Async.js

Collections

A collection of async functions for manipulating collections, such as arrays and objects.

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Methods

(static) concat(coll, iteratee, callback(err)opt)

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.

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

A function to apply to each item in coll, which should use an array as its result. Invoked with (item, callback).
callback(err) function <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. Invoked with (err, results).
Example
async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files) {
    // files is now a list of filenames that exist in the 3 directories
});
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(static) concatLimit(coll, limit, iteratee, callbackopt)

import concatLimit from 'async/concatLimit';

The same as concat but runs a maximum of limit async operations at a time.

Parameters:
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 coll, which should use an array as its result. Invoked with (item, callback).
callback function <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. Invoked with (err, results).
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(static) concatSeries(coll, iteratee, callback(err)opt)

import concatSeries from 'async/concatSeries';

The same as concat but runs only a single async operation at a time.

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

A function to apply to each item in coll. The iteratee should complete with an array an array of results. Invoked with (item, callback).
callback(err) function <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. Invoked with (err, results).
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(static) detect(coll, iteratee, callbackopt)

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.

Alias:
  • find
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

A truth test to apply to each item in coll. The iteratee must complete with a boolean value as its result. Invoked with (item, callback).
callback function <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. Invoked with (err, result).
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
});
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(static) detectLimit(coll, limit, iteratee, callbackopt)

import detectLimit from 'async/detectLimit';

The same as detect but runs a maximum of limit async operations at a time.

Alias:
  • findLimit
Parameters:
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 coll. The iteratee must complete with a boolean value as its result. Invoked with (item, callback).
callback function <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. Invoked with (err, result).
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(static) detectSeries(coll, iteratee, callbackopt)

import detectSeries from 'async/detectSeries';

The same as detect but runs only a single async operation at a time.

Alias:
  • findSeries
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

A truth test to apply to each item in coll. The iteratee must complete with a boolean value as its result. Invoked with (item, callback).
callback function <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. Invoked with (err, result).
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(static) each(coll, iteratee, callbackopt)

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.

Alias:
  • forEach
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. Invoked with (item, callback). The array index is not passed to the iteratee. If you need the index, use eachOf.
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
Example
// 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');
    }
});
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(static) eachLimit(coll, limit, iteratee, callbackopt)

import eachLimit from 'async/eachLimit';

The same as each but runs a maximum of limit async operations at a time.

Alias:
  • forEachLimit
Parameters:
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 coll. The array index is not passed to the iteratee. If you need the index, use eachOfLimit. Invoked with (item, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
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(static) eachOf(coll, iteratee, callbackopt)

import eachOf from 'async/eachOf';

Like each, except that it passes the key (or index) as the second argument to the iteratee.

Alias:
  • forEachOf
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

A function to apply to each item in coll. The key is the item's key, or index in the case of an array. Invoked with (item, key, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
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);
});
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(static) eachOfLimit(coll, limit, iteratee, callbackopt)

import eachOfLimit from 'async/eachOfLimit';

The same as eachOf but runs a maximum of limit async operations at a time.

Alias:
  • forEachOfLimit
Parameters:
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 coll. The key is the item's key, or index in the case of an array. Invoked with (item, key, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
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(static) eachOfSeries(coll, iteratee, callbackopt)

import eachOfSeries from 'async/eachOfSeries';

The same as eachOf but runs only a single async operation at a time.

Alias:
  • forEachOfSeries
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. Invoked with (item, key, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
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(static) eachSeries(coll, iteratee, callbackopt)

import eachSeries from 'async/eachSeries';

The same as each but runs only a single async operation at a time.

Alias:
  • forEachSeries
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. The array index is not passed to the iteratee. If you need the index, use eachOfSeries. Invoked with (item, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
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(static) every(coll, iteratee, callbackopt)

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.

Alias:
  • all
Parameters:
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 iteratee functions have finished. Result will be either true or false depending on the values of the async tests. Invoked with (err, result).
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
});
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(static) everyLimit(coll, limit, iteratee, callbackopt)

import everyLimit from 'async/everyLimit';

The same as every but runs a maximum of limit async operations at a time.

Alias:
  • allLimit
Parameters:
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 iteratee functions have finished. Result will be either true or false depending on the values of the async tests. Invoked with (err, result).
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(static) everySeries(coll, iteratee, callbackopt)

import everySeries from 'async/everySeries';

The same as every but runs only a single async operation at a time.

Alias:
  • allSeries
Parameters:
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 iteratee functions have finished. Result will be either true or false depending on the values of the async tests. Invoked with (err, result).
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(static) filter(coll, iteratee, callbackopt)

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.

Alias:
  • select
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee function

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. Invoked with (item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Invoked with (err, results).
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
});
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(static) filterLimit(coll, limit, iteratee, callbackopt)

import filterLimit from 'async/filterLimit';

The same as filter but runs a maximum of limit async operations at a time.

Alias:
  • selectLimit
Parameters:
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 coll. The iteratee is passed a callback(err, truthValue), which must be called with a boolean argument once it has completed. Invoked with (item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Invoked with (err, results).
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(static) filterSeries(coll, iteratee, callbackopt)

import filterSeries from 'async/filterSeries';

The same as filter but runs only a single async operation at a time.

Alias:
  • selectSeries
Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee function

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. Invoked with (item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Invoked with (err, results)
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(static) groupBy(coll, iteratee, callbackopt)

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).

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. The iteratee should complete with a key to group the value under. Invoked with (value, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Result is an Object whoses properties are arrays of values which returned the corresponding key.
Example
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']};
});
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(static) groupByLimit(coll, limit, iteratee, callbackopt)

import groupByLimit from 'async/groupByLimit';

The same as groupBy but runs a maximum of limit async operations at a time.

Parameters:
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 coll. The iteratee should complete with a key to group the value under. Invoked with (value, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Result is an Object whoses properties are arrays of values which returned the corresponding key.
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(static) groupBySeries(coll, limit, iteratee, callbackopt)

import groupBySeries from 'async/groupBySeries';

The same as groupBy but runs only a single async operation at a time.

Parameters:
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 coll. The iteratee should complete with a key to group the value under. Invoked with (value, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. Result is an Object whoses properties are arrays of values which returned the corresponding key.
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(static) map(coll, iteratee, callbackopt)

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).

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. The iteratee should complete with the transformed item. Invoked with (item, callback).
callback function <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. Invoked with (err, results).
Example
async.map(['file1','file2','file3'], fs.stat, function(err, results) {
    // results is now an array of stats for each file
});
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(static) mapLimit(coll, limit, iteratee, callbackopt)

import mapLimit from 'async/mapLimit';

The same as map but runs a maximum of limit async operations at a time.

Parameters:
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 coll. The iteratee should complete with the transformed item. Invoked with (item, callback).
callback function <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. Invoked with (err, results).
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(static) mapSeries(coll, iteratee, callbackopt)

import mapSeries from 'async/mapSeries';

The same as map but runs only a single async operation at a time.

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. The iteratee should complete with the transformed item. Invoked with (item, callback).
callback function <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. Invoked with (err, results).
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(static) mapValues(obj, iteratee, callbackopt)

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)

Parameters:
Name Type Description
obj Object

A collection to iterate over.
iteratee AsyncFunction

A function to apply to each value and key in coll. The iteratee should complete with the transformed value as its result. Invoked with (value, key, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. result is a new object consisting of each key from obj, with each transformed value on the right-hand side. Invoked with (err, result).
Example
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]
    // }
});
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(static) mapValuesLimit(obj, limit, iteratee, callbackopt)

import mapValuesLimit from 'async/mapValuesLimit';

The same as mapValues but runs a maximum of limit async operations at a time.

Parameters:
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 coll. The iteratee should complete with the transformed value as its result. Invoked with (value, key, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. result is a new object consisting of each key from obj, with each transformed value on the right-hand side. Invoked with (err, result).
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(static) mapValuesSeries(obj, iteratee, callbackopt)

import mapValuesSeries from 'async/mapValuesSeries';

The same as mapValues but runs only a single async operation at a time.

Parameters:
Name Type Description
obj Object

A collection to iterate over.
iteratee AsyncFunction

A function to apply to each value and key in coll. The iteratee should complete with the transformed value as its result. Invoked with (value, key, callback).
callback function <optional>

A callback which is called when all iteratee functions have finished, or an error occurs. result is a new object consisting of each key from obj, with each transformed value on the right-hand side. Invoked with (err, result).
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(static) reduce(coll, memo, iteratee, callbackopt)

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.

Alias:
  • foldl
Parameters:
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 iteratee should complete with the next state of the reduction. If the iteratee complete with an error, the reduction is stopped and the main callback is immediately called with the error. Invoked with (memo, item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Result is the reduced value. Invoked with (err, result).
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
});
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(static) reduceRight(array, memo, iteratee, callbackopt)

import reduceRight from 'async/reduceRight';

Same as reduce, only operates on array in reverse order.

Alias:
  • foldr
Parameters:
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 iteratee should complete with the next state of the reduction. If the iteratee complete with an error, the reduction is stopped and the main callback is immediately called with the error. Invoked with (memo, item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Result is the reduced value. Invoked with (err, result).
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(static) reject(coll, iteratee, callbackopt)

import reject from 'async/reject';

The opposite of filter. Removes values that pass an async truth test.

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee function

An async truth test to apply to each item in coll. The should complete with a boolean value as its result. Invoked with (item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Invoked with (err, results).
Example
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);
});
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(static) rejectLimit(coll, limit, iteratee, callbackopt)

import rejectLimit from 'async/rejectLimit';

The same as reject but runs a maximum of limit async operations at a time.

Parameters:
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 coll. The should complete with a boolean value as its result. Invoked with (item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Invoked with (err, results).
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(static) rejectSeries(coll, iteratee, callbackopt)

import rejectSeries from 'async/rejectSeries';

The same as reject but runs only a single async operation at a time.

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee function

An async truth test to apply to each item in coll. The should complete with a boolean value as its result. Invoked with (item, callback).
callback function <optional>

A callback which is called after all the iteratee functions have finished. Invoked with (err, results).
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(static) some(coll, iteratee, callbackopt)

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.

Alias:
  • any
Parameters:
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 result value. Invoked with (item, callback).
callback function <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. Invoked with (err, result).
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
});
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(static) someLimit(coll, limit, iteratee, callbackopt)

import someLimit from 'async/someLimit';

The same as some but runs a maximum of limit async operations at a time.

Alias:
  • anyLimit
Parameters:
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 result value. Invoked with (item, callback).
callback function <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. Invoked with (err, result).
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(static) someSeries(coll, iteratee, callbackopt)

import someSeries from 'async/someSeries';

The same as some but runs only a single async operation at a time.

Alias:
  • anySeries
Parameters:
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 result value. Invoked with (item, callback).
callback function <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. Invoked with (err, result).
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(static) sortBy(coll, iteratee, callback)

import sortBy from 'async/sortBy';

Sorts a list by the results of running each coll value through an async iteratee.

Parameters:
Name Type Description
coll Array | Iterable | Object

A collection to iterate over.
iteratee AsyncFunction

An async function to apply to each item in coll. The iteratee should complete with a value to use as the sort criteria as its result. Invoked with (item, callback).
callback function

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. Invoked with (err, results).
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
});

// 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
});
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(static) transform(coll, accumulatoropt, iteratee, callbackopt)

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.

Parameters:
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 coll
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 iteratee functions have finished. Result is the transformed accumulator. Invoked with (err, result).
Examples
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}
})
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