path: root/intro.md

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*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](https://nodejs.org/) and installable via `npm install --save async`,
it can also be used directly in the browser.

Async is also installable via:

- [yarn](https://yarnpkg.com/en/): `yarn add async`
- [bower](http://bower.io/): `bower 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(callback) { ... },
    function(callback) { ... }
], function(err, results) {
    // optional callback
});

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

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.

### Using ES2017 `async` functions

Async accepts `async` functions wherever we accept a Node-style callback function.  However, we do not pass them a callback, and instead use the return value and handle any promise rejections or errors thrown.

```js
async.mapLimit(files, 10, async file => { // <- no callback!
    const text = await util.promisify(fs.readFile)(dir + file, 'utf8')
    const body = JSON.parse(text) // <- a parse error herre will be caught automatically
    if (!(await checkValidity(body))) {
        throw new Error(`${file} has invalid contents`) // <- this error will also be caught
    }
    return body // <- return a value!
}, (err, contents) => {
    if (err) throw err
    console.log(contents)
})
```

We can only detect native `async` functions, not transpiled versions (e.g. with Babel).  Otherwise, you can wrap `async` functions in `async.asyncify()`.

### 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.
});
```

### Subtle Memory Leaks

There are cases where you might want to exit early from async flow, when calling an Async method inside another async function:

```javascript
function myFunction (args, outerCallback) {
    async.waterfall([
        //...
        function (arg, next) {
            if (someImportantCondition()) {
                return outerCallback(null)
            }
        },
        function (arg, next) {/*...*/}
    ], function done (err) {
        //...
    })
}
```

Something happened in a waterfall where you want to skip the rest of the execution, so you call an outer callack.  However, Async will still wait for that inner `next` callback to be called, leaving some closure scope allocated.

As of version 3.0, you can call any Async callback with `false` as the `error` argument, and the rest of the execution of the Async method will be stopped or ignored.

```javascript
        function (arg, next) {
            if (someImportantCondition()) {
                outerCallback(null)
                return next(false) // ← signal that you called an outer callback
            }
        },
```

### Mutating collections

If you pass an array to a collection method (such as `each`, `mapLimit`, or `filterSeries`), and then attempt to `push`, `pop`, or `splice` additional items on to the array, this could lead to unexpected or undefined behavior.  Async will iterate until the original `length` of the array is met, and the indexes of items `pop()`ed or `splice()`d could already have been processed. Therefore, it is not recommended to modify the array after Async has begun iterating over it.  If you do need to `push`, `pop`, or `splice`, use a `queue` instead.


## Download

The source is available for download from
[GitHub](https://raw.githubusercontent.com/caolan/async/master/dist/async.min.js).
Alternatively, you can install using npm:

```bash
$ npm install --save async
```

As well as using Bower:

```bash
$ 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://raw.githubusercontent.com/caolan/async/master/dist/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>
```

The portable versions of Async, including `async.js` and `async.min.js`, are
included in the `/dist` folder. Async can also be found on the [jsDelivr CDN](http://www.jsdelivr.com/projects/async).

### ES Modules

We also provide Async as a collection of ES2015 modules, in an alternative `async-es` package on npm.

```bash
$ npm install --save async-es
```

```js
import waterfall from 'async-es/waterfall';
import async from 'async-es';
```

## Other Libraries

* [`limiter`](https://www.npmjs.com/package/limiter) a package for rate-limiting based on requests per sec/hour.
* [`neo-async`](https://www.npmjs.com/package/neo-async) an altername implementation of Async, focusing on speed.
* [`co-async`](https://www.npmjs.com/package/co-async) a library inspired by Async for use with [`co`](https://www.npmjs.com/package/co) and generator functions.
*  [`promise-async`](https://www.npmjs.com/package/promise-async) a version of Async where all the methods are Promisified.