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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.
Higher-order functions and common patterns for asynchronous code in node.js
I've so far avoided using the existing async modules in favour of the standard callbacks provided by node. When writing modules, I find sticking to the convention of using a single callback makes the API easier to understand, and allows people to wrap the module with other methods of handling async code if they so wish.
However, I've found myself repeating a number of patterns, so I've decided to abstract some of the more common ones into a separate module. What I've ended up with is a few higher-order functions that operate on async code using the convention of a single callback. This includes the usual 'functional' suspects (map, reduce, filter, forEach...) as well as some common patterns for running blocks of async code (parallel, series, waterfall...).
This is not an attempt to replace the standard callback mechanism in node. In fact, it is designed to work as seamlessly as possible with the existing node modules, and any other module which follows those conventions. If you're interested in other ways to manage async code, then you may like to take a look at the new implementation of the old node Promise objects or alternative modules like node-continuables.
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.
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 inline 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 inline 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 inline 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
});
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.
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']
});
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.
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){
// in this case, the results array will equal ['two','one']
// because the functions were run in parallel and the second
// function had a shorter timeout before calling the callback.
});
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');
}
]);
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 and 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.
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
}],
email_link: ['write_file', function(callback){
// once the file is written let's email a link to it...
}]
});
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: ['write_file', 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'
FAQs
Higher-order functions and common patterns for asynchronous code
The npm package async receives a total of 45,311,347 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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