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, it can also be used directly in the browser.
Async provides around 20 functions that include the usual 'functional' suspects (map, reduce, filter, forEach…) 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'], path.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.
Releases are available for download from GitHub. Alternatively, you can install using Node Package Manager (npm):
npm install async
Development: async.js - 17.5kb Uncompressed
Production: async.min.js - 1.7kb Packed and Gzipped
So far its 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>
Applies an iterator function to each item in an array, 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 this callback, the main callback for the forEach 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.
Arguments
Example
// assuming openFiles is an array of file names and saveFile is a function
// to save the modified contents of that file:
async.forEach(openFiles, saveFile, function(err){
// if any of the saves produced an error, err would equal that error
});
The same as forEach only the iterator is applied to each item in the array in series. The next iterator is only called once the current one has completed processing. This means the iterator functions will complete in order.
The same as forEach only the iterator is applied to batches of items in the array, in series. The next batch of iterators is only called once the current one has completed processing.
Arguments
Example
// Assume documents is an array of JSON objects and requestApi is a
// function that interacts with a rate-limited REST api.
async.forEachLimit(documents, 20, requestApi, function(err){
// if any of the saves produced an error, err would equal that error
});
Produces a new array of values by mapping each value in the given array through the iterator function. The iterator is called with an item from the array and a callback for when it has finished processing. The callback takes 2 arguments, an error and the transformed item from the array. If the iterator passes an error to this 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 array.
Arguments
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 the array in series. The next iterator is only called once the current one has completed processing. The results array will be in the same order as the original.
Alias: select
Returns a new array of all the values 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 path.exists. This operation is performed in parallel, but the results array will be in the same order as the original.
Arguments
Example
async.filter(['file1','file2','file3'], path.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 the array in series. The next iterator is only called once the current one has completed processing. 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 filter, only the iterator is applied to each item in the array in series.
aliases: inject, foldl
Reduces a list of values 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, 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 its probably a good idea to do so.
Arguments
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 the items in the array in reverse order.
Returns the first value in a list 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 array (in terms of order) that passes the test.
If order within the original array is important then look at detectSeries.
Arguments
Example
async.detect(['file1','file2','file3'], path.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 the array in series. This means the result is always the first in the original array (in terms of array order) that passes the truth test.
Sorts a list by the results of running each value through an async iterator.
Arguments
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
});
Alias: any
Returns true if at least one element in the array 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 path.exists. Once any iterator call returns true, the main callback is immediately called.
Arguments
Example
async.some(['file1','file2','file3'], path.exists, function(result){
// if result is true then at least one of the files exists
});
Alias: all
Returns true if every element in the array 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 path.exists.
Arguments
Example
async.every(['file1','file2','file3'], path.exists, function(result){
// if result is true then every file exists
});
Applies an iterator to each item in a list, 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 the arguments passed to the iterator function.
Arguments
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 async.concat, but executes in series instead of parallel.
Run 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 callback for the series 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 async.series.
Arguments
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 an 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 async.parallel.
Arguments
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}
});
Repeatedly call fn, while test returns true. Calls the callback when stopped, or an error occurs.
Arguments
Example
var count = 0;
async.whilst(
function () { return count < 5; },
function (callback) {
count++;
setTimeout(callback, 1000);
},
function (err) {
// 5 seconds have passed
}
);
Repeatedly call fn, until test returns true. Calls the callback when stopped, or an error occurs.
The inverse of async.whilst.
Runs an array of functions in series, each passing their results to the next in the array. However, if any of the functions pass an error to the callback, the next function is not executed and the main callback is immediately called with the error.
Arguments
Example
async.waterfall([
function(callback){
callback(null, 'one', 'two');
},
function(arg1, arg2, callback){
callback(null, 'three');
},
function(arg1, callback){
// arg1 now equals 'three'
callback(null, 'done');
}
], function (err, result) {
// result now equals 'done'
});
Creates a queue object with the specified concurrency. Tasks added to the queue will be processed in parallel (up to the concurrency limit). If all workers are in progress, the task is queued until one is available. Once a worker has completed a task, the task's callback is called.
Arguments
Queue objects
The queue object returned by this function has the following properties and methods:
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 bar');
});
Determines the best order for running functions 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, that function will not complete (so any other functions depending on it will not run) and the main callback will be called immediately with the error. Functions also receive an object containing the results of functions which have completed so far.
Arguments
Example
async.auto({
get_data: function(callback){
// async code to get some data
},
make_folder: function(callback){
// async code to create a directory to store a file in
// this is run at the same time as getting the data
},
write_file: ['get_data', 'make_folder', function(callback){
// 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){
// once the file is written let's email a link to it...
// results.write_file contains the filename returned by write_file.
}]
});
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){
// async code to get some data
},
function(callback){
// async code to create a directory to store a file in
// this is run at the same time as getting the data
}
],
function(results){
async.series([
function(callback){
// once there is some data and the directory exists,
// write the data to a file in the directory
},
email_link: function(callback){
// once the file is written let's email a link to it...
}
]);
});
For a complicated series of async tasks using the auto function makes adding new tasks much easier and makes the code more readable.
Creates an iterator function which calls the next function in the array, returning a continuation to call the next one after that. Its also possible to 'peek' the next iterator by doing 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
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, a useful 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
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 the callback on a later loop around the event loop. In node.js this just calls process.nextTick, in the browser it falls back to setTimeout(callback, 0), which means other higher priority events may precede the execution of the callback.
This is used internally for browser-compatibility purposes.
Arguments
Example
var call_order = [];
async.nextTick(function(){
call_order.push('two');
// call_order now equals ['one','two]
});
call_order.push('one')
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.
Arguments
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. Comes handy in tests.
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
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
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.
FAQs
Higher-order functions and common patterns for asynchronous code
The npm package async receives a total of 56,514,885 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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