synchronous iteratees for performance reasons. 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. ### Multiple callbacks Make sure to always `return` when calling a callback early, otherwise you will cause multiple callbacks and unpredictable behavior in many cases. ```js async.waterfall([ function (callback) { getSomething(options, function (err, result) { if (err) { callback(new Error("failed getting something:" + err.message)); // we should return here } // since we did not return, this callback still will be called and // `processData` will be called twice callback(null, result); }); }, processData ], done) ``` It is always good practice to `return callback(err, result)` whenever a callback call is not the last statement of a function. ### Binding a context to an iteratee This section is really about `bind`, not about `async`. If you are wondering how to make `async` execute your iteratees in a given context, or are confused as to why a method of another library isn't working as an iteratee, 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 iteratee 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 npm: npm install --save async As well as using Bower: bower install async You can then `require()` async as normal: ```js var async = require("async"); ``` Or require individual methods: ```js var waterfall = require("async/waterfall"); var map = require("async/map"); ``` __Development:__ [async.js](https://github.com/caolan/async/raw/master/lib/async.js) - 29.6kb Uncompressed ### In the Browser Async should work in any ES5 environment (IE9 and above). Usage: ```html <script type="text/javascript" src="async.js"></script> <script type="text/javascript"> async.map(data, asyncProcess, function(err, results){ alert(results); }); </script> ``` ### ES Modules We also provide async as a collection of ES2015 modules, in an alternative `async-es` package on npm. npm i -S async-es ```js import waterfall from 'async-es/waterfall'; import async from 'async-es'; ``` ## Documentation Some functions are also available in the following forms: * `<name>Series` - the same as `<name>` but runs only a single async operation at a time * `<name>Limit` - the same as `<name>` but runs a maximum of `limit` async operations at a time ### Collections * [`each`](#each), `eachSeries`, `eachLimit` * [`forEachOf`](#forEachOf), `forEachOfSeries`, `forEachOfLimit` * [`map`](#map), `mapSeries`, `mapLimit` * [`filter`](#filter), `filterSeries`, `filterLimit` * [`reject`