For Async v1.5.x documentation, go HERE
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 --save async,
it can also be used directly in the browser.
Async is also installable via:
Async provides around 70 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 asynchronous function -- a callback which expects an Error as its first argument -- and calling the callback once.
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'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, 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.
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 iteratee. 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 iteratee(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 iteratee(item, callback) {
if (inCache(item)) {
async.setImmediate(function () {
callback(null, cache[item]);
});
} else {
doSomeIO(item, callback);
//...
}
});
Async does not guard against synchronous iteratees for performance reasons. 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.
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.
This section is really about bind, not about async. If you are wondering how to
make async execute your iteratees in a given context, or are confused as to why
a method of another library isn't working as an iteratee, 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 iteratee 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 npm:
npm install --save async
As well as using Bower:
bower install async
You can then require() async as normal:
var async = require("async");
Or require individual methods:
var waterfall = require("async/waterfall");
var map = require("async/map");
Development: async.js - 29.6kb Uncompressed
Async should work in any ES5 environment (IE9 and above).
Usage:
<script type="text/javascript" src="async.js"></script>
<script type="text/javascript">
async.map(data, asyncProcess, function(err, results){
alert(results);
});
</script>
We also provide async as a collection of ES2015 modules, in an alternative async-es package on npm.
npm i -S async-es
import waterfall from 'async-es/waterfall';
import async from 'async-es';
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 of limit async operations at a timeeach, eachSeries, eachLimitforEachOf, forEachOfSeries, forEachOfLimitmap, mapSeries, mapLimitfilter, filterSeries, filterLimitreject, rejectSeries, rejectLimitreduce, reduceRighttransformdetect, detectSeries, detectLimitsortBysome, someLimit, someSeriesevery, everyLimit, everySeriesconcat, concatSeriesseriesparallel, parallelLimitwhilst, doWhilstuntil, doUntilduring, doDuringforeverwaterfallcomposeseqapplyEach, applyEachSeriesqueue, priorityQueuecargoautoautoInjectretryretryableiteratortimes, timesSeries, timesLimitraceapplynextTickmemoizeunmemoizeensureAsyncconstantasyncifywrapSynclogdirnoConflicttimeoutreflectreflectAllCollection methods can iterate over Arrays, Objects, Maps, Sets, and any object that implements the ES2015 iterator protocol.
Applies the function iteratee to each item in coll, in parallel.
The iteratee is called with an item from the list, and a callback for when it
has finished. If the iteratee 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 iteratee to each item in parallel,
there is no guarantee that the iteratee functions will complete in order.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A function to apply to each item in coll.
The iteratee 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 iteratee. If you need the index, use forEachOf.callback(err) - Optional A callback which is called when all iteratee 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
Like each, except that it passes the key (or index) as the second argument to the iteratee.
Arguments
coll - A collection to iterate over.iteratee(item, key, callback) - A function to apply to each item in coll.
The key is the item's key, or index in the case of an array. The iteratee 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) - Optional A callback which is called when all iteratee 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
Produces a new collection of values by mapping each value in coll through
the iteratee function. The iteratee is called with an item from coll and a
callback for when it has finished processing. Each of these callback takes 2 arguments:
an error, and the transformed item from coll. If iteratee 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 iteratee to each item in parallel,
there is no guarantee that the iteratee functions will complete in order.
However, the results array will be in the same order as the original coll.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A function to apply to each item in coll.
The iteratee 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) - Optional A callback which is called when all iteratee
functions have finished, or an error occurs. Results is an array of the
transformed items from the coll.Example
async.map(['file1','file2','file3'], fs.stat, function(err, results){
// results is now an array of stats for each file
});
Related
Alias: select
Returns a new array of all the values in coll which pass an async truth test.
This operation is performed in parallel,
but the results array will be in the same order as the original.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A truth test to apply to each item in coll.
The iteratee is passed a callback(err, truthValue) , which must be called with a boolean argument once it has completed. Callback arguments changed in 2.0callback(err, results) - Optional A callback which is called after all the iteratee
functions have finished.Example
async.filter(['file1','file2','file3'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, results){
// results now equals an array of the existing files
});
Related
The opposite of filter. Removes values that pass an async truth test.
Related
Aliases: inject, foldl
Reduces coll into a single value using an async iteratee 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
coll - A collection to iterate over.memo - The initial state of the reduction.iteratee(memo, item, callback) - A function applied to each item in the
array to produce the next step in the reduction. The iteratee 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) - Optional A callback which is called after all the iteratee
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 coll in reverse order.
Alias: find
Returns the first value in coll that passes an async truth test. The
iteratee is applied in parallel, meaning the first iteratee to return true will
fire the detect callback with that result. That means the result might not be
the first item in the original coll (in terms of order) that passes the test.
If order within the original coll is important, then look at detectSeries.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A truth test to apply to each item in coll. The iteratee is passed a callback(err, truthValue) which must be called with a boolean argument once it has completed. Callback arguments changed in 2.0callback(err, result) - Optional A callback which is called as soon as any iteratee returns
true, or after all the iteratee functions have finished. Result will be
the first item in the array that passes the truth test (iteratee) or the
value undefined if none passed.Example
async.detect(['file1','file2','file3'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, result){
// result now equals the first file in the list that exists
});
Related
Sorts a list by the results of running each coll value through an async iteratee.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A function to apply to each item in coll.
The iteratee 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) - Optional A callback which is called after all the iteratee
functions have finished, or an error occurs. Results is the items from
the original coll sorted by the values returned by the iteratee 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 coll satisfies an async test.
If any iteratee call returns true, the main callback is immediately called.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A truth test to apply to each item in the array
in parallel. The iteratee is passed a callback(err, truthValue) which must be called with a boolean argument once it has completed. Callback arguments changed in 2.0callback(err, result) - Optional A callback which is called as soon as any iteratee returns
true, or after all the iteratee functions have finished. Result will be
either true or false depending on the values of the async tests.Example
async.some(['file1','file2','file3'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, result){
// if result is true then at least one of the files exists
});
Related
Alias: all
Returns true if every element in coll satisfies an async test.
If any iteratee call returns false, the main callback is immediately called.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A truth test to apply to each item in the collection in parallel. The iteratee is passed a callback(err, truthValue) which must be called with a boolean argument once it has completed. Callback arguments changed in 2.0callback(err, result) - Optional A callback which is called after all the iteratee
functions have finished. Result will be either true or false depending on
the values of the async tests.Example
async.every(['file1','file2','file3'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, result){
// if result is true then every file exists
});
Related
Applies iteratee to each item in coll, concatenating the results. Returns the
concatenated list. The iteratees 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 coll passed to the iteratee function.
Arguments
coll - A collection to iterate over.iteratee(item, callback) - A function to apply to each item in coll.
The iteratee 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) - Optional A callback which is called after all the iteratee
functions have finished, or an error occurs. Results is an array containing
the concatenated results of the iteratee 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
Run the functions in the tasks collection 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 - A collection 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 collection 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 - A collection containing functions to run. Each function is passed
a callback(err, result) which it must call on completion with an error err
(which can be null) and an optional result value.callback(err, results) - An optional callback to run once all the functions
have completed successfully. This function gets a results array (or object) containing all
the result arguments passed to the task callbacks.Example
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
Repeatedly call fn, while test returns true. Calls callback when stopped,
or an error occurs.
Arguments
test() - synchronous truth test to perform before each execution of fn.fn(callback) - A function which is called each time test passes. The function is
passed a callback(err), which must be called once it has completed with an
optional err argument.callback(err, [results]) - A callback which is called after the test
function has failed and repeated execution of fn has stopped. callback
will be passed an error and any arguments passed to the final fn's callback.Example
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
}
);
The post-check version of whilst. To reflect the difference in
the order of operations, the arguments test and fn are switched. The test function is also passed the non-error callback results of fn.
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. callback will be passed an error and any arguments passed
to the final fn's callback.
The inverse of whilst.
Like doWhilst, except the test is inverted. Note the argument ordering differs from until. The test function is also passed the non-error callback results of fn.
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.
Additionaly during passes any arguments passed by the iteratee function (2nd function) to the test function (1st function). The test callback will allways be the last parameter.
Example
var count = 0;
async.during(
function (result, callback) {
if(!callback) {
callback = result;
result = null;
}
return callback(null, !result || result.counter < 5);
},
function (callback) {
count++;
setTimeout(function() {
callback(null, { counter: count });
}, 1000);
},
function (err) {
// 5 seconds have passed
}
);
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.
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'
});
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');
}
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
functions... - the asynchronous functions to composeExample
// 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 - A collection of 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
);
Related
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. If you want to handle errors from an individual task, pass a callback to q.push().concurrency - An integer for determining how many worker functions should be
run in parallel. If omitted, the concurrency defaults to 1. If the concurrency is 0, an error is thrown.Queue objects
The queue object returned by this function has the following properties and
methods:
length() - a function returning the number of items waiting to be processed.started - a function returning whether or not any items have been pushed and processed by the 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 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 number of running workers hits the concurrency limit, and further tasks will be queued.unsaturated - a callback that is called when the number of running workers is less than the concurrency & buffer limits, and further tasks will not be queued.buffer A minimum threshold buffer in order to say that the queue is unsaturated.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 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 its callback(err) argument when finished, with
an optional err argument.payload - An optional integer for determining how many tasks should be
processed per round; if omitted, the default is unlimited.Cargo objects
The cargo object returned by this function has the following properties and
methods:
length() - A function returning the number of items waiting to be processed.payload - An integer for determining how many tasks should be
process per round. This property can be changed after a cargo is created to
alter the payload on-the-fly.push(task, [callback]) - Adds task to the queue. The callback is called
once the worker has finished processing the task. Instead of a single task, an array of tasks
can be submitted. The respective callback is used for every task in the list.saturated - A callback that is called when the queue.length() hits the concurrency and further tasks will be queued.empty - A callback that is called when the last item from the queue is given to a worker.drain - A callback that is called when the last item from the queue has returned from the worker.idle(), pause(), resume(), kill() - cargo inherits all of the same methods and event callbacks as queueExample
// 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, 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 as the first argument, if they have dependencies. If a task function has no dependencies, it will only be passed a callback.
async.auto({
// this function will just be passed a callback
readData: async.apply(fs.readFile, 'data.txt', 'utf-8')
showData: ['readData', function (results, cb) {
// results.readData is the file's contents
// ...
}]
}, 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 one or two arguments:
results object, containing the results of the previously executed functions, only passed if the task has any dependencies, Argument order changed in 2.0callback(err, result) function, which must be called when finished, passing an error (which can be null) and the result of the function's execution. Argument order changed in 2.0concurrency - An optional integer 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 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(results, 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
callback(null, 'filename');
}],
email_link: ['write_file', function(results, callback){
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).
A dependency-injected version of the auto function. Dependent tasks are specified as parameters to the function, before the usual callback parameter, with the parameter names matching the names of the tasks it depends on. This can provide even more readable task graphs which can be easier to maintain.
If a final callback is specified, the task results are still provided as a composite results object, exactly like auto.
The autoInject function is purely syntactic sugar and its semantics are otherwise equivalent to auto.
Arguments
tasks - An object, each of whose properties is a function of the form
'func([dependencies...], callback). 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.
callback parameter is 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. The remaining parameters name other tasks on which the task is dependent, and the results from those tasks are the arguments of those parameters.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
The example from auto can be rewritten as follows:
async.autoInject({
get_data: function(callback){
// async code to get some data
callback(null, 'data', 'converted to array');
},
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
callback(null, 'folder');
},
write_file: function(get_data, make_folder, callback){
// once there is some data and the directory exists,
// write the data to a file in the directory
callback(null, 'filename');
},
email_link: function(write_file, callback){
// once the file is written let's email a link to it...
// write_file contains the filename returned by write_file.
callback(null, {'file':write_file, 'email':'user@example.com'});
}
}, function(err, results) {
console.log('err = ', err);
console.log('email_link = ', results.email_link);
});
If you are using a JS minifier that mangles parameter names, autoInject will not work with plain functions, since the parameter names will be collapsed to a single letter identifier. To work around this, you can explicitly specify the names of the parameters your task function needs in an array, similar to Angular.js dependency injection. The final results callback can be provided as an array in the same way.
async.autoInject({
//...
write_file: ['get_data', 'make_folder', function(get_data, make_folder, callback){
callback(null, 'filename');
}],
email_link: ['write_file', function(write_file, callback){
callback(null, {'file':write_file, 'email':'user@example.com'});
}]
//...
}, function(err, results) {
console.log('err = ', err);
console.log('email_link = ', results.email_link);
});
This still has an advantage over plain auto, since the results a task depends on are still spread into arguments.
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 with times and interval or a number.
times - The number of attempts to make before giving up. The default is 5.interval - The time to wait between retries, in milliseconds. The default is 0. The interval may also be specified as a function of the retry count (see example).opts is a number, the number specifies the number of times to retry, with the default interval of 0.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. Callback made optional in 2.0, use retryable for previous behavior.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
});
The interval may also be specified as a function of the retry count:
// try calling apiMethod 10 times with exponential backoff
// (i.e. intervals of 100, 200, 400, 800, 1600, ... milliseconds)
async.retry({
times: 10,
interval: function(retryCount) {
return 50 * Math.pow(2, retryCount);
}
}, 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
});
A close relative of retry. This method wraps a task and makes it retryable, rather than immediately calling it with retries.
Arguments
opts - optional options, exactly the same as from retrytask - the asynchronous function to wrapExample
async.auto({
dep1: async.retryable(3, getFromFlakyService),
process: ["dep1", async.retryable(3, function (results, cb) {
maybeProcessData(results.dep1, cb)
})]
}, callback)
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 setImmediate. In the browser it will use setImmediate 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.args... - any number of additional arguments to pass to the callback on the next tickExample
var call_order = [];
async.nextTick(function(){
call_order.push('two');
// call_order now equals ['one','two']
});
call_order.push('one')
async.setImmediate(function (a, b, c) {
// a, b, and c equal 1, 2, and 3
}, 1, 2, 3)
Calls the iteratee 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.iteratee - 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
});
Related
Runs the tasks array of functions in parallel, without waiting until the
previous function has completed. Once any the tasks completed or pass an
error to its callback, the main callback is immediately called. It's
equivalent to Promise.race().
Arguments
tasks - An array 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, result) - A callback to run once any of the
functions have completed. This function gets an error or result from the
first function that completed.Example
async.race([
function(callback){
setTimeout(function(){
callback(null, 'one');
}, 200);
},
function(callback){
setTimeout(function(){
callback(null, 'two');
}, 100);
}
],
// main callback
function(err, result){
// the result will be equal to 'two' as it finishes earlier
});
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 functionWrap 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 argumentReturns 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);
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 (options, cb) {
startServer(options, cb);
}],
//...
}, callback);
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);
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'
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.
Sets a time limit on an asynchronous function. If the function does not call its callback within the specified miliseconds, it will be called with a timeout error. The code property for the error object will be 'ETIMEDOUT'.
Returns a wrapped function that can be used with any of the control flow functions.
Arguments
function - The asynchronous function you want to set the time limit.miliseconds - The specified time limit.info - Optional Any variable you want attached (string, object, etc) to
timeout Error for more information.Example
async.timeout(function nameOfCallback(callback) {
doAsyncTask(callback);
}, 1000, 'more info about timeout');
Wraps the function in another function that always returns data even when it errors.
The object returned has either the property error or value.
Arguments
function - The function you want to wrapExample
async.parallel([
async.reflect(function(callback){
// do some stuff ...
callback(null, 'one');
}),
async.reflect(function(callback){
// do some more stuff but error ...
callback('bad stuff happened');
}),
async.reflect(function(callback){
// do some more stuff ...
callback(null, 'two');
})
],
// optional callback
function(err, results){
// values
// results[0].value = 'one'
// results[1].error = 'bad stuff happened'
// results[2].value = 'two'
});
A helper function that wraps an array of functions with reflect.
Arguments
tasks - The array of functions to wrap in reflect.Example
let tasks = [
function(callback){
setTimeout(function(){
callback(null, 'one');
}, 200);
},
function(callback){
// do some more stuff but error ...
callback(new Error('bad stuff happened'));
}
function(callback){
setTimeout(function(){
callback(null, 'two');
}, 100);
}
];
async.parallel(async.reflectAll(tasks),
// optional callback
function(err, results){
// values
// results[0].value = 'one'
// results[1].error = Error('bad stuff happened')
// results[2].value = 'two'
});
FAQs
Higher-order functions and common patterns for asynchronous code
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We found that async-es demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 0 open source maintainers collaborating on the project.
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