The async npm package provides utility functions for working with asynchronous JavaScript. It offers a variety of powerful control flow functions and utilities to work with asynchronous operations, helping to manage callbacks, reduce boilerplate code, and increase readability.
Control Flow
Execute an array of functions 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 the main callback is immediately called with the value of the error.
async.series([
function(callback) {
// do some stuff ...
callback(null, 'one');
},
function(callback) {
// do some more stuff ...
callback(null, 'two');
}
],
function(err, results) {
// results is now equal to ['one', 'two']
});Collections
Apply a function to each item in a collection and collect the results. For example, you can use `async.map` to get the file stats for an array of file names.
async.map(['file1','file2','file3'], fs.stat, function(err, results) {
// results is now an array of stats for each file
});Utilities
Repeatedly call a function a set number of times and collect the results. It's useful for seeding databases, among other things.
async.times(5, function(n, next) {
createUser(n, function(err, user) {
next(err, user);
});
}, function(err, users) {
// we should now have 5 users
});Bluebird is a fully-featured promise library with a focus on innovative features and performance. It allows for promise-based asynchronous control flow, which can lead to cleaner and more readable code compared to traditional callback patterns. Bluebird also provides utility functions similar to async but uses promises instead of callbacks.
Q is a tool for making and composing asynchronous promises in JavaScript. It helps in organizing asynchronous code with more maintainable and readable structures. Q is similar to async in that it helps manage asynchronous operations but does so through promises rather than callbacks.
Co is a generator based flow-control utility for Node.js and the browser, using promises. It allows you to work with generators for asynchronous control flow, which can make your code more readable and maintainable. Co is different from async in that it leverages generator functions to pause and resume execution, while async uses callbacks.
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 install asynccomponent install caolan/asyncjam install asyncspm install asyncAsync 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.
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.
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.
});
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
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>
eacheachSerieseachLimitforEachOfforEachOfSeriesforEachOfLimitmapmapSeriesmapLimitfilterfilterSeriesrejectrejectSeriesreducereduceRightdetectdetectSeriessortBysomeeveryconcatconcatSeriesseriesparallelparallelLimitwhilstdoWhilstuntildoUntilforeverwaterfallcomposeseqapplyEachapplyEachSeriesqueuepriorityQueuecargoautoretryiteratorapplynextTicktimestimesSeriesApplies 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.callback(err) - A callback which is called when all iterator 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');
}
});
The same as each, only iterator is applied to each item in arr in
series. The next iterator is only called once the current one has completed.
This means the iterator functions will complete in order.
The same as each, only no more than limit iterators will be simultaneously
running at any time.
Note that the items in arr are not processed in batches, so there is no guarantee that
the first limit iterator functions will complete before any others are started.
Arguments
arr - An array to iterate over.limit - The maximum number of iterators to run at any time.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.callback(err) - A callback which is called when all iterator functions
have finished, or an error occurs.Example
// Assume documents is an array of JSON objects and requestApi is a
// function that interacts with a rate-limited REST api.
async.eachLimit(documents, 20, requestApi, function(err){
// if any of the saves produced an error, err would equal that error
});
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) - A callback which is called when all iterator 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);
})
Like forEachOf, except only one iterator is run at a time. The order of execution is not guaranteed for objects, but it will be guaranteed for arrays.
Like forEachOf, except the number of iterators running at a given time is controlled by limit.
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) - 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
async.map(['file1','file2','file3'], fs.stat, function(err, results){
// results is now an array of stats for each file
});
The same as map, only the iterator is applied to each item in arr in
series. The next iterator is only called once the current one has completed.
The results array will be in the same order as the original.
The same as map, only no more than limit iterators will be simultaneously
running at any time.
Note that the items are not processed in batches, so there is no guarantee that
the first limit iterator functions will complete before any others are started.
Arguments
arr - An array to iterate over.limit - The maximum number of iterators to run at any time.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) - A callback which is called when all iterator
calls have finished, or an error occurs. The result is an array of the
transformed items from the original arr.Example
async.mapLimit(['file1','file2','file3'], 1, fs.stat, function(err, results){
// results is now an array of stats for each file
});
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) - A callback which is called after all the iterator
functions have finished.Example
async.filter(['file1','file2','file3'], fs.exists, function(results){
// results now equals an array of the existing files
});
Alias: selectSeries
The same as filter only the iterator is applied to each item in arr in
series. The next iterator is only called once the current one has completed.
The results array will be in the same order as the original.
The opposite of filter. Removes values that pass an async truth test.
The same as reject, only the iterator is applied to each item in arr
in series.
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) - A callback which is called after all the iterator
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
});
Alias: foldr
Same as reduce, only operates on arr in reverse order.
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 in arr.
The iterator is passed a callback(truthValue) which must be called with a
boolean argument once it has completed.callback(result) - 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.Example
async.detect(['file1','file2','file3'], fs.exists, function(result){
// result now equals the first file in the list that exists
});
The same as detect, only the iterator is applied to each item in arr
in series. This means the result is always the first in the original arr (in
terms of array order) that passes the truth test.
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) - 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
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
} );
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) - 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.Example
async.some(['file1','file2','file3'], fs.exists, function(result){
// if result is true then at least one of the files exists
});
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) - A callback which is called after all the iterator
functions have finished. Result will be either true or false depending on
the values of the async tests.Example
async.every(['file1','file2','file3'], fs.exists, function(result){
// if result is true then every file exists
});
Applies iterator to each item in arr, concatenating the results. Returns the
concatenated list. The iterators 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) - 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
async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files){
// files is now a list of filenames that exist in the 3 directories
});
Same as concat, but executes in series instead of parallel.
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 Specifcation 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
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}
});
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.
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
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. 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}
});
The same as parallel, only tasks are executed in parallel
with a maximum of limit tasks executing at any time.
Note that the tasks are not executed in batches, so there is no guarantee that
the first limit tasks will complete before any others are started.
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.limit - The maximum number of tasks to run at any time.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.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) - A callback which is called after the test fails and repeated
execution of fn has stopped.Example
var count = 0;
async.whilst(
function () { return count < 5; },
function (callback) {
count++;
setTimeout(callback, 1000);
},
function (err) {
// 5 seconds have passed
}
);
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.
Repeatedly call fn until test returns true. Calls callback when stopped,
or an error occurs.
Like doWhilst, except the test is inverted. Note the argument ordering differs from until.
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.
}
);
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
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'
});
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 composeExample
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
});
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
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 });
}
});
});
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 processingExample
async.applyEach([enableSearch, updateSchema], 'bucket', callback);
// partial application example:
async.each(
buckets,
async.applyEach([enableSearch, updateSchema]),
callback
);
The same as applyEach only the functions are applied in series.
Creates a queue object with the specified concurrency. Tasks added to the
queue are processed in parallel (up to the concurrency limit). If all
workers 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.concurrency - An integer for determining how many worker functions should be
run in parallel.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.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 statepause() - 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
// 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');
});
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 of
tasks is given, all tasks will be assigned the same priority.unshift method was removed.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 this animation 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 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.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');
});
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, it will not
complete (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) a callback(err, result) which must be
called when finished, passing an error (which can be null) and the result of
the function's execution, and (2) a results object, containing the results of
the previously executed functions.callback(err, results) - An optional callback which is called when all the
tasks have been completed. It receives the err argument if any tasks
pass an error to their callback. Results are always returned; however, if
an error occurs, no further tasks 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).
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
times - An integer indicating how many times to attempt the task before giving up. Defaults to 5.task(callback, results) - A function which receives two arguments: (1) a callback(err, result)
which must be called when finished, passing err (which can be null) and the result of
the function's execution, and (2) a results 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 the err and result arguments of the last attempt at completing the task.The retry function can be used as a stand-alone control flow by passing a
callback, as shown below:
async.retry(3, apiMethod, function(err, result) {
// do something with the result
});
It can also be embeded 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
});
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'
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
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')
Calls the callback 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 call n times.callback - see mapExample
// 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
});
The same as times, only the iterator is applied to each item in arr in
series. The next iterator is only called once the current one has completed.
The results array will be in the same order as the original.
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
});
Undoes a memoized function, reverting it to the original, unmemoized
form. Handy for testing.
Arguments
fn - the memoized functionLogs 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'
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'}
Changes the value of async back to its original value, returning a reference to the
async object.
v1.0.0
No known breaking changes, we are simply complying with semver from here on out.
Changes:
forEachOf for iterating over Objects (or to iterate Arrays with indexes available) (#168 #704 #321)auto (#663)0 (#714)queue.resume() (#758)setImmediate (#609 #611)_each, _map and _keys functions.FAQs
Higher-order functions and common patterns for asynchronous code
The npm package async receives a total of 54,495,207 weekly downloads. As such, async popularity was classified as popular.
We found that async demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 5 open source maintainers collaborating on the project.
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