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README.md
Async.js
Async is a utility module which provides straight-forward, powerful functions
for working with asynchronous JavaScript. Although originally designed for
use with Node.js and installable via npm install async
,
it can also be used directly in the browser.
Async is also installable via:
- bower:
bower install async
- component:
component install caolan/async
- jam:
jam install async
- spm:
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
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 (StackOverflow)
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:
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:
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
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 or this talk 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.
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:
// 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.
Alternatively, you can install using Node Package Manager (npm
):
npm install async
As well as using Bower:
bower install async
Development: async.js - 29.6kb Uncompressed
In the Browser
So far it's been tested in IE6, IE7, IE8, FF3.6 and Chrome 5.
Usage:
<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 oflimit
async operations at a time
Collections
each
,eachSeries
,eachLimit
forEachOf
,forEachOfSeries
,forEachOfLimit
map
,mapSeries
,mapLimit
filter
,filterSeries
,filterLimit
reject
,rejectSeries
,rejectLimit
reduce
,reduceRight
detect
,detectSeries
,detectLimit
sortBy
some
,someLimit
every
,everyLimit
concat
,concatSeries
Control Flow
series
parallel
,parallelLimit
whilst
,doWhilst
until
,doUntil
during
,doDuring
forever
waterfall
compose
seq
applyEach
,applyEachSeries
queue
,priorityQueue
cargo
auto
retry
iterator
times
,timesSeries
,timesLimit
Utils
Collections
### 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 inarr
. The iterator is passed acallback(err)
which must be called once it has completed. If no error has occurred, thecallback
should be run without arguments or with an explicitnull
argument. The array index is not passed to the iterator. If you need the index, useforEachOf
.callback(err)
- Optional A callback which is called when alliterator
functions have finished, or an error occurs.
Examples
// 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
});
// 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])
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 inobj
. Thekey
is the item's key, or index in the case of an array. The iterator is passed acallback(err)
which must be called once it has completed. If no error has occurred, the callback should be run without arguments or with an explicitnull
argument.callback(err)
- Optional A callback which is called when alliterator
functions have finished, or an error occurs.
Example
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])
### 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 inarr
. The iterator is passed acallback(err, transformed)
which must be called once it has completed with an error (which can benull
) and a transformed item.callback(err, results)
- Optional A callback which is called when alliterator
functions have finished, or an error occurs. Results is an array of the transformed items from thearr
.
Example
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])
### 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 inarr
. Theiterator
is passed acallback(truthValue)
, which must be called with a boolean argument once it has completed.callback(results)
- Optional A callback which is called after all theiterator
functions have finished.
Example
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])
### reject(arr, iterator, [callback])
The opposite of filter
. Removes values that pass an async
truth test.
Related
- rejectSeries(arr, iterator, [callback])
- rejectLimit(arr, limit, iterator, [callback])
### 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. Theiterator
is passed acallback(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 maincallback
is immediately called with the error.callback(err, result)
- Optional A callback which is called after all theiterator
functions have finished. Result is the reduced value.
Example
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
});
### reduceRight(arr, memo, iterator, [callback])
Alias: foldr
Same as reduce
, only operates on arr
in reverse order.
### 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
.
Arguments
arr
- An array to iterate over.iterator(item, callback)
- A truth test to apply to each item inarr
. The iterator is passed acallback(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 returnstrue
, or after all theiterator
functions have finished. Result will be the first item in the array that passes the truth test (iterator) or the valueundefined
if none passed. Note: this callback does not take an error as its first argument.
Example
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])
### 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 inarr
. The iterator is passed acallback(err, sortValue)
which must be called once it has completed with an error (which can benull
) and a value to use as the sort criteria.callback(err, results)
- Optional A callback which is called after all theiterator
functions have finished, or an error occurs. Results is the items from the originalarr
sorted by the values returned by theiterator
calls.
Example
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:
//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
} );
### 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 returnstrue
, or after all the iterator functions have finished. Result will be eithertrue
orfalse
depending on the values of the async tests.
Note: the callbacks do not take an error as their first argument. Example
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)
### 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 acallback(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 returnsfalse
, or after all the iterator functions have finished. Result will be eithertrue
orfalse
depending on the values of the async tests.
Note: the callbacks do not take an error as their first argument.
Example
async.every(['file1','file2','file3'], fs.exists, function(result){
// if result is true then every file exists
});
Related
- everyLimit(arr, limit, iterator, callback)
### 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 inarr
. The iterator is passed acallback(err, results)
which must be called once it has completed with an error (which can benull
) and an array of results.callback(err, results)
- Optional A callback which is called after all theiterator
functions have finished, or an error occurs. Results is an array containing the concatenated results of theiterator
function.
Example
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
### 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
.
Note that while many implementations preserve the order of object properties, the ECMAScript Language Specification 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 acallback(err, result)
it must call on completion with an errorerr
(which can benull
) and an optionalresult
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 thetask
callbacks.
Example
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}
});
### 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
.
Arguments
tasks
- An array or object containing functions to run. Each function is passed acallback(err, result)
which it must call on completion with an errorerr
(which can benull
) and an optionalresult
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
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])
### 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 offn
.fn(callback)
- A function which is called each timetest
passes. The function is passed acallback(err)
, which must be called once it has completed with an optionalerr
argument.callback(err, [results])
- A callback which is called after the test function has failed and repeated execution offn
has stopped.callback
will be passed an error and any arguments passed to the finalfn
's callback.
Example
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
}
);
### doWhilst(fn, test, callback)
The post-check version of 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.
### 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.
### doUntil(fn, test, callback)
Like doWhilst
, except the test
is inverted. Note the argument ordering differs from until
.
### during(test, fn, callback)
Like 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
var count = 0;
async.during(
function (callback) {
return callback(null, count < 5);
},
function (callback) {
count++;
setTimeout(callback, 1000);
},
function (err) {
// 5 seconds have passed
}
);
### doDuring(fn, test, callback)
The post-check version of during
. To reflect the difference in
the order of operations, the arguments test
and fn
are switched.
Also a version of doWhilst
with asynchronous test
function.
### 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.
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.
}
);
### 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 acallback(err, result1, result2, ...)
it must call on completion. The first argument is an error (which can benull
) 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
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:
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:
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');
}
### 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
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
});
### 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
with the arguments reversed.
Each function is executed with the this
binding of the composed function.
Arguments
functions...
- the asynchronous functions to compose
Example
// 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))
}
)(req.session.user_id, function (err, cats) {
if (err) {
console.error(err);
response.json({ status: 'error', message: err.message });
} else {
response.json({ status: 'ok', message: 'Cats found', data: cats });
}
});
});
### applyEach(fns, args..., callback)
Applies the provided arguments to each function in the array, calling
callback
after all functions have completed. If you only provide the first
argument, then it will return a function which lets you pass in the
arguments as if it were a single function call.
Arguments
fns
- the asynchronous functions to all call with the same argumentsargs...
- any number of separate arguments to pass to the functioncallback
- the final argument should be the callback, called when all functions have completed processing
Example
async.applyEach([enableSearch, updateSchema], 'bucket', callback);
// partial application example:
async.each(
buckets,
async.applyEach([enableSearch, updateSchema]),
callback
);
Related
- applyEachSeries(tasks, args..., [callback])
### queue(worker, [concurrency])
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 itscallback(err)
argument when finished, with an optionalerror
as an argument. If you want to handle errors from an individual task, pass a callback toq.push()
.concurrency
- Aninteger
for determining how manyworker
functions should be run in parallel. If omitted, the concurrency defaults to1
. If the concurrency is0
, 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 queuerunning()
- 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 manyworker
functions should be run in parallel. This property can be changed after aqueue
is created to alter the concurrency on-the-fly.push(task, [callback])
- add a new task to thequeue
. Callscallback
once theworker
has finished processing the task. Instead of a single task, atasks
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 thequeue
.saturated
- a callback that is called when thequeue
length hits theconcurrency
limit, and further tasks will be queued.empty
- a callback that is called when the last item from thequeue
is given to aworker
.drain
- a callback that is called when the last item from thequeue
has returned from theworker
.paused
- a boolean for determining whether the queue is in a paused statepause()
- a function that pauses the processing of tasks untilresume()
is called.resume()
- a function that resumes the processing of queued tasks when the queue is paused.kill()
- a function that removes thedrain
callback and empties remaining tasks from the queue forcing it to go idle.
Example
// 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');
});
### priorityQueue(worker, concurrency)
The same as 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 oftasks
is given, all tasks will be assigned the same priority.- The
unshift
method was removed.
### 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 animations for how cargo
and queue
work.
While 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 itscallback(err)
argument when finished, with an optionalerr
argument.payload
- An optionalinteger
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
- Aninteger
for determining how many tasks should be process per round. This property can be changed after acargo
is created to alter the payload on-the-fly.push(task, [callback])
- Addstask
to thequeue
. The callback is called once theworker
has finished processing the task. Instead of a single task, an array oftasks
can be submitted. The respective callback is used for every task in the list.saturated
- A callback that is called when thequeue.length()
hits the concurrency and further tasks will be queued.empty
- A callback that is called when the last item from thequeue
is given to aworker
.drain
- A callback that is called when the last item from thequeue
has returned from theworker
.idle()
,pause()
,resume()
,kill()
- cargo inherits all of the same methods and event calbacks asqueue
Example
// 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');
});
### 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
extra argument.
For example, this snippet of code:
async.auto({
readData: async.apply(fs.readFile, 'data.txt', 'utf-8')
}, callback);
will have the effect of calling readFile
with the results object as the last
argument, which will fail:
fs.readFile('data.txt', 'utf-8', cb, {});
Instead, wrap the call to readFile
in a function which does not forward the
results
object:
async.auto({
readData: function(cb, results){
fs.readFile('data.txt', 'utf-8', cb);
}
}, callback);
Arguments
tasks
- An object. Each of its properties is either a function or an array of requirements, with the function itself the last item in the array. The object's key of a property serves as the name of the task defined by that property, i.e. can be used when specifying requirements for other tasks. The function receives two arguments: (1) acallback(err, result)
which must be called when finished, passing anerror
(which can benull
) and the result of the function's execution, and (2) aresults
object, containing the results of the previously executed functions.concurrency
- An optionalinteger
for determining the maximum number of tasks that can be run in parallel. By default, as many as possible.callback(err, results)
- An optional callback which is called when all the tasks have been completed. It receives theerr
argument if anytasks
pass an error to their callback. Results are always returned; however, if an error occurs, no furthertasks
will be performed, and the results object will only contain partial results.
Example
async.auto({
get_data: function(callback){
console.log('in get_data');
// async code to get some data
callback(null, 'data', 'converted to array');
},
make_folder: function(callback){
console.log('in make_folder');
// async code to create a directory to store a file in
// this is run at the same time as getting the data
callback(null, 'folder');
},
write_file: ['get_data', 'make_folder', function(callback, results){
console.log('in write_file', JSON.stringify(results));
// once there is some data and the directory exists,
// write the data to a file in the directory
callback(null, 'filename');
}],
email_link: ['write_file', function(callback, results){
console.log('in email_link', JSON.stringify(results));
// once the file is written let's email a link to it...
// results.write_file contains the filename returned by write_file.
callback(null, {'file':results.write_file, 'email':'user@example.com'});
}]
}, function(err, results) {
console.log('err = ', err);
console.log('results = ', results);
});
This is a fairly trivial example, but to do this using the basic parallel and series functions would look like this:
async.parallel([
function(callback){
console.log('in get_data');
// async code to get some data
callback(null, 'data', 'converted to array');
},
function(callback){
console.log('in make_folder');
// async code to create a directory to store a file in
// this is run at the same time as getting the data
callback(null, 'folder');
}
],
function(err, results){
async.series([
function(callback){
console.log('in write_file', JSON.stringify(results));
// once there is some data and the directory exists,
// write the data to a file in the directory
results.push('filename');
callback(null);
},
function(callback){
console.log('in email_link', JSON.stringify(results));
// once the file is written let's email a link to it...
callback(null, {'file':results.pop(), 'email':'user@example.com'});
}
]);
});
For a complicated series of async
tasks, using the auto
function makes adding
new tasks much easier (and the code more readable).
### retry([opts = {times: 5, interval: 0}| 5], task, [callback])
Attempts to get a successful response from task
no more than times
times before
returning an error. If the task is successful, the callback
will be passed the result
of the successful task. If all attempts fail, the callback will be passed the error and
result (if any) of the final attempt.
Arguments
opts
- Can be either an object withtimes
andinterval
or a number.times
- The number of attempts to make before giving up. The default is5
.interval
- The time to wait between retries, in milliseconds. The default is0
.- If
opts
is a number, the number specifies the number of times to retry, with the default interval of0
.
task(callback, results)
- A function which receives two arguments: (1) acallback(err, result)
which must be called when finished, passingerr
(which can benull
) and theresult
of the function's execution, and (2) aresults
object, containing the results of the previously executed functions (if nested inside another control flow).callback(err, results)
- An optional callback which is called when the task has succeeded, or after the final failed attempt. It receives theerr
andresult
arguments of the last attempt at completing thetask
.
The retry
function can be used as a stand-alone control flow by passing a callback, as shown below:
// try calling apiMethod 3 times
async.retry(3, apiMethod, function(err, result) {
// do something with the result
});
// try calling apiMethod 3 times, waiting 200 ms between each retry
async.retry({times: 3, interval: 200}, apiMethod, function(err, result) {
// do something with the result
});
// try calling apiMethod the default 5 times no delay between each retry
async.retry(apiMethod, function(err, result) {
// do something with the result
});
It can also be embedded within other control flow functions to retry individual methods that are not as reliable, like this:
async.auto({
users: api.getUsers.bind(api),
payments: async.retry(3, api.getPayments.bind(api))
}, function(err, results) {
// do something with the results
});
### iterator(tasks)
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
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'
### 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
// 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:
node> var fn = async.apply(sys.puts, 'one');
node> fn('two', 'three');
one
two
three
### 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
var call_order = [];
async.nextTick(function(){
call_order.push('two');
// call_order now equals ['one','two']
});
call_order.push('one')
### times(n, iterator, [callback])
Calls the iterator
function n
times, and accumulates results in the same manner
you would use with map
.
Arguments
n
- The number of times to run the function.iterator
- The function to calln
times.callback
- seemap
Example
// 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
### 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
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
});
### unmemoize(fn)
Undoes a memoize
d function, reverting it to the original, unmemoized
form. Handy for testing.
Arguments
fn
- the memoized function
### 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 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
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,
// preventing stack overflows
async.mapSeries(args, async.ensureAsync(sometimesAsync), done);
### constant(values...)
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
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);
### 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
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:
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.
var q = async.queue(async.asyncify(async function (file) {
var intermediateStep = await processFile(file);
return await somePromise(intermediateStep)
}));
q.push(files);
### 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
var hello = function(name, callback){
setTimeout(function(){
callback(null, 'hello ' + name);
}, 1000);
};
node> async.log(hello, 'world');
'hello world'
### 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
var hello = function(name, callback){
setTimeout(function(){
callback(null, {hello: name});
}, 1000);
};
node> async.dir(hello, 'world');
{hello: 'world'}
### noConflict()
Changes the value of async
back to its original value, returning a reference to the
async
object.