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.
A collection of async functions for manipulating collections, such as
arrays and objects.
import concat from 'async/concat';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.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | A function to apply to each item in |
callback(err) |
function <optional> | A callback which is called after all the
|
async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files) {
// files is now a list of filenames that exist in the 3 directories
});import concatLimit from 'async/concatLimit';The same as concat but runs a maximum of limit async operations at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | A function to apply to each item in |
callback |
function <optional> | A callback which is called after all the
|
import concatSeries from 'async/concatSeries';The same as concat but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | A function to apply to each item in |
callback(err) |
function <optional> | A callback which is called after all the
|
import detect from 'async/detect';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.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | A truth test to apply to each item in |
callback |
function <optional> | A callback which is called as soon as any
iteratee returns |
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
});import detectLimit from 'async/detectLimit';The same as detect but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | A truth test to apply to each item in |
callback |
function <optional> | A callback which is called as soon as any
iteratee returns |
import detectSeries from 'async/detectSeries';The same as detect but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | A truth test to apply to each item in |
callback |
function <optional> | A callback which is called as soon as any
iteratee returns |
import each from 'async/each';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.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to
each item in |
callback |
function <optional> | A callback which is called when all
|
// 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');
}
});import eachLimit from 'async/eachLimit';The same as each but runs a maximum of limit async operations at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all
|
import eachOf from 'async/eachOf';Like each, except that it passes the key (or index) as the second argument
to the iteratee.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | A function to apply to each
item in |
callback |
function <optional> | A callback which is called when all
|
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);
});import eachOfLimit from 'async/eachOfLimit';The same as eachOf but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async function to apply to each
item in |
callback |
function <optional> | A callback which is called when all
|
import eachOfSeries from 'async/eachOfSeries';The same as eachOf but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
import eachSeries from 'async/eachSeries';The same as each but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to each
item in |
callback |
function <optional> | A callback which is called when all
|
import every from 'async/every';Returns true if every element in coll satisfies an async test. If any
iteratee call returns false, the main callback is immediately called.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async truth test to apply to each item in the collection in parallel. The iteratee must complete with a boolean result value. Invoked with (item, callback). |
callback |
function <optional> | A callback which is called after all the
|
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
});import everyLimit from 'async/everyLimit';The same as every but runs a maximum of limit async operations at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async truth test to apply to each item in the collection in parallel. The iteratee must complete with a boolean result value. Invoked with (item, callback). |
callback |
function <optional> | A callback which is called after all the
|
import everySeries from 'async/everySeries';The same as every but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async truth test to apply to each item in the collection in series. The iteratee must complete with a boolean result value. Invoked with (item, callback). |
callback |
function <optional> | A callback which is called after all the
|
import filter from 'async/filter';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.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
function | A truth test to apply to each item in |
callback |
function <optional> | A callback which is called after all the
|
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
});import filterLimit from 'async/filterLimit';The same as filter but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
function | A truth test to apply to each item in |
callback |
function <optional> | A callback which is called after all the
|
import filterSeries from 'async/filterSeries';The same as filter but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
function | A truth test to apply to each item in |
callback |
function <optional> | A callback which is called after all the
|
import groupBy from 'async/groupBy';Returns a new object, where each value corresponds to an array of items, from
coll, that returned the corresponding key. That is, the keys of the object
correspond to the values passed to the iteratee callback.
Note: 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 values for each key in the result will be in the same order as
the original coll. For Objects, the values will roughly be in the order of
the original Objects' keys (but this can vary across JavaScript engines).
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
async.groupBy(['userId1', 'userId2', 'userId3'], function(userId, callback) {
db.findById(userId, function(err, user) {
if (err) return callback(err);
return callback(null, user.age);
});
}, function(err, result) {
// result is object containing the userIds grouped by age
// e.g. { 30: ['userId1', 'userId3'], 42: ['userId2']};
});import groupByLimit from 'async/groupByLimit';The same as groupBy but runs a maximum of limit async operations at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
import groupBySeries from 'async/groupBySeries';The same as groupBy but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
import map from 'async/map';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.
If map is passed an Object, the results will be an Array. The results
will roughly be in the order of the original Objects' keys (but this can
vary across JavaScript engines).
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
async.map(['file1','file2','file3'], fs.stat, function(err, results) {
// results is now an array of stats for each file
});import mapLimit from 'async/mapLimit';The same as map but runs a maximum of limit async operations at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
import mapSeries from 'async/mapSeries';The same as map but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function <optional> | A callback which is called when all |
import mapValues from 'async/mapValues';A relative of map, designed for use with objects.
Produces a new Object by mapping each value of obj through the iteratee
function. The iteratee is called each value and key from obj and a
callback for when it has finished processing. Each of these callbacks takes
two arguments: an error, and the transformed item from obj. If iteratee
passes an error to its callback, the main callback (for the mapValues
function) is immediately called with the error.
Note, the order of the keys in the result is not guaranteed. The keys will be roughly in the order they complete, (but this is very engine-specific)
| Name | Type | Description |
|---|---|---|
obj |
Object | A collection to iterate over. |
iteratee |
AsyncFunction | A function to apply to each value and key
in |
callback |
function <optional> | A callback which is called when all |
async.mapValues({
f1: 'file1',
f2: 'file2',
f3: 'file3'
}, function (file, key, callback) {
fs.stat(file, callback);
}, function(err, result) {
// result is now a map of stats for each file, e.g.
// {
// f1: [stats for file1],
// f2: [stats for file2],
// f3: [stats for file3]
// }
});import mapValuesLimit from 'async/mapValuesLimit';The same as mapValues but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
obj |
Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | A function to apply to each value and key
in |
callback |
function <optional> | A callback which is called when all |
import mapValuesSeries from 'async/mapValuesSeries';The same as mapValues but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
obj |
Object | A collection to iterate over. |
iteratee |
AsyncFunction | A function to apply to each value and key
in |
callback |
function <optional> | A callback which is called when all |
import reduce from 'async/reduce';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.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
memo |
* | The initial state of the reduction. |
iteratee |
AsyncFunction | A function applied to each item in the
array to produce the next step in the reduction.
The |
callback |
function <optional> | A callback which is called after all the
|
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
});import reduceRight from 'async/reduceRight';Same as reduce, only operates on array in reverse order.
| Name | Type | Description |
|---|---|---|
array |
Array | A collection to iterate over. |
memo |
* | The initial state of the reduction. |
iteratee |
AsyncFunction | A function applied to each item in the
array to produce the next step in the reduction.
The |
callback |
function <optional> | A callback which is called after all the
|
import reject from 'async/reject';The opposite of filter. Removes values that pass an async truth test.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
function | An async truth test to apply to each item in
|
callback |
function <optional> | A callback which is called after all the
|
async.reject(['file1','file2','file3'], function(filePath, callback) {
fs.access(filePath, function(err) {
callback(null, !err)
});
}, function(err, results) {
// results now equals an array of missing files
createFiles(results);
});import rejectLimit from 'async/rejectLimit';The same as reject but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
function | An async truth test to apply to each item in
|
callback |
function <optional> | A callback which is called after all the
|
import rejectSeries from 'async/rejectSeries';The same as reject but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
function | An async truth test to apply to each item in
|
callback |
function <optional> | A callback which is called after all the
|
import some from 'async/some';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.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async truth test to apply to each item
in the collections in parallel.
The iteratee should complete with a boolean |
callback |
function <optional> | A callback which is called as soon as any
iteratee returns |
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
});import someLimit from 'async/someLimit';The same as some but runs a maximum of limit async operations at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | An async truth test to apply to each item
in the collections in parallel.
The iteratee should complete with a boolean |
callback |
function <optional> | A callback which is called as soon as any
iteratee returns |
import someSeries from 'async/someSeries';The same as some but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async truth test to apply to each item
in the collections in series.
The iteratee should complete with a boolean |
callback |
function <optional> | A callback which is called as soon as any
iteratee returns |
import sortBy from 'async/sortBy';Sorts a list by the results of running each coll value through an async
iteratee.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
iteratee |
AsyncFunction | An async function to apply to each item in
|
callback |
function | A callback which is called after all the
|
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
});
// 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
});import transform from 'async/transform';A relative of reduce. Takes an Object or Array, and iterates over each
element in series, each step potentially mutating an accumulator value.
The type of the accumulator defaults to the type of collection passed in.
| Name | Type | Description |
|---|---|---|
coll |
Array | Iterable | Object | A collection to iterate over. |
accumulator |
* <optional> | The initial state of the transform. If omitted,
it will default to an empty Object or Array, depending on the type of |
iteratee |
AsyncFunction | A function applied to each item in the collection that potentially modifies the accumulator. Invoked with (accumulator, item, key, callback). |
callback |
function <optional> | A callback which is called after all the
|
async.transform([1,2,3], function(acc, item, index, callback) {
// pointless async:
process.nextTick(function() {
acc.push(item * 2)
callback(null)
});
}, function(err, result) {
// result is now equal to [2, 4, 6]
});async.transform({a: 1, b: 2, c: 3}, function (obj, val, key, callback) {
setImmediate(function () {
obj[key] = val * 2;
callback();
})
}, function (err, result) {
// result is equal to {a: 2, b: 4, c: 6}
})A collection of async functions for controlling the flow through a script.
import applyEach from 'async/applyEach';Applies the provided arguments to each function in the array, calling
callback after all functions have completed. If you only provide the first
argument, fns, then it will return a function which lets you pass in the
arguments as if it were a single function call. If more arguments are
provided, callback is required while args is still optional.
| Name | Type | Description |
|---|---|---|
fns |
Array | Iterable | Object | A collection of AsyncFunctions to all call with the same arguments |
args |
* <optional> | any number of separate arguments to pass to the function. |
callback |
function <optional> | the final argument should be the callback, called when all functions have completed processing. |
fns, is provided, it will
return a function which lets you pass in the arguments as if it were a single
function call. The signature is (..args, callback). If invoked with any
arguments, callback is required.async.applyEach([enableSearch, updateSchema], 'bucket', callback);
// partial application example:
async.each(
buckets,
async.applyEach([enableSearch, updateSchema]),
callback
);import applyEachSeries from 'async/applyEachSeries';The same as applyEach but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
fns |
Array | Iterable | Object | A collection of AsyncFunctions to all call with the same arguments |
args |
* <optional> | any number of separate arguments to pass to the function. |
callback |
function <optional> | the final argument should be the callback, called when all functions have completed processing. |
import auto from 'async/auto';Determines the best order for running the AsyncFunctions 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 AsyncFunctions 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.
AsyncFunctions 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.
| Name | Type | Default | Description |
|---|---|---|---|
tasks |
Object | An object. Each of its properties is either a function or an array of requirements, with the AsyncFunction 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:
|
|
concurrency |
number <optional> | Infinity | An optional |
callback |
function <optional> | An optional callback which is called when all
the tasks have been completed. It receives the |
undefined
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);
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);
});import autoInject from 'async/autoInject';A dependency-injected version of the async.auto function. Dependent tasks are specified as parameters to the function, after 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 similarly injected, specified as named parameters after the initial error parameter.
The autoInject function is purely syntactic sugar and its semantics are otherwise equivalent to async.auto.
| Name | Type | Description |
|---|---|---|
tasks |
Object | An object, each of whose properties is an AsyncFunction 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 |
function <optional> | An optional callback which is called when all
the tasks have been completed. It receives the |
// 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.
// This still has an advantage over plain `auto`, since the results a task
// depends on are still spread into arguments.
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);
});import cargo from 'async/cargo';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.
| Name | Type | Default | Description |
|---|---|---|---|
worker |
AsyncFunction | An asynchronous function for processing an array
of queued tasks. Invoked with |
|
payload |
number <optional> | Infinity | An optional |
A cargo object to manage the tasks. Callbacks can attached as certain properties to listen for specific events during the lifecycle of the cargo and inner queue.
// 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');
});import compose from 'async/compose';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.
| Name | Type | Description |
|---|---|---|
functions |
AsyncFunction | the asynchronous functions to compose |
an asynchronous function that is the composed
asynchronous functions
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
});import doDuring from 'async/doDuring';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.
| Name | Type | Description |
|---|---|---|
fn |
AsyncFunction | An async function which is called each time
|
test |
AsyncFunction | asynchronous truth test to perform before each
execution of |
callback |
function <optional> | A callback which is called after the test
function has failed and repeated execution of |
import doUntil from 'async/doUntil';Like 'doWhilst', except the test is inverted. Note the
argument ordering differs from until.
| Name | Type | Description |
|---|---|---|
iteratee |
AsyncFunction | An async function which is called each time
|
test |
function | synchronous truth test to perform after each
execution of |
callback |
function <optional> | A callback which is called after the test
function has passed and repeated execution of |
import doWhilst from 'async/doWhilst';The post-check version of whilst. To reflect the difference in
the order of operations, the arguments test and iteratee are switched.
doWhilst is to whilst as do while is to while in plain JavaScript.
| Name | Type | Description |
|---|---|---|
iteratee |
AsyncFunction | A function which is called each time |
test |
function | synchronous truth test to perform after each
execution of |
callback |
function <optional> | A callback which is called after the test
function has failed and repeated execution of |
import during from 'async/during';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.
| Name | Type | Description |
|---|---|---|
test |
AsyncFunction | asynchronous truth test to perform before each
execution of |
fn |
AsyncFunction | An async function which is called each time
|
callback |
function <optional> | A callback which is called after the test
function has failed and repeated execution of |
var count = 0;
async.during(
function (callback) {
return callback(null, count < 5);
},
function (callback) {
count++;
setTimeout(callback, 1000);
},
function (err) {
// 5 seconds have passed
}
);import forever from 'async/forever';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.
| Name | Type | Description |
|---|---|---|
fn |
AsyncFunction | an async function to call repeatedly. Invoked with (next). |
errback |
function <optional> | when |
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.
}
);import parallel from 'async/parallel';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.
Hint: Use reflect to continue the
execution of other tasks when a task fails.
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.
| Name | Type | Description |
|---|---|---|
tasks |
Array | Iterable | Object | A collection of
async functions to run.
Each async function can complete with any number of optional |
callback |
function <optional> | 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. Invoked with (err, results). |
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}
});import parallelLimit from 'async/parallelLimit';The same as parallel but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
tasks |
Array | Iterable | Object | A collection of
async functions to run.
Each async function can complete with any number of optional |
limit |
number | The maximum number of async operations at a time. |
callback |
function <optional> | 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. Invoked with (err, results). |
import priorityQueue from 'async/priorityQueue';The same as async.queue only tasks are assigned a priority and completed in ascending priority order.
| Name | Type | Description |
|---|---|---|
worker |
AsyncFunction | An async function for processing a queued task.
If you want to handle errors from an individual task, pass a callback to
|
concurrency |
number | An |
A priorityQueue object to manage the tasks. 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.import queue from 'async/queue';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.
| Name | Type | Default | Description |
|---|---|---|---|
worker |
AsyncFunction | An async function for processing a queued task.
If you want to handle errors from an individual task, pass a callback to
|
|
concurrency |
number <optional> | 1 | An |
A queue object to manage the tasks. Callbacks can attached as certain properties to listen for specific events during the lifecycle of the queue.
// 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');
});import race from 'async/race';Runs the tasks array of functions in parallel, without waiting until the
previous function has completed. Once any of the tasks complete or pass an
error to its callback, the main callback is immediately called. It's
equivalent to Promise.race().
| Name | Type | Description |
|---|---|---|
tasks |
Array | An array containing async functions
to run. Each function can complete with an optional |
callback |
function | A callback to run once any of the functions have completed. This function gets an error or result from the first function that completed. Invoked with (err, result). |
undefined
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
});import retry from 'async/retry';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.
| Name | Type | Default | Description |
|---|---|---|---|
opts |
Object | number <optional> | {times: 5, interval: 0}| 5 | Can be either an
object with
|
task |
AsyncFunction | An async function to retry. Invoked with (callback). |
|
callback |
function <optional> | An optional callback which is called when the
task has succeeded, or after the final failed attempt. It receives the |
// The `retry` function can be used as a stand-alone control flow by passing
// a callback, as shown below:
// try calling apiMethod 3 times
async.retry(3, apiMethod, function(err, result) {
// do something with the result
});
// try calling apiMethod 3 times, waiting 200 ms between each retry
async.retry({times: 3, interval: 200}, apiMethod, function(err, result) {
// do something with the result
});
// try calling apiMethod 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
});
// try calling apiMethod only when error condition satisfies, all other
// errors will abort the retry control flow and return to final callback
async.retry({
errorFilter: function(err) {
return err.message === 'Temporary error'; // only retry on a specific error
}
}, apiMethod, function(err, result) {
// do something with the result
});
// to retry individual methods that are not as reliable within other
// control flow functions, use the `retryable` wrapper:
async.auto({
users: api.getUsers.bind(api),
payments: async.retryable(3, api.getPayments.bind(api))
}, function(err, results) {
// do something with the results
});import retryable from 'async/retryable';A close relative of retry. This method
wraps a task and makes it retryable, rather than immediately calling it
with retries.
| Name | Type | Default | Description |
|---|---|---|---|
opts |
Object | number <optional> | {times: 5, interval: 0}| 5 | optional
options, exactly the same as from |
task |
AsyncFunction | the asynchronous function to wrap. This function will be passed any arguments passed to the returned wrapper. Invoked with (...args, callback). |
The wrapped function, which when invoked, will
retry on an error, based on the parameters specified in opts.
This function will accept the same parameters as task.
async.auto({
dep1: async.retryable(3, getFromFlakyService),
process: ["dep1", async.retryable(3, function (results, cb) {
maybeProcessData(results.dep1, cb);
})]
}, callback);import seq from 'async/seq';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.
| Name | Type | Description |
|---|---|---|
functions |
AsyncFunction | the asynchronous functions to compose |
a function that composes the functions in order
// 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 });
}
});
});import series from 'async/series';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 async.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.
| Name | Type | Description |
|---|---|---|
tasks |
Array | Iterable | Object | A collection containing
async functions to run in series.
Each function can complete with any number of optional |
callback |
function <optional> | 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 |
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']
});
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}
});import times from 'async/times';Calls the iteratee function n times, and accumulates results in the same
manner you would use with map.
| Name | Type | Description |
|---|---|---|
n |
number | The number of times to run the function. |
iteratee |
AsyncFunction | The async function to call |
callback |
function | see map. |
// 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
});import timesLimit from 'async/timesLimit';The same as times but runs a maximum of limit async operations at a
time.
| Name | Type | Description |
|---|---|---|
count |
number | The number of times to run the function. |
limit |
number | The maximum number of async operations at a time. |
iteratee |
AsyncFunction | The async function to call |
callback |
function | see async.map. |
import timesSeries from 'async/timesSeries';The same as times but runs only a single async operation at a time.
| Name | Type | Description |
|---|---|---|
n |
number | The number of times to run the function. |
iteratee |
AsyncFunction | The async function to call |
callback |
function | see map. |
import tryEach from 'async/tryEach';It runs each task in series but stops whenever any of the functions were
successful. If one of the tasks were successful, the callback will be
passed the result of the successful task. If all tasks fail, the callback
will be passed the error and result (if any) of the final attempt.
| Name | Type | Description |
|---|---|---|
tasks |
Array | Iterable | Object | A collection containing functions to
run, each function is passed a |
callback |
function <optional> | An optional callback which is called when one
of the tasks has succeeded, or all have failed. It receives the |
async.tryEach([
function getDataFromFirstWebsite(callback) {
// Try getting the data from the first website
callback(err, data);
},
function getDataFromSecondWebsite(callback) {
// First website failed,
// Try getting the data from the backup website
callback(err, data);
}
],
// optional callback
function(err, results) {
Now do something with the data.
});import until from 'async/until';Repeatedly call iteratee 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 iteratee's callback.
The inverse of whilst.
| Name | Type | Description |
|---|---|---|
test |
function | synchronous truth test to perform before each
execution of |
iteratee |
AsyncFunction | An async function which is called each time
|
callback |
function <optional> | A callback which is called after the test
function has passed and repeated execution of |
import waterfall from 'async/waterfall';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.
| Name | Type | Description |
|---|---|---|
tasks |
Array | An array of async functions
to run.
Each function should complete with any number of |
callback |
function <optional> | An optional callback to run once all the functions have completed. This will be passed the results of the last task's callback. Invoked with (err, [results]). |
undefined
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');
}import whilst from 'async/whilst';Repeatedly call iteratee, while test returns true. Calls callback when
stopped, or an error occurs.
| Name | Type | Description |
|---|---|---|
test |
function | synchronous truth test to perform before each
execution of |
iteratee |
AsyncFunction | An async function which is called each time
|
callback |
function <optional> | A callback which is called after the test
function has failed and repeated execution of |
undefined
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
}
);import cargo from 'async/cargo';A cargo of tasks for the worker function to complete. Cargo inherits all of
the same methods and event callbacks as queue.
| Name | Type | Description |
|---|---|---|
length |
function | A function returning the number of items
waiting to be processed. Invoke like |
payload |
number | An |
push |
function | Adds |
saturated |
function | A callback that is called when the
|
empty |
function | A callback that is called when the last item
from the |
drain |
function | A callback that is called when the last item
from the |
idle |
function | a function returning false if there are items
waiting or being processed, or true if not. Invoke like |
pause |
function | a function that pauses the processing of tasks
until |
resume |
function | a function that resumes the processing of
queued tasks when the queue is paused. Invoke like |
kill |
function | a function that removes the |
import queue from 'async/queue';A queue of tasks for the worker function to complete.
| Name | Type | Description |
|---|---|---|
length |
function | a function returning the number of items
waiting to be processed. Invoke with |
started |
boolean | a boolean indicating whether or not any items have been pushed and processed by the queue. |
running |
function | a function returning the number of items
currently being processed. Invoke with |
workersList |
function | a function returning the array of items
currently being processed. Invoke with |
idle |
function | a function returning false if there are items
waiting or being processed, or true if not. Invoke with |
concurrency |
number | an integer for determining how many |
push |
function | add a new task to the |
unshift |
function | add a new task to the front of the |
remove |
function | remove items from the queue that match a test
function. The test function will be passed an object with a |
saturated |
function | a callback that is called when the number of
running workers hits the |
unsaturated |
function | a callback that is called when the number
of running workers is less than the |
buffer |
number | A minimum threshold buffer in order to say that
the |
empty |
function | a callback that is called when the last item
from the |
drain |
function | a callback that is called when the last item
from the |
error |
function | a callback that is called when a task errors.
Has the signature |
paused |
boolean | a boolean for determining whether the queue is in a paused state. |
pause |
function | a function that pauses the processing of tasks
until |
resume |
function | a function that resumes the processing of
queued tasks when the queue is paused. Invoke with |
kill |
function | a function that removes the |
import apply from 'async/apply';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.
| Name | Type | Description |
|---|---|---|
fn |
function | The function you want to eventually apply all arguments to. Invokes with (arguments...). |
arguments... |
* | Any number of arguments to automatically apply when the continuation is called. |
the partially-applied function
// 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
threeimport asyncify from 'async/asyncify';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.
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.
This also means you can asyncify ES2017 async functions.
| Name | Type | Description |
|---|---|---|
func |
function | The synchronous function, or Promise-returning function to convert to an AsyncFunction. |
An asynchronous wrapper of the func. To be
invoked with (args..., callback).
// passing a regular synchronous function
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);
// passing a function returning a promise
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);
// es2017 example, though `asyncify` is not needed if your JS environment
// supports async functions out of the box
var q = async.queue(async.asyncify(async function(file) {
var intermediateStep = await processFile(file);
return await somePromise(intermediateStep)
}));
q.push(files);import constant from 'async/constant';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.
| Name | Type | Description |
|---|---|---|
arguments... |
* | Any number of arguments to automatically invoke callback with. |
Returns a function that when invoked, automatically invokes the callback with the previous given arguments.
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);import dir from 'async/dir';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.
| Name | Type | Description |
|---|---|---|
function |
AsyncFunction | The function you want to eventually apply all arguments to. |
arguments... |
* | Any number of arguments to apply to the function. |
// in a module
var hello = function(name, callback) {
setTimeout(function() {
callback(null, {hello: name});
}, 1000);
};
// in the node repl
node> async.dir(hello, 'world');
{hello: 'world'}import ensureAsync from 'async/ensureAsync';Wrap an async function and ensure it calls its callback on a later tick of
the event loop. If the function already calls its callback on a next tick,
no extra deferral is added. This is useful for preventing stack overflows
(RangeError: Maximum call stack size exceeded) and generally keeping
Zalgo
contained. ES2017 async functions are returned as-is -- they are immune
to Zalgo's corrupting influences, as they always resolve on a later tick.
| Name | Type | Description |
|---|---|---|
fn |
AsyncFunction | an async function, one that expects a node-style callback as its last argument. |
Returns a wrapped function with the exact same call signature as the function passed in.
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);import log from 'async/log';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.
| Name | Type | Description |
|---|---|---|
function |
AsyncFunction | The function you want to eventually apply all arguments to. |
arguments... |
* | Any number of arguments to apply to the function. |
// in a module
var hello = function(name, callback) {
setTimeout(function() {
callback(null, 'hello ' + name);
}, 1000);
};
// in the node repl
node> async.log(hello, 'world');
'hello world'import memoize from 'async/memoize';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.
| Name | Type | Description |
|---|---|---|
fn |
AsyncFunction | The async function to proxy and cache results from. |
hasher |
function | 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. |
a memoized version of fn
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
});import nextTick from 'async/nextTick';Calls callback on a later loop around the event loop. In Node.js this just
calls process.nextTick. 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.
| Name | Type | Description |
|---|---|---|
callback |
function | The function to call on a later loop around the event loop. Invoked with (args...). |
args... |
* | any number of additional arguments to pass to the callback on the next tick. |
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);import reflect from 'async/reflect';Wraps the async function in another function that always completes with a result object, even when it errors.
The result object has either the property error or value.
| Name | Type | Description |
|---|---|---|
fn |
AsyncFunction | The async function you want to wrap |
object with
either an error or a value property.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'
});import reflectAll from 'async/reflectAll';A helper function that wraps an array or an object of functions with reflect.
| Name | Type | Description |
|---|---|---|
tasks |
Array | Object | Iterable | The collection of
async functions to wrap in |
Returns an array of async functions, each wrapped in
async.reflect
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'
});
// an example using an object instead of an array
let tasks = {
one: function(callback) {
setTimeout(function() {
callback(null, 'one');
}, 200);
},
two: function(callback) {
callback('two');
},
three: function(callback) {
setTimeout(function() {
callback(null, 'three');
}, 100);
}
};
async.parallel(async.reflectAll(tasks),
// optional callback
function(err, results) {
// values
// results.one.value = 'one'
// results.two.error = 'two'
// results.three.value = 'three'
});import setImmediate from 'async/setImmediate';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.
| Name | Type | Description |
|---|---|---|
callback |
function | The function to call on a later loop around the event loop. Invoked with (args...). |
args... |
* | any number of additional arguments to pass to the callback on the next tick. |
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);import timeout from 'async/timeout';Sets a time limit on an asynchronous function. If the function does not call
its callback within the specified milliseconds, it will be called with a
timeout error. The code property for the error object will be 'ETIMEDOUT'.
| Name | Type | Description |
|---|---|---|
asyncFn |
AsyncFunction | The async function to limit in time. |
milliseconds |
number | The specified time limit. |
info |
* <optional> | Any variable you want attached ( |
Returns a wrapped function that can be used with any
of the control flow functions.
Invoke this function with the same parameters as you would asyncFunc.
function myFunction(foo, callback) {
doAsyncTask(foo, function(err, data) {
// handle errors
if (err) return callback(err);
// do some stuff ...
// return processed data
return callback(null, data);
});
}
var wrapped = async.timeout(myFunction, 1000);
// call `wrapped` as you would `myFunction`
wrapped({ bar: 'bar' }, function(err, data) {
// if `myFunction` takes < 1000 ms to execute, `err`
// and `data` will have their expected values
// else `err` will be an Error with the code 'ETIMEDOUT'
});import unmemoize from 'async/unmemoize';Undoes a memoized function, reverting it to the original, unmemoized form. Handy for testing.
| Name | Type | Description |
|---|---|---|
fn |
AsyncFunction | the memoized function |
a function that calls the original unmemoized function