README.md

# Async.js

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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 <sub>[(StackOverflow)](http://stackoverflow.com/questions/tagged/async.js)</sub>
### 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.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 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.


### 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 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

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`](#reject), `rejectSeries`, `rejectLimit`
* [`reduce`](#reduce), [`reduceRight`](#reduceRight)
* [`detect`](#detect), `detectSeries`, `detectLimit`
* [`sortBy`](#sortBy)
* [`some`](#some), `someLimit`
* [`every`](#every), `everyLimit`
* [`concat`](#concat), `concatSeries`

### Control Flow

* [`series`](#seriestasks-callback)
* [`parallel`](#parallel), `parallelLimit`
* [`whilst`](#whilst), [`doWhilst`](#doWhilst)
* [`until`](#until), [`doUntil`](#doUntil)
* [`during`](#during), [`doDuring`](#doDuring)
* [`forever`](#forever)
* [`waterfall`](#waterfall)
* [`compose`](#compose)
* [`seq`](#seq)
* [`applyEach`](#applyEach), `applyEachSeries`
* [`queue`](#queue), [`priorityQueue`](#priorityQueue)
* [`cargo`](#cargo)
* [`auto`](#auto)
* [`retry`](#retry)
* [`iterator`](#iterator)
* [`times`](#times), `timesSeries`, `timesLimit`

### Utils

* [`apply`](#apply)
* [`nextTick`](#nextTick)
* [`memoize`](#memoize)
* [`unmemoize`](#unmemoize)
* [`ensureAsync`](#ensureAsync)
* [`constant`](#constant)
* [`asyncify`](#asyncify)
* [`wrapSync`](#wrapSync)
* [`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 is not passed
  to the iterator.  If you need the index, use [`forEachOf`](#forEachOf).
* `callback(err)` - *Optional* A callback which is called when all `iterator` functions
  have finished, or an error occurs.

__Examples__


```js
// assuming openFiles is an array of file names and saveFile is a function
// to save the modified contents of that file:

async.each(openFiles, saveFile, function(err){
    // if any of the saves produced an error, err would equal that error
});
```

```js
// assuming openFiles is an array of file names

async.each(openFiles, function(file, callback) {

  // Perform operation on file here.
  console.log('Processing file ' + file);

  if( file.length > 32 ) {
    console.log('This file name is too long');
    callback('File name too long');
  } else {
    // Do work to process file here
    console.log('File processed');
    callback();
  }
}, function(err){
    // if any of the file processing produced an error, err would equal that error
    if( err ) {
      // One of the iterations produced an error.
      // All processing will now stop.
      console.log('A file failed to process');
    } else {
      console.log('All files have been processed successfully');
    }
});
```

__Related__

* eachSeries(arr, iterator, [callback])
* eachLimit(arr, limit, iterator, [callback])

---------------------------------------

<a name="forEachOf" />
<a name="eachOf" />

### forEachOf(obj, iterator, [callback])

Like `each`, except that it iterates over objects, and passes the key as the second argument to the iterator.

__Arguments__

* `obj` - An object or array to iterate over.
* `iterator(item, key, callback)` - A function to apply to each item in `obj`.
The `key` is the item's key, or index in the case of an array. 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.
* `callback(err)` - *Optional* A callback which is called when all `iterator` functions have finished, or an error occurs.

__Example__

```js
var obj = {dev: "/dev.json", test: "/test.json", prod: "/prod.json"};
var configs = {};

async.forEachOf(obj, function (value, key, callback) {
  fs.readFile(__dirname + value, "utf8", function (err, data) {
    if (err) return callback(err);
    try {
      configs[key] = JSON.parse(data);
    } catch (e) {
      return callback(e);
    }
    callback();
  })
}, function (err) {
  if (err) console.error(err.message);
  // configs is now a map of JSON data
  doSomethingWith(configs);
})
```

__Related__

* forEachOfSeries(obj, iterator, [callback])
* forEachOfLimit(obj, limit, iterator, [callback])

---------------------------------------

<a name="map" />
### map(arr, iterator, [callback])

Produces a new array of values by mapping each value in `arr` through
the `iterator` function. The `iterator` is called with an item from `arr` and a
callback for when it has finished processing. Each of these callback takes 2 arguments:
an `error`, and the transformed item from `arr`. If `iterator` passes an error to its
callback, the main `callback` (for the `map` function) is immediately called with the error.

Note, that since this function applies the `iterator` to each item in parallel,
there is no guarantee that the `iterator` functions will complete in order.
However, the results array will be in the same order as the original `arr`.

__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, transformed)` which must be called once
  it has completed with an error (which can be `null`) and a transformed item.
* `callback(err, results)` - *Optional* A callback which is called when all `iterator`
  functions have finished, or an error occurs. Results is an array of the
  transformed items from the `arr`.

__Example__

```js
async.map(['file1','file2','file3'], fs.stat, function(err, results){
    // results is now an array of stats for each file
});
```

__Related__
* mapSeries(arr, iterator, [callback])
* mapLimit(arr, limit, iterator, [callback])

---------------------------------------

<a name="select" />
<a name="filter" />
### filter(arr, iterator, [callback])

__Alias:__ `select`

Returns a new array of all the values in `arr` which pass an async truth test.
_The callback for each `iterator` call only accepts a single argument of `true` or
`false`; it does not accept an error argument first!_ This is in-line with the
way node libraries work with truth tests like `fs.exists`. This operation is
performed in parallel, but the results array will be in the same order as the
original.

__Arguments__

* `arr` - An array to iterate over.
* `iterator(item, callback)` - A truth test to apply to each item in `arr`.
  The `iterator` is passed a `callback(truthValue)`, which must be called with a
  boolean argument once it has completed.
* `callback(results)` - *Optional* A callback which is called after all the `iterator`
  functions have finished.

__Example__

```js
async.filter(['file1','file2','file3'], fs.exists, function(results){
    // results now equals an array of the existing files
});
```

__Related__

* filterSeries(arr, iterator, [callback])
* filterLimit(arr, limit, iterator, [callback])

---------------------------------------

<a name="reject" />
### reject(arr, iterator, [callback])

The opposite of [`filter`](#filter). Removes values that pass an `async` truth test.

__Related__

* rejectSeries(arr, iterator, [callback])
* rejectLimit(arr, limit, iterator, [callback])

---------------------------------------

<a name="reduce" />
### reduce(arr, memo, iterator, [callback])

__Aliases:__ `inject`, `foldl`

Reduces `arr` into a single value using an async `iterator` to return
each successive step. `memo` is the initial state of the reduction.
This function only operates in series.

For performance reasons, it may make sense to split a call to this function into
a parallel map, and then use the normal `Array.prototype.reduce` on the results.
This function is for situations where each step in the reduction needs to be async;
if you can get the data before reducing it, then it's probably a good idea to do so.

__Arguments__

* `arr` - An array to iterate over.
* `memo` - The initial state of the reduction.
* `iterator(memo, item, callback)` - A function applied to each item in the
  array to produce the next step in the reduction. The `iterator` is passed a
  `callback(err, reduction)` which accepts an optional error as its first
  argument, and the state of the reduction as the second. If an error is
  passed to the callback, the reduction is stopped and the main `callback` is
  immediately called with the error.
* `callback(err, result)` - *Optional* A callback which is called after all the `iterator`
  functions have finished. Result is the reduced value.

__Example__

```js
async.reduce([1,2,3], 0, function(memo, item, callback){
    // pointless async:
    process.nextTick(function(){
        callback(null, memo + item)
    });
}, function(err, result){
    // result is now equal to the last value of memo, which is 6
});
```

---------------------------------------

<a name="reduceRight" />
### reduceRight(arr, memo, iterator, [callback])

__Alias:__ `foldr`

Same as [`reduce`](#reduce), only operates on `arr` in reverse order.


---------------------------------------

<a name="detect" />
### detect(arr, iterator, [callback])

Returns the first value in `arr` that passes an async truth test. The
`iterator` is applied in parallel, meaning the first iterator to return `true` will
fire the detect `callback` with that result. That means the result might not be
the first item in the original `arr` (in terms of order) that passes the test.

If order within the original `arr` is important, then look at [`detectSeries`](#detectSeries).

__Arguments__

* `arr` - An array to iterate over.
* `iterator(item, callback)` - A truth test to apply to each item in `arr`.
  The iterator is passed a `callback(truthValue)` which must be called with a
  boolean argument once it has completed. **Note: this callback does not take an error as its first argument.**
* `callback(result)` - *Optional* A callback which is called as soon as any iterator returns
  `true`, or after all the `iterator` functions have finished. Result will be
  the first item in the array that passes the truth test (iterator) or the
  value `undefined` if none passed.  **Note: this callback does not take an error as its first argument.**

__Example__

```js
async.detect(['file1','file2','file3'], fs.exists, function(result){
    // result now equals the first file in the list that exists
});
```

__Related__

* detectSeries(arr, iterator, [callback])
* detectLimit(arr, limit, iterator, [callback])

---------------------------------------

<a name="sortBy" />
### sortBy(arr, iterator, [callback])

Sorts a list by the results of running each `arr` value through an async `iterator`.

__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, sortValue)` which must be called once it
  has completed with an error (which can be `null`) and a value to use as the sort
  criteria.
* `callback(err, results)` - *Optional* A callback which is called after all the `iterator`
  functions have finished, or an error occurs. Results is the items from
  the original `arr` sorted by the values returned by the `iterator` calls.

__Example__

```js
async.sortBy(['file1','file2','file3'], function(file, callback){
    fs.stat(file, function(err, stats){
        callback(err, stats.mtime);
    });
}, function(err, results){
    // results is now the original array of files sorted by
    // modified date
});
```

__Sort Order__

By modifying the callback parameter the sorting order can be influenced:

```js
//ascending order
async.sortBy([1,9,3,5], function(x, callback){
    callback(null, x);
}, function(err,result){
    //result callback
} );

//descending order
async.sortBy([1,9,3,5], function(x, callback){
    callback(null, x*-1);    //<- x*-1 instead of x, turns the order around
}, function(err,result){
    //result callback
} );
```

---------------------------------------

<a name="some" />
### some(arr, iterator, [callback])

__Alias:__ `any`

Returns `true` if at least one element in the `arr` satisfies an async test.
_The callback for each iterator call only accepts a single argument of `true` or
`false`; it does not accept an error argument first!_ This is in-line with the
way node libraries work with truth tests like `fs.exists`. Once any iterator
call returns `true`, the main `callback` is immediately called.

__Arguments__

* `arr` - An array to iterate over.
* `iterator(item, callback)` - A truth test to apply to each item in the array
  in parallel. The iterator is passed a `callback(truthValue)`` which must be
  called with a boolean argument once it has completed.
* `callback(result)` - *Optional* A callback which is called as soon as any iterator returns
  `true`, or after all the iterator functions have finished. Result will be
  either `true` or `false` depending on the values of the async tests.

 **Note: the callbacks do not take an error as their first argument.**
__Example__

```js
async.some(['file1','file2','file3'], fs.exists, function(result){
    // if result is true then at least one of the files exists
});
```

__Related__

* someLimit(arr, limit, iterator, callback)

---------------------------------------

<a name="every" />
### every(arr, iterator, [callback])

__Alias:__ `all`

Returns `true` if every element in `arr` satisfies an async test.
_The callback for each `iterator` call only accepts a single argument of `true` or
`false`; it does not accept an error argument first!_ This is in-line with the
way node libraries work with truth tests like `fs.exists`.

__Arguments__

* `arr` - An array to iterate over.
* `iterator(item, callback)` - A truth test to apply to each item in the array
  in parallel. The iterator is passed a `callback(truthValue)` which must be
  called with a  boolean argument once it has completed.
* `callback(result)` - *Optional* A callback which is called as soon as any iterator returns
  `false`, or after all the iterator functions have finished. Result will be
  either `true` or `false` depending on the values of the async tests.

 **Note: the callbacks do not take an error as their first argument.**

__Example__

```js
async.every(['file1','file2','file3'], fs.exists, function(result){
    // if result is true then every file exists
});
```

__Related__

* everyLimit(arr, limit, iterator, callback)

---------------------------------------

<a name="concat" />
### concat(arr, iterator, [callback])

Applies `iterator` to each item in `arr`, concatenating the results. Returns the
concatenated list. The `iterator`s are called in parallel, and the results are
concatenated as they return. There is no guarantee that the results array will
be returned in the original order of `arr` passed to the `iterator` function.

__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, results)` which must be called once it
  has completed with an error (which can be `null`) and an array of results.
* `callback(err, results)` - *Optional* A callback which is called after all the `iterator`
  functions have finished, or an error occurs. Results is an array containing
  the concatenated results of the `iterator` function.

__Example__

```js
async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files){
    // files is now a list of filenames that exist in the 3 directories
});
```

__Related__

* concatSeries(arr, iterator, [callback])


## Control Flow

<a name="series" />
### series(tasks, [callback])

Run the functions in the `tasks` array in series, each one running once the previous
function has completed. If any functions in the series pass an error to its
callback, no more functions are run, and `callback` is immediately called with the value of the error.
Otherwise, `callback` receives an array of results when `tasks` have completed.

It is also possible to use an object instead of an array. Each property will be
run as a function, and the results will be passed to the final `callback` as an object
instead of an array. This can be a more readable way of handling results from
[`series`](#series).

**Note** that while many implementations preserve the order of object properties, the
[ECMAScript Language Specification](http://www.ecma-international.org/ecma-262/5.1/#sec-8.6)
explicitly states that

> The mechanics and order of enumerating the properties is not specified.

So if you rely on the order in which your series of functions are executed, and want
this to work on all platforms, consider using an array.

__Arguments__

* `tasks` - An array or object containing functions to run, each function is passed
  a `callback(err, result)` it must call on completion with an error `err` (which can
  be `null`) and an optional `result` value.
* `callback(err, results)` - An optional callback to run once all the functions
  have completed. This function gets a results array (or object) containing all
  the result arguments passed to the `task` callbacks.

__Example__

```js
async.series([
    function(callback){
        // do some stuff ...
        callback(null, 'one');
    },
    function(callback){
        // do some more stuff ...
        callback(null, 'two');
    }
],
// optional callback
function(err, results){
    // results is now equal to ['one', 'two']
});


// an example using an object instead of an array
async.series({
    one: function(callback){
        setTimeout(function(){
            callback(null, 1);
        }, 200);
    },
    two: function(callback){
        setTimeout(function(){
            callback(null, 2);
        }, 100);
    }
},
function(err, results) {
    // results is now equal to: {one: 1, two: 2}
});
```

---------------------------------------

<a name="parallel" />
### parallel(tasks, [callback])

Run the `tasks` array of functions in parallel, without waiting until the previous
function has completed. If any of the functions pass an error to its
callback, the main `callback` is immediately called with the value of the error.
Once the `tasks` have completed, the results are passed to the final `callback` as an
array.

**Note:** `parallel` is about kicking-off I/O tasks in parallel, not about parallel execution of code.  If your tasks do not use any timers or perform any I/O, they will actually be executed in series.  Any synchronous setup sections for each task will happen one after the other.  JavaScript remains single-threaded.

It is also possible to use an object instead of an array. Each property will be
run as a function and the results will be passed to the final `callback` as an object
instead of an array. This can be a more readable way of handling results from
[`parallel`](#parallel).


__Arguments__

* `tasks` - An array or object containing functions to run. Each function is passed
  a `callback(err, result)` which it must call on completion with an error `err`
  (which can be `null`) and an optional `result` value.
* `callback(err, results)` - An optional callback to run once all the functions
  have completed successfully. This function gets a results array (or object) containing all
  the result arguments passed to the task callbacks.

__Example__

```js
async.parallel([
    function(callback){
        setTimeout(function(){
            callback(null, 'one');
        }, 200);
    },
    function(callback){
        setTimeout(function(){
            callback(null, 'two');
        }, 100);
    }
],
// optional callback
function(err, results){
    // the results array will equal ['one','two'] even though
    // the second function had a shorter timeout.
});


// an example using an object instead of an array
async.parallel({
    one: function(callback){
        setTimeout(function(){
            callback(null, 1);
        }, 200);
    },
    two: function(callback){
        setTimeout(function(){
            callback(null, 2);
        }, 100);
    }
},
function(err, results) {
    // results is now equals to: {one: 1, two: 2}
});
```

__Related__

* parallelLimit(tasks, limit, [callback])

---------------------------------------

<a name="whilst" />
### whilst(test, fn, callback)

Repeatedly call `fn`, while `test` returns `true`. Calls `callback` when stopped,
or an error occurs.

__Arguments__

* `test()` - synchronous truth test to perform before each execution of `fn`.
* `fn(callback)` - A function which is called each time `test` passes. The function is
  passed a `callback(err)`, which must be called once it has completed with an
  optional `err` argument.
* `callback(err, [results])` - A callback which is called after the test
  function has failed and repeated execution of `fn` has stopped. `callback`
  will be passed an error and any arguments passed to the final `fn`'s callback.

__Example__

```js
var count = 0;

async.whilst(
    function () { return count < 5; },
    function (callback) {
        count++;
        setTimeout(function () {
            callback(null, count);
        }, 1000);
    },
    function (err, n) {
        // 5 seconds have passed, n = 5
    }
);
```

---------------------------------------

<a name="doWhilst" />
### doWhilst(fn, test, callback)

The post-check version of [`whilst`](#whilst). To reflect the difference in
the order of operations, the arguments `test` and `fn` are switched.

`doWhilst` is to `whilst` as `do while` is to `while` in plain JavaScript.

---------------------------------------

<a name="until" />
### until(test, fn, callback)

Repeatedly call `fn` until `test` returns `true`. Calls `callback` when stopped,
or an error occurs. `callback` will be passed an error and any arguments passed
to the final `fn`'s callback.

The inverse of [`whilst`](#whilst).

---------------------------------------

<a name="doUntil" />
### doUntil(fn, test, callback)

Like [`doWhilst`](#doWhilst), except the `test` is inverted. Note the argument ordering differs from `until`.

---------------------------------------

<a name="during" />
### during(test, fn, callback)

Like [`whilst`](#whilst), except the `test` is an asynchronous function that is passed a callback in the form of `function (err, truth)`. If error is passed to `test` or `fn`, the main callback is immediately called with the value of the error.

__Example__

```js
var count = 0;

async.during(
    function (callback) {
      return callback(null, count < 5);
    },
    function (callback) {
        count++;
        setTimeout(callback, 1000);
    },
    function (err) {
        // 5 seconds have passed
    }
);
```

---------------------------------------

<a name="doDuring" />
### doDuring(fn, test, callback)

The post-check version of [`during`](#during). To reflect the difference in
the order of operations, the arguments `test` and `fn` are switched.

Also a version of [`doWhilst`](#doWhilst) with asynchronous `test` function.

---------------------------------------

<a name="forever" />
### forever(fn, [errback])

Calls the asynchronous function `fn` with a callback parameter that allows it to
call itself again, in series, indefinitely.

If an error is passed to the callback then `errback` is called with the
error, and execution stops, otherwise it will never be called.

```js
async.forever(
    function(next) {
        // next is suitable for passing to things that need a callback(err [, whatever]);
        // it will result in this function being called again.
    },
    function(err) {
        // if next is called with a value in its first parameter, it will appear
        // in here as 'err', and execution will stop.
    }
);
```

---------------------------------------

<a name="waterfall" />
### waterfall(tasks, [callback])

Runs the `tasks` array of functions in series, each passing their results to the next in
the array. However, if any of the `tasks` pass an error to their own callback, the
next function is not executed, and the main `callback` is immediately called with
the error.

__Arguments__

* `tasks` - An array of functions to run, each function is passed a
  `callback(err, result1, result2, ...)` it must call on completion. The first
  argument is an error (which can be `null`) and any further arguments will be
  passed as arguments in order to the next task.
* `callback(err, [results])` - An optional callback to run once all the functions
  have completed. This will be passed the results of the last task's callback.



__Example__

```js
async.waterfall([
    function(callback) {
        callback(null, 'one', 'two');
    },
    function(arg1, arg2, callback) {
      // arg1 now equals 'one' and arg2 now equals 'two'
        callback(null, 'three');
    },
    function(arg1, callback) {
        // arg1 now equals 'three'
        callback(null, 'done');
    }
], function (err, result) {
    // result now equals 'done'
});
```
Or, with named functions:

```js
async.waterfall([
    myFirstFunction,
    mySecondFunction,
    myLastFunction,
], function (err, result) {
    // result now equals 'done'
});
function myFirstFunction(callback) {
  callback(null, 'one', 'two');
}