[](https://gitter.im/caolan/async?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
// 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.
// files is now a list of filenames that exist in the 3 directories
});
```
__Related__
* concatSeries(arr, iterator, [callback])
## Control Flow
<aname="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}
});
```
---------------------------------------
<aname="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])
---------------------------------------
<aname="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
}
);
```
---------------------------------------
<aname="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.
---------------------------------------
<aname="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).
---------------------------------------
<aname="doUntil"/>
### doUntil(fn, test, callback)
Like [`doWhilst`](#doWhilst), except the `test` is inverted. Note the argument ordering differs from `until`.
---------------------------------------
<aname="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
}
);
```
---------------------------------------
<aname="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.
---------------------------------------
<aname="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.
}
);
```
---------------------------------------
<aname="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');
}
function mySecondFunction(arg1, arg2, callback) {
// arg1 now equals 'one' and arg2 now equals 'two'
callback(null, 'three');
}
function myLastFunction(arg1, callback) {
// arg1 now equals 'three'
callback(null, 'done');
}
```
Or, if you need to pass any argument to the first function:
```js
async.waterfall([
async.apply(myFirstFunction, 'zero'),
mySecondFunction,
myLastFunction,
], function (err, result) {
// result now equals 'done'
});
function myFirstFunction(arg1, callback) {
// arg1 now equals 'zero'
callback(null, 'one', 'two');
}
function mySecondFunction(arg1, arg2, callback) {
// arg1 now equals 'one' and arg2 now equals 'two'
callback(null, 'three');
}
function myLastFunction(arg1, callback) {
// arg1 now equals 'three'
callback(null, 'done');
}
```
---------------------------------------
<aname="compose"/>
### compose(fn1, fn2...)
Creates a function which is a composition of the passed asynchronous
functions. Each function consumes the return value of the function that
follows. Composing functions `f()`, `g()`, and `h()` would produce the result of
`f(g(h()))`, only this version uses callbacks to obtain the return values.
Each function is executed with the `this` binding of the composed function.
__Arguments__
*`functions...` - the asynchronous functions to compose
__Example__
```js
function add1(n, callback) {
setTimeout(function () {
callback(null, n + 1);
}, 10);
}
function mul3(n, callback) {
setTimeout(function () {
callback(null, n * 3);
}, 10);
}
var add1mul3 = async.compose(mul3, add1);
add1mul3(4, function (err, result) {
// result now equals 15
});
```
---------------------------------------
<aname="seq"/>
### seq(fn1, fn2...)
Version of the compose function that is more natural to read.
Each function consumes the return value of the previous function.
It is the equivalent of [`compose`](#compose) with the arguments reversed.
Each function is executed with the `this` binding of the composed function.
__Arguments__
*`functions...` - the asynchronous functions to compose
__Example__
```js
// Requires lodash (or underscore), express3 and dresende's orm2.
// Part of an app, that fetches cats of the logged user.
// This example uses `seq` function to avoid overnesting and error
// handling clutter.
app.get('/cats', function(request, response) {
var User = request.models.User;
async.seq(
_.bind(User.get, User), // 'User.get' has signature (id, callback(err, data))
function(user, fn) {
user.getCats(fn); // 'getCats' has signature (callback(err, data))
Creates a `queue` object with the specified `concurrency`. Tasks added to the
`queue` are processed in parallel (up to the `concurrency` limit). If all
`worker`s are in progress, the task is queued until one becomes available.
Once a `worker` completes a `task`, that `task`'s callback is called.
__Arguments__
*`worker(task, callback)` - An asynchronous function for processing a queued
task, which must call its `callback(err)` argument when finished, with an
optional `error` as an argument. If you want to handle errors from an individual task, pass a callback to `q.push()`.
*`concurrency` - An `integer` for determining how many `worker` functions should be
run in parallel. If omitted, the concurrency defaults to `1`. If the concurrency is `0`, an error is thrown.
__Queue objects__
The `queue` object returned by this function has the following properties and
methods:
*`length()` - a function returning the number of items waiting to be processed.
*`started` - a function returning whether or not any items have been pushed and processed by the queue
*`running()` - a function returning the number of items currently being processed.
*`workersList()` - a function returning the array of items currently being processed.
*`idle()` - a function returning false if there are items waiting or being processed, or true if not.
*`concurrency` - an integer for determining how many `worker` functions should be
run in parallel. This property can be changed after a `queue` is created to
alter the concurrency on-the-fly.
*`push(task, [callback])` - add a new task to the `queue`. Calls `callback` once
the `worker` has finished processing the task. Instead of a single task, a `tasks` array
can be submitted. The respective callback is used for every task in the list.
*`unshift(task, [callback])` - add a new task to the front of the `queue`.
*`saturated` - a callback that is called when the `queue` length hits the `concurrency` limit,
and further tasks will be queued.
*`empty` - a callback that is called when the last item from the `queue` is given to a `worker`.
*`drain` - a callback that is called when the last item from the `queue` has returned from the `worker`.
*`paused` - a boolean for determining whether the queue is in a paused state
*`pause()` - a function that pauses the processing of tasks until `resume()` is called.
*`resume()` - a function that resumes the processing of queued tasks when the queue is paused.
*`kill()` - a function that removes the `drain` callback and empties remaining tasks from the queue forcing it to go idle.
__Example__
```js
// create a queue object with concurrency 2
var q = async.queue(function (task, callback) {
console.log('hello ' + task.name);
callback();
}, 2);
// assign a callback
q.drain = function() {
console.log('all items have been processed');
}
// add some items to the queue
q.push({name: 'foo'}, function (err) {
console.log('finished processing foo');
});
q.push({name: 'bar'}, function (err) {
console.log('finished processing bar');
});
// add some items to the queue (batch-wise)
q.push([{name: 'baz'},{name: 'bay'},{name: 'bax'}], function (err) {
console.log('finished processing item');
});
// add some items to the front of the queue
q.unshift({name: 'bar'}, function (err) {
console.log('finished processing bar');
});
```
---------------------------------------
<aname="priorityQueue"/>
### priorityQueue(worker, concurrency)
The same as [`queue`](#queue) only tasks are assigned a priority and completed in ascending priority order. There are two differences between `queue` and `priorityQueue` objects:
*`push(task, priority, [callback])` - `priority` should be a number. If an array of
`tasks` is given, all tasks will be assigned the same priority.
* The `unshift` method was removed.
---------------------------------------
<aname="cargo"/>
### cargo(worker, [payload])
Creates a `cargo` object with the specified payload. Tasks added to the
cargo will be processed altogether (up to the `payload` limit). If the
`worker` is in progress, the task is queued until it becomes available. Once
the `worker` has completed some tasks, each callback of those tasks is called.
Check out [these](https://camo.githubusercontent.com/6bbd36f4cf5b35a0f11a96dcd2e97711ffc2fb37/68747470733a2f2f662e636c6f75642e6769746875622e636f6d2f6173736574732f313637363837312f36383130382f62626330636662302d356632392d313165322d393734662d3333393763363464633835382e676966) [animations](https://camo.githubusercontent.com/f4810e00e1c5f5f8addbe3e9f49064fd5d102699/68747470733a2f2f662e636c6f75642e6769746875622e636f6d2f6173736574732f313637363837312f36383130312f38346339323036362d356632392d313165322d383134662d3964336430323431336266642e676966) for how `cargo` and `queue` work.
While [queue](#queue) passes only one task to one of a group of workers
at a time, cargo passes an array of tasks to a single worker, repeating
when the worker is finished.
__Arguments__
*`worker(tasks, callback)` - An asynchronous function for processing an array of
queued tasks, which must call its `callback(err)` argument when finished, with
an optional `err` argument.
*`payload` - An optional `integer` for determining how many tasks should be
processed per round; if omitted, the default is unlimited.
__Cargo objects__
The `cargo` object returned by this function has the following properties and
methods:
*`length()` - A function returning the number of items waiting to be processed.
*`payload` - An `integer` for determining how many tasks should be
process per round. This property can be changed after a `cargo` is created to
alter the payload on-the-fly.
*`push(task, [callback])` - Adds `task` to the `queue`. The callback is called
once the `worker` has finished processing the task. Instead of a single task, an array of `tasks`
can be submitted. The respective callback is used for every task in the list.
*`saturated` - A callback that is called when the `queue.length()` hits the concurrency and further tasks will be queued.
*`empty` - A callback that is called when the last item from the `queue` is given to a `worker`.
*`drain` - A callback that is called when the last item from the `queue` has returned from the `worker`.
*`idle()`, `pause()`, `resume()`, `kill()` - cargo inherits all of the same methods and event calbacks as [`queue`](#queue)
__Example__
```js
// create a cargo object with payload 2
var cargo = async.cargo(function (tasks, callback) {
for(var i=0; i<tasks.length;i++){
console.log('hello ' + tasks[i].name);
}
callback();
}, 2);
// add some items
cargo.push({name: 'foo'}, function (err) {
console.log('finished processing foo');
});
cargo.push({name: 'bar'}, function (err) {
console.log('finished processing bar');
});
cargo.push({name: 'baz'}, function (err) {
console.log('finished processing baz');
});
```
---------------------------------------
<aname="auto"/>
### auto(tasks, [concurrency], [callback])
Determines the best order for running the functions in `tasks`, based on their requirements. Each function can optionally depend on other functions being completed first, and each function is run as soon as its requirements are satisfied.
If any of the functions pass an error to their callback, the `auto` sequence will stop. Further tasks will not execute (so any other functions depending on it will not run), and the main `callback` is immediately called with the error. Functions also receive an object containing the results of functions which have completed so far.
Note, all functions are called with a `results` object as a second argument,
so it is unsafe to pass functions in the `tasks` object which cannot handle the
Creates an iterator function which calls the next function in the `tasks` array,
returning a continuation to call the next one after that. It's also possible to
“peek” at the next iterator with `iterator.next()`.
This function is used internally by the `async` module, but can be useful when
you want to manually control the flow of functions in series.
__Arguments__
*`tasks` - An array of functions to run.
__Example__
```js
var iterator = async.iterator([
function(){ sys.p('one'); },
function(){ sys.p('two'); },
function(){ sys.p('three'); }
]);
node> var iterator2 = iterator();
'one'
node> var iterator3 = iterator2();
'two'
node> iterator3();
'three'
node> var nextfn = iterator2.next();
node> nextfn();
'three'
```
---------------------------------------
<aname="apply"/>
### apply(function, arguments..)
Creates a continuation function with some arguments already applied.
Useful as a shorthand when combined with other control flow functions. Any arguments
passed to the returned function are added to the arguments originally passed
to apply.
__Arguments__
*`function` - The function you want to eventually apply all arguments to.
*`arguments...` - Any number of arguments to automatically apply when the
continuation is called.
__Example__
```js
// using apply
async.parallel([
async.apply(fs.writeFile, 'testfile1', 'test1'),
async.apply(fs.writeFile, 'testfile2', 'test2'),
]);
// the same process without using apply
async.parallel([
function(callback){
fs.writeFile('testfile1', 'test1', callback);
},
function(callback){
fs.writeFile('testfile2', 'test2', callback);
}
]);
```
It's possible to pass any number of additional arguments when calling the
continuation:
```js
node> var fn = async.apply(sys.puts, 'one');
node> fn('two', 'three');
one
two
three
```
---------------------------------------
<aname="nextTick"/>
### nextTick(callback), setImmediate(callback)
Calls `callback` on a later loop around the event loop. In Node.js this just
calls `process.nextTick`; in the browser it falls back to `setImmediate(callback)`
if available, otherwise `setTimeout(callback, 0)`, which means other higher priority
events may precede the execution of `callback`.
This is used internally for browser-compatibility purposes.
__Arguments__
*`callback` - The function to call on a later loop around the event loop.
__Example__
```js
var call_order = [];
async.nextTick(function(){
call_order.push('two');
// call_order now equals ['one','two']
});
call_order.push('one')
```
<aname="times"/>
### times(n, iterator, [callback])
Calls the `iterator` function `n` times, and accumulates results in the same manner
you would use with [`map`](#map).
__Arguments__
*`n` - The number of times to run the function.
*`iterator` - The function to call `n` times.
*`callback` - see [`map`](#map)
__Example__
```js
// Pretend this is some complicated async factory
var createUser = function(id, callback) {
callback(null, {
id: 'user' + id
})
}
// generate 5 users
async.times(5, function(n, next){
createUser(n, function(err, user) {
next(err, user)
})
}, function(err, users) {
// we should now have 5 users
});
```
__Related__
* timesSeries(n, iterator, [callback])
* timesLimit(n, limit, iterator, [callback])
## Utils
<aname="memoize"/>
### memoize(fn, [hasher])
Caches the results of an `async` function. When creating a hash to store function
results against, the callback is omitted from the hash and an optional hash
function can be used.
If no hash function is specified, the first argument is used as a hash key, which may work reasonably if it is a string or a data type that converts to a distinct string. Note that objects and arrays will not behave reasonably. Neither will cases where the other arguments are significant. In such cases, specify your own hash function.
The cache of results is exposed as the `memo` property of the function returned
by `memoize`.
__Arguments__
*`fn` - The function to proxy and cache results from.
*`hasher` - An optional function for generating a custom hash for storing
results. It has all the arguments applied to it apart from the callback, and
must be synchronous.
__Example__
```js
var slow_fn = function (name, callback) {
// do something
callback(null, result);
};
var fn = async.memoize(slow_fn);
// fn can now be used as if it were slow_fn
fn('some name', function () {
// callback
});
```
<aname="unmemoize"/>
### unmemoize(fn)
Undoes a [`memoize`](#memoize)d function, reverting it to the original, unmemoized
form. Handy for testing.
__Arguments__
*`fn` - the memoized function
---------------------------------------
<aname="ensureAsync"/>
### ensureAsync(fn)
Wrap an async function and ensure it calls its callback on a later tick of the event loop. If the function already calls its callback on a next tick, no extra deferral is added. This is useful for preventing stack overflows (`RangeError: Maximum call stack size exceeded`) and generally keeping [Zalgo](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony) contained.
__Arguments__
*`fn` - an async function, one that expects a node-style callback as its last argument
Returns a wrapped function with the exact same call signature as the function passed in.
__Example__
```js
function sometimesAsync(arg, callback) {
if (cache[arg]) {
return callback(null, cache[arg]); // this would be synchronous!!
} else {
doSomeIO(arg, callback); // this IO would be asynchronous
}
}
// this has a risk of stack overflows if many results are cached in a row
async.mapSeries(args, sometimesAsync, done);
// this will defer sometimesAsync's callback if necessary,
Returns a function that when called, calls-back with the values provided. Useful as the first function in a `waterfall`, or for plugging values in to `auto`.
__Example__
```js
async.waterfall([
async.constant(42),
function (value, next) {
// value === 42
},
//...
], callback);
async.waterfall([
async.constant(filename, "utf8"),
fs.readFile,
function (fileData, next) {
//...
}
//...
], callback);
async.auto({
hostname: async.constant("https://server.net/"),
port: findFreePort,
launchServer: ["hostname", "port", function (cb, options) {
startServer(options, cb);
}],
//...
}, callback);
```
---------------------------------------
<aname="asyncify">
<aname="wrapSync">
### asyncify(func)
__Alias:__ `wrapSync`
Take a sync function and make it async, passing its return value to a callback. This is useful for plugging sync functions into a waterfall, series, or other async functions. Any arguments passed to the generated function will be passed to the wrapped function (except for the final callback argument). Errors thrown will be passed to the callback.
__Example__
```js
async.waterfall([
async.apply(fs.readFile, filename, "utf8"),
async.asyncify(JSON.parse),
function (data, next) {
// data is the result of parsing the text.
// If there was a parsing error, it would have been caught.
}
], callback)
```
If the function passed to `asyncify` returns a Promise, that promises's resolved/rejected state will be used to call the callback, rather than simply the synchronous return value. Example:
```js
async.waterfall([
async.apply(fs.readFile, filename, "utf8"),
async.asyncify(function (contents) {
return db.model.create(contents);
}),
function (model, next) {
// `model` is the instantiated model object.
// If there was an error, this function would be skipped.
}
], callback)
```
This also means you can asyncify ES2016 `async` functions.
```js
var q = async.queue(async.asyncify(async function (file) {
var intermediateStep = await processFile(file);
return await somePromise(intermediateStep)
}));
q.push(files);
```
---------------------------------------
<aname="log"/>
### log(function, arguments)
Logs the result of an `async` function to the `console`. Only works in Node.js or
in browsers that support `console.log` and `console.error` (such as FF and Chrome).
If multiple arguments are returned from the async function, `console.log` is
called on each argument in order.
__Arguments__
*`function` - The function you want to eventually apply all arguments to.
*`arguments...` - Any number of arguments to apply to the function.
__Example__
```js
var hello = function(name, callback){
setTimeout(function(){
callback(null, 'hello ' + name);
}, 1000);
};
```
```js
node> async.log(hello, 'world');
'hello world'
```
---------------------------------------
<aname="dir"/>
### dir(function, arguments)
Logs the result of an `async` function to the `console` using `console.dir` to
display the properties of the resulting object. Only works in Node.js or
in browsers that support `console.dir` and `console.error` (such as FF and Chrome).
If multiple arguments are returned from the async function, `console.dir` is
called on each argument in order.
__Arguments__
*`function` - The function you want to eventually apply all arguments to.
*`arguments...` - Any number of arguments to apply to the function.
__Example__
```js
var hello = function(name, callback){
setTimeout(function(){
callback(null, {hello: name});
}, 1000);
};
```
```js
node> async.dir(hello, 'world');
{hello: 'world'}
```
---------------------------------------
<aname="noConflict"/>
### noConflict()
Changes the value of `async` back to its original value, returning a reference to the
