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Async.js

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 async, it can also be used directly in the browser.

Async is also installable via:

bower: bower install async
component: component install caolan/async
jam: jam install async
spm: spm install async

Async provides around 20 functions that include the usual 'functional' suspects (map, reduce, filter, each…) as well as some common patterns for asynchronous control flow (parallel, series, waterfall…). All these functions assume you follow the Node.js convention of providing a single callback as the last argument of your async function.

Quick Examples

async.map(['file1','file2','file3'], fs.stat, function(err, results){
    // results is now an array of stats for each file
});

async.filter(['file1','file2','file3'], fs.exists, function(results){
    // results now equals an array of the existing files
});

async.parallel([
    function(){ ... },
    function(){ ... }
], callback);

async.series([
    function(){ ... },
    function(){ ... }
]);

There are many more functions available so take a look at the docs below for a full list. This module aims to be comprehensive, so if you feel anything is missing please create a GitHub issue for it.

Common Pitfalls _{(StackOverflow)}

Synchronous iteration functions

If you get an error like RangeError: Maximum call stack size exceeded. or other stack overflow issues when using async, you are likely using a synchronous iterator. By synchronous we mean a function that calls its callback on the same tick in the javascript event loop, without doing any I/O or using any timers. Calling many callbacks iteratively will quickly overflow the stack. If you run into this issue, just defer your callback with async.setImmediate to start a new call stack on the next tick of the event loop.

This can also arise by accident if you callback early in certain cases:

async.eachSeries(hugeArray, function iterator(item, callback) {
  if (inCache(item)) {
    callback(null, cache[item]); // if many items are cached, you'll overflow
  } else {
    doSomeIO(item, callback);
  }
}, function done() {
  //...
});

Just change it to:

async.eachSeries(hugeArray, function iterator(item, callback) {
  if (inCache(item)) {
    async.setImmediate(function () {
      callback(null, cache[item]);
    });
  } else {
    doSomeIO(item, callback);
  //...

Async guards against synchronous functions in some, but not all, cases. If you are still running into stack overflows, you can defer as suggested above, or wrap functions with async.ensureAsync Functions that are asynchronous by their nature do not have this problem and don't need the extra callback deferral.

If JavaScript's event loop is still a bit nebulous, check out this article or this talk for more detailed information about how it works.

Multiple callbacks

Make sure to always return when calling a callback early, otherwise you will cause multiple callbacks and unpredictable behavior in many cases.

async.waterfall([
    function (callback) {
        getSomething(options, function (err, result) {
          if (err) {
            callback(new Error("failed getting something:" + err.message));
            // we should return here
          }
          // since we did not return, this callback still will be called and
          // `processData` will be called twice
          callback(null, result);
        });
    },
    processData
], done)

It is always good practice to return callback(err, result) whenever a callback call is not the last statement of a function.

Binding a context to an iterator

This section is really about bind, not about async. If you are wondering how to make async execute your iterators in a given context, or are confused as to why a method of another library isn't working as an iterator, study this example:

// Here is a simple object with an (unnecessarily roundabout) squaring method
var AsyncSquaringLibrary = {
  squareExponent: 2,
  square: function(number, callback){
    var result = Math.pow(number, this.squareExponent);
    setTimeout(function(){
      callback(null, result);
    }, 200);
  }
};

async.map([1, 2, 3], AsyncSquaringLibrary.square, function(err, result){
  // result is [NaN, NaN, NaN]
  // This fails because the `this.squareExponent` expression in the square
  // function is not evaluated in the context of AsyncSquaringLibrary, and is
  // therefore undefined.
});

async.map([1, 2, 3], AsyncSquaringLibrary.square.bind(AsyncSquaringLibrary), function(err, result){
  // result is [1, 4, 9]
  // With the help of bind we can attach a context to the iterator before
  // passing it to async. Now the square function will be executed in its
  // 'home' AsyncSquaringLibrary context and the value of `this.squareExponent`
  // will be as expected.
});

Download

The source is available for download from GitHub. Alternatively, you can install using Node Package Manager (npm):

npm install async

As well as using Bower:

bower install async

Development: async.js - 29.6kb Uncompressed

In the Browser

So far it's been tested in IE6, IE7, IE8, FF3.6 and Chrome 5.

Usage:

<script type="text/javascript" src="async.js"></script>
<script type="text/javascript">

    async.map(data, asyncProcess, function(err, results){
        alert(results);
    });

</script>

Documentation

Some functions are also available in the following forms:

<name>Series - the same as <name> but runs only a single async operation at a time
<name>Limit - the same as <name> but runs a maximum of limit async operations at a time

Collections

each, eachSeries, eachLimit
forEachOf, forEachOfSeries, forEachOfLimit
map, mapSeries, mapLimit
filter, filterSeries, filterLimit
reject, rejectSeries, rejectLimit
reduce, reduceRight
detect, detectSeries, detectLimit
sortBy
some, someLimit
every, everyLimit
concat, concatSeries

Control Flow

series
parallel, parallelLimit
whilst, doWhilst
until, doUntil
during, doDuring
forever
waterfall
compose
seq
applyEach, applyEachSeries
queue, priorityQueue
cargo
auto
retry
iterator
times, timesSeries, timesLimit

Collections

each(arr, iterator, [callback])

Applies the function iterator to each item in arr, in parallel. The iterator is called with an item from the list, and a callback for when it has finished. If the iterator passes an error to its callback, the main callback (for the each function) is immediately called with the error.

Note, that since this function applies iterator to each item in parallel, there is no guarantee that the iterator functions will complete in order.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A function to apply to each item in arr. The iterator is passed a callback(err) which must be called once it has completed. If no error has occurred, the callback should be run without arguments or with an explicit null argument. The array index is not passed to the iterator. If you need the index, use forEachOf.
callback(err) - Optional A callback which is called when all iterator functions have finished, or an error occurs.

Examples

// assuming openFiles is an array of file names and saveFile is a function
// to save the modified contents of that file:

async.each(openFiles, saveFile, function(err){
    // if any of the saves produced an error, err would equal that error
});

// assuming openFiles is an array of file names

async.each(openFiles, function(file, callback) {

  // Perform operation on file here.
  console.log('Processing file ' + file);

  if( file.length > 32 ) {
    console.log('This file name is too long');
    callback('File name too long');
  } else {
    // Do work to process file here
    console.log('File processed');
    callback();
  }
}, function(err){
    // if any of the file processing produced an error, err would equal that error
    if( err ) {
      // One of the iterations produced an error.
      // All processing will now stop.
      console.log('A file failed to process');
    } else {
      console.log('All files have been processed successfully');
    }
});

Related

eachSeries(arr, iterator, [callback])
eachLimit(arr, limit, iterator, [callback])

forEachOf(obj, iterator, [callback])

Like each, except that it iterates over objects, and passes the key as the second argument to the iterator.

Arguments

obj - An object or array to iterate over.
iterator(item, key, callback) - A function to apply to each item in obj. The key is the item's key, or index in the case of an array. The iterator is passed a callback(err) which must be called once it has completed. If no error has occurred, the callback should be run without arguments or with an explicit null argument.
callback(err) - Optional A callback which is called when all iterator functions have finished, or an error occurs.

Example

var obj = {dev: "/dev.json", test: "/test.json", prod: "/prod.json"};
var configs = {};

async.forEachOf(obj, function (value, key, callback) {
  fs.readFile(__dirname + value, "utf8", function (err, data) {
    if (err) return callback(err);
    try {
      configs[key] = JSON.parse(data);
    } catch (e) {
      return callback(e);
    }
    callback();
  })
}, function (err) {
  if (err) console.error(err.message);
  // configs is now a map of JSON data
  doSomethingWith(configs);
})

Related

forEachOfSeries(obj, iterator, [callback])
forEachOfLimit(obj, limit, iterator, [callback])

map(arr, iterator, [callback])

Produces a new array of values by mapping each value in arr through the iterator function. The iterator is called with an item from arr and a callback for when it has finished processing. Each of these callback takes 2 arguments: an error, and the transformed item from arr. If iterator 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 iterator to each item in parallel, there is no guarantee that the iterator functions will complete in order. However, the results array will be in the same order as the original arr.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A function to apply to each item in arr. The iterator is passed a callback(err, transformed) which must be called once it has completed with an error (which can be null) and a transformed item.
callback(err, results) - Optional A callback which is called when all iterator functions have finished, or an error occurs. Results is an array of the transformed items from the arr.

Example

async.map(['file1','file2','file3'], fs.stat, function(err, results){
    // results is now an array of stats for each file
});

Related

mapSeries(arr, iterator, [callback])
mapLimit(arr, limit, iterator, [callback])

filter(arr, iterator, [callback])

Alias: select

Returns a new array of all the values in arr which pass an async truth test. The callback for each iterator call only accepts a single argument of true or false; it does not accept an error argument first! This is in-line with the way node libraries work with truth tests like fs.exists. This operation is performed in parallel, but the results array will be in the same order as the original.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A truth test to apply to each item in arr. The iterator is passed a callback(truthValue), which must be called with a boolean argument once it has completed.
callback(results) - Optional A callback which is called after all the iterator functions have finished.

Example

async.filter(['file1','file2','file3'], fs.exists, function(results){
    // results now equals an array of the existing files
});

Related

filterSeries(arr, iterator, [callback])
filterLimit(arr, limit, iterator, [callback])

reject(arr, iterator, [callback])

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

Related

rejectSeries(arr, iterator, [callback])
rejectLimit(arr, limit, iterator, [callback])

reduce(arr, memo, iterator, [callback])

Aliases: inject, foldl

Reduces arr into a single value using an async iterator to return each successive step. memo is the initial state of the reduction. This function only operates in series.

For performance reasons, it may make sense to split a call to this function into a parallel map, and then use the normal Array.prototype.reduce on the results. This function is for situations where each step in the reduction needs to be async; if you can get the data before reducing it, then it's probably a good idea to do so.

Arguments

arr - An array to iterate over.
memo - The initial state of the reduction.
iterator(memo, item, callback) - A function applied to each item in the array to produce the next step in the reduction. The iterator is passed a callback(err, reduction) which accepts an optional error as its first argument, and the state of the reduction as the second. If an error is passed to the callback, the reduction is stopped and the main callback is immediately called with the error.
callback(err, result) - Optional A callback which is called after all the iterator functions have finished. Result is the reduced value.

Example

async.reduce([1,2,3], 0, function(memo, item, callback){
    // pointless async:
    process.nextTick(function(){
        callback(null, memo + item)
    });
}, function(err, result){
    // result is now equal to the last value of memo, which is 6
});

reduceRight(arr, memo, iterator, [callback])

Alias: foldr

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

detect(arr, iterator, [callback])

Returns the first value in arr that passes an async truth test. The iterator is applied in parallel, meaning the first iterator to return true will fire the detect callback with that result. That means the result might not be the first item in the original arr (in terms of order) that passes the test.

If order within the original arr is important, then look at detectSeries.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A truth test to apply to each item in arr. The iterator is passed a callback(truthValue) which must be called with a boolean argument once it has completed. Note: this callback does not take an error as its first argument.
callback(result) - Optional A callback which is called as soon as any iterator returns true, or after all the iterator functions have finished. Result will be the first item in the array that passes the truth test (iterator) or the value undefined if none passed. Note: this callback does not take an error as its first argument.

Example

async.detect(['file1','file2','file3'], fs.exists, function(result){
    // result now equals the first file in the list that exists
});

Related

detectSeries(arr, iterator, [callback])
detectLimit(arr, limit, iterator, [callback])

sortBy(arr, iterator, [callback])

Sorts a list by the results of running each arr value through an async iterator.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A function to apply to each item in arr. The iterator is passed a callback(err, sortValue) which must be called once it has completed with an error (which can be null) and a value to use as the sort criteria.
callback(err, results) - Optional A callback which is called after all the iterator functions have finished, or an error occurs. Results is the items from the original arr sorted by the values returned by the iterator calls.

Example

async.sortBy(['file1','file2','file3'], function(file, callback){
    fs.stat(file, function(err, stats){
        callback(err, stats.mtime);
    });
}, function(err, results){
    // results is now the original array of files sorted by
    // modified date
});

Sort Order

By modifying the callback parameter the sorting order can be influenced:

//ascending order
async.sortBy([1,9,3,5], function(x, callback){
    callback(null, x);
}, function(err,result){
    //result callback
} );

//descending order
async.sortBy([1,9,3,5], function(x, callback){
    callback(null, x*-1);    //<- x*-1 instead of x, turns the order around
}, function(err,result){
    //result callback
} );

some(arr, iterator, [callback])

Alias: any

Returns true if at least one element in the arr satisfies an async test. The callback for each iterator call only accepts a single argument of true or false; it does not accept an error argument first! This is in-line with the way node libraries work with truth tests like fs.exists. Once any iterator call returns true, the main callback is immediately called.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A truth test to apply to each item in the array in parallel. The iterator is passed a callback(truthValue)` which must be called with a boolean argument once it has completed.
callback(result) - Optional A callback which is called as soon as any iterator returns true, or after all the iterator functions have finished. Result will be either true or false depending on the values of the async tests.

Note: the callbacks do not take an error as their first argument. Example

async.some(['file1','file2','file3'], fs.exists, function(result){
    // if result is true then at least one of the files exists
});

Related

someLimit(arr, limit, iterator, callback)

every(arr, iterator, [callback])

Alias: all

Returns true if every element in arr satisfies an async test. The callback for each iterator call only accepts a single argument of true or false; it does not accept an error argument first! This is in-line with the way node libraries work with truth tests like fs.exists.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A truth test to apply to each item in the array in parallel. The iterator is passed a callback(truthValue) which must be called with a boolean argument once it has completed.
callback(result) - Optional A callback which is called as soon as any iterator returns false, or after all the iterator functions have finished. Result will be either true or false depending on the values of the async tests.

Note: the callbacks do not take an error as their first argument.

Example

async.every(['file1','file2','file3'], fs.exists, function(result){
    // if result is true then every file exists
});

Related

everyLimit(arr, limit, iterator, callback)

concat(arr, iterator, [callback])

Applies iterator to each item in arr, concatenating the results. Returns the concatenated list. The iterators are called in parallel, and the results are concatenated as they return. There is no guarantee that the results array will be returned in the original order of arr passed to the iterator function.

Arguments

arr - An array to iterate over.
iterator(item, callback) - A function to apply to each item in arr. The iterator is passed a callback(err, results) which must be called once it has completed with an error (which can be null) and an array of results.
callback(err, results) - Optional A callback which is called after all the iterator functions have finished, or an error occurs. Results is an array containing the concatenated results of the iterator function.

Example

async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files){
    // files is now a list of filenames that exist in the 3 directories
});

Related

concatSeries(arr, iterator, [callback])

Control Flow

series(tasks, [callback])

Run the functions in the tasks array in series, each one running once the previous function has completed. If any functions in the series pass an error to its callback, no more functions are run, and callback is immediately called with the value of the error. Otherwise, callback receives an array of results when tasks have completed.

It is also possible to use an object instead of an array. Each property will be run as a function, and the results will be passed to the final callback as an object instead of an array. This can be a more readable way of handling results from series.

Note that while many implementations preserve the order of object properties, the ECMAScript Language Specification explicitly states that

The mechanics and order of enumerating the properties is not specified.

So if you rely on the order in which your series of functions are executed, and want this to work on all platforms, consider using an array.

Arguments

tasks - An array or object containing functions to run, each function is passed a callback(err, result) it must call on completion with an error err (which can be null) and an optional result value.
callback(err, results) - An optional callback to run once all the functions have completed. This function gets a results array (or object) containing all the result arguments passed to the task callbacks.

Example

async.series([
    function(callback){
        // do some stuff ...
        callback(null, 'one');
    },
    function(callback){
        // do some more stuff ...
        callback(null, 'two');
    }
],
// optional callback
function(err, results){
    // results is now equal to ['one', 'two']
});


// an example using an object instead of an array
async.series({
    one: function(callback){
        setTimeout(function(){
            callback(null, 1);
        }, 200);
    },
    two: function(callback){
        setTimeout(function(){
            callback(null, 2);
        }, 100);
    }
},
function(err, results) {
    // results is now equal to: {one: 1, two: 2}
});

parallel(tasks, [callback])

Run the tasks array of functions in parallel, without waiting until the previous function has completed. If any of the functions pass an error to its callback, the main callback is immediately called with the value of the error. Once the tasks have completed, the results are passed to the final callback as an array.

Note: parallel is about kicking-off I/O tasks in parallel, not about parallel execution of code. If your tasks do not use any timers or perform any I/O, they will actually be executed in series. Any synchronous setup sections for each task will happen one after the other. JavaScript remains single-threaded.

It is also possible to use an object instead of an array. Each property will be run as a function and the results will be passed to the final callback as an object instead of an array. This can be a more readable way of handling results from parallel.

Arguments

tasks - An array or object containing functions to run. Each function is passed a callback(err, result) which it must call on completion with an error err (which can be null) and an optional result value.
callback(err, results) - An optional callback to run once all the functions have completed successfully. This function gets a results array (or object) containing all the result arguments passed to the task callbacks.

Example

async.parallel([
    function(callback){
        setTimeout(function(){
            callback(null, 'one');
        }, 200);
    },
    function(callback){
        setTimeout(function(){
            callback(null, 'two');
        }, 100);
    }
],
// optional callback
function(err, results){
    // the results array will equal ['one','two'] even though
    // the second function had a shorter timeout.
});


// an example using an object instead of an array
async.parallel({
    one: function(callback){
        setTimeout(function(){
            callback(null, 1);
        }, 200);
    },
    two: function(callback){
        setTimeout(function(){
            callback(null, 2);
        }, 100);
    }
},
function(err, results) {
    // results is now equals to: {one: 1, two: 2}
});

Related

parallelLimit(tasks, limit, [callback])

whilst(test, fn, callback)

Repeatedly call fn, while test returns true. Calls callback when stopped, or an error occurs.

Arguments

test() - synchronous truth test to perform before each execution of fn.
fn(callback) - A function which is called each time test passes. The function is passed a callback(err), which must be called once it has completed with an optional err argument.
callback(err, [results]) - A callback which is called after the test function has failed and repeated execution of fn has stopped. callback will be passed an error and any arguments passed to the final fn's callback.

Example

var count = 0;

async.whilst(
    function () { return count < 5; },
    function (callback) {
        count++;
        setTimeout(function () {
            callback(null, count);
        }, 1000);
    },
    function (err, n) {
        // 5 seconds have passed, n = 5
    }
);

doWhilst(fn, test, callback)

The post-check version of whilst. To reflect the difference in the order of operations, the arguments test and fn are switched.

doWhilst is to whilst as do while is to while in plain JavaScript.

until(test, fn, callback)

Repeatedly call fn until test returns true. Calls callback when stopped, or an error occurs. callback will be passed an error and any arguments passed to the final fn's callback.

The inverse of whilst.

doUntil(fn, test, callback)

Like doWhilst, except the test is inverted. Note the argument ordering differs from until.

during(test, fn, callback)

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.

Example

var count = 0;

async.during(
    function (callback) {
      return callback(null, count < 5);
    },
    function (callback) {
        count++;
        setTimeout(callback, 1000);
    },
    function (err) {
        // 5 seconds have passed
    }
);

doDuring(fn, test, callback)

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.

forever(fn, [errback])

Calls the asynchronous function fn with a callback parameter that allows it to call itself again, in series, indefinitely.

If an error is passed to the callback then errback is called with the error, and execution stops, otherwise it will never be called.

async.forever(
    function(next) {
        // next is suitable for passing to things that need a callback(err [, whatever]);
        // it will result in this function being called again.
    },
    function(err) {
        // if next is called with a value in its first parameter, it will appear
        // in here as 'err', and execution will stop.
    }
);

waterfall(tasks, [callback])

Runs the tasks array of functions in series, each passing their results to the next in the array. However, if any of the tasks pass an error to their own callback, the next function is not executed, and the main callback is immediately called with the error.

Arguments

tasks - An array of functions to run, each function is passed a callback(err, result1, result2, ...) it must call on completion. The first argument is an error (which can be null) and any further arguments will be passed as arguments in order to the next task.
callback(err, [results]) - An optional callback to run once all the functions have completed. This will be passed the results of the last task's callback.

Example

async.waterfall([
    function(callback) {
        callback(null, 'one', 'two');
    },
    function(arg1, arg2, callback) {
      // arg1 now equals 'one' and arg2 now equals 'two'
        callback(null, 'three');
    },
    function(arg1, callback) {
        // arg1 now equals 'three'
        callback(null, 'done');
    }
], function (err, result) {
    // result now equals 'done'
});

Or, with named functions:

async.waterfall([
    myFirstFunction,
    mySecondFunction,
    myLastFunction,
], function (err, result) {
    // result now equals 'done'
});
function myFirstFunction(callback) {
  callback(null, 'one', 'two');
}
function mySecondFunction(arg1, arg2, callback) {
  // arg1 now equals 'one' and arg2 now equals 'two'
  callback(null, 'three');
}
function myLastFunction(arg1, callback) {
  // arg1 now equals 'three'
  callback(null, 'done');
}

Or, if you need to pass any argument to the first function:

async.waterfall([
    async.apply(myFirstFunction, 'zero'),
    mySecondFunction,
    myLastFunction,
], function (err, result) {
    // result now equals 'done'
});
function myFirstFunction(arg1, callback) {
  // arg1 now equals 'zero'
  callback(null, 'one', 'two');
}
function mySecondFunction(arg1, arg2, callback) {
  // arg1 now equals 'one' and arg2 now equals 'two'
  callback(null, 'three');
}
function myLastFunction(arg1, callback) {
  // arg1 now equals 'three'
  callback(null, 'done');
}

compose(fn1, fn2...)

Creates a function which is a composition of the passed asynchronous functions. Each function consumes the return value of the function that follows. Composing functions f(), g(), and h() would produce the result of f(g(h())), only this version uses callbacks to obtain the return values.

Each function is executed with the this binding of the composed function.

Arguments

functions... - the asynchronous functions to compose

Example

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
});

seq(fn1, fn2...)

Version of the compose function that is more natural to read. Each function consumes the return value of the previous function. It is the equivalent of compose with the arguments reversed.

Each function is executed with the this binding of the composed function.

Arguments

functions... - the asynchronous functions to compose

Example

// 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 });
    }
  });
});

applyEach(fns, args..., callback)

Applies the provided arguments to each function in the array, calling callback after all functions have completed. If you only provide the first argument, then it will return a function which lets you pass in the arguments as if it were a single function call.

Arguments

fns - the asynchronous functions to all call with the same arguments
args... - any number of separate arguments to pass to the function
callback - the final argument should be the callback, called when all functions have completed processing

Example

async.applyEach([enableSearch, updateSchema], 'bucket', callback);

// partial application example:
async.each(
    buckets,
    async.applyEach([enableSearch, updateSchema]),
    callback
);

Related

applyEachSeries(tasks, args..., [callback])

queue(worker, [concurrency])

Creates a queue object with the specified concurrency. Tasks added to the queue are processed in parallel (up to the concurrency limit). If all workers are in progress, the task is queued until one becomes available. Once a worker completes a task, that task's callback is called.

Arguments

worker(task, callback) - An asynchronous function for processing a queued task, which must call its callback(err) argument when finished, with an optional error as an argument. If you want to handle errors from an individual task, pass a callback to q.push().
concurrency - An integer for determining how many worker functions should be run in parallel. If omitted, the concurrency defaults to 1. If the concurrency is 0, an error is thrown.

Queue objects

The queue object returned by this function has the following properties and methods:

length() - a function returning the number of items waiting to be processed.
started - a function returning whether or not any items have been pushed and processed by the queue
running() - a function returning the number of items currently being processed.
workersList() - a function returning the array of items currently being processed.
idle() - a function returning false if there are items waiting or being processed, or true if not.
concurrency - an integer for determining how many worker functions should be run in parallel. This property can be changed after a queue is created to alter the concurrency on-the-fly.
push(task, [callback]) - add a new task to the queue. Calls callback once the worker has finished processing the task. Instead of a single task, a tasks array can be submitted. The respective callback is used for every task in the list.
unshift(task, [callback]) - add a new task to the front of the queue.
saturated - a callback that is called when the queue length hits the concurrency limit, and further tasks will be queued.
empty - a callback that is called when the last item from the queue is given to a worker.
drain - a callback that is called when the last item from the queue has returned from the worker.
paused - a boolean for determining whether the queue is in a paused state
pause() - a function that pauses the processing of tasks until resume() is called.
resume() - a function that resumes the processing of queued tasks when the queue is paused.
kill() - a function that removes the drain callback and empties remaining tasks from the queue forcing it to go idle.

Example

// create a queue object with concurrency 2

var q = async.queue(function (task, callback) {
    console.log('hello ' + task.name);
    callback();
}, 2);


// assign a callback
q.drain = function() {
    console.log('all items have been processed');
}

// add some items to the queue

q.push({name: 'foo'}, function (err) {
    console.log('finished processing foo');
});
q.push({name: 'bar'}, function (err) {
    console.log('finished processing bar');
});

// add some items to the queue (batch-wise)

q.push([{name: 'baz'},{name: 'bay'},{name: 'bax'}], function (err) {
    console.log('finished processing item');
});

// add some items to the front of the queue

q.unshift({name: 'bar'}, function (err) {
    console.log('finished processing bar');
});

priorityQueue(worker, concurrency)

The same as queue only tasks are assigned a priority and completed in ascending priority order. There are two differences between queue and priorityQueue objects:

push(task, priority, [callback]) - priority should be a number. If an array of tasks is given, all tasks will be assigned the same priority.
The unshift method was removed.

cargo(worker, [payload])

Creates a cargo object with the specified payload. Tasks added to the cargo will be processed altogether (up to the payload limit). If the worker is in progress, the task is queued until it becomes available. Once the worker has completed some tasks, each callback of those tasks is called. Check out these animations for how cargo and queue work.

While queue passes only one task to one of a group of workers at a time, cargo passes an array of tasks to a single worker, repeating when the worker is finished.

Arguments

worker(tasks, callback) - An asynchronous function for processing an array of queued tasks, which must call its callback(err) argument when finished, with an optional err argument.
payload - An optional integer for determining how many tasks should be processed per round; if omitted, the default is unlimited.

Cargo objects

The cargo object returned by this function has the following properties and methods:

length() - A function returning the number of items waiting to be processed.
payload - An integer for determining how many tasks should be process per round. This property can be changed after a cargo is created to alter the payload on-the-fly.
push(task, [callback]) - Adds task to the queue. The callback is called once the worker has finished processing the task. Instead of a single task, an array of tasks can be submitted. The respective callback is used for every task in the list.
saturated - A callback that is called when the queue.length() hits the concurrency and further tasks will be queued.
empty - A callback that is called when the last item from the queue is given to a worker.
drain - A callback that is called when the last item from the queue has returned from the worker.
idle(), pause(), resume(), kill() - cargo inherits all of the same methods and event calbacks as queue

Example

// 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');
});

auto(tasks, [concurrency], [callback])

Determines the best order for running the functions in tasks, based on their requirements. Each function can optionally depend on other functions being completed first, and each function is run as soon as its requirements are satisfied.

If any of the functions pass an error to their callback, the auto sequence will stop. Further tasks will not execute (so any other functions depending on it will not run), and the main callback is immediately called with the error. Functions also receive an object containing the results of functions which have completed so far.

Note, all functions are called with a results object as a second argument, so it is unsafe to pass functions in the tasks object which cannot handle the extra argument.

For example, this snippet of code:

async.auto({
  readData: async.apply(fs.readFile, 'data.txt', 'utf-8')
}, callback);

will have the effect of calling readFile with the results object as the last argument, which will fail:

fs.readFile('data.txt', 'utf-8', cb, {});

Instead, wrap the call to readFile in a function which does not forward the results object:

async.auto({
  readData: function(cb, results){
    fs.readFile('data.txt', 'utf-8', cb);
  }
}, callback);

Arguments

tasks - An object. Each of its properties is either a function or an array of requirements, with the function itself the last item in the array. The object's key of a property serves as the name of the task defined by that property, i.e. can be used when specifying requirements for other tasks. The function receives two arguments: (1) a callback(err, result) which must be called when finished, passing an error (which can be null) and the result of the function's execution, and (2) a results object, containing the results of the previously executed functions.
concurrency - An optional integer for determining the maximum number of tasks that can be run in parallel. By default, as many as possible.
callback(err, results) - An optional callback which is called when all the tasks have been completed. It receives the err argument if any tasks pass an error to their callback. Results are always returned; however, if an error occurs, no further tasks will be performed, and the results object will only contain partial results.

Example

async.auto({
    get_data: function(callback){
        console.log('in get_data');
        // async code to get some data
        callback(null, 'data', 'converted to array');
    },
    make_folder: function(callback){
        console.log('in make_folder');
        // async code to create a directory to store a file in
        // this is run at the same time as getting the data
        callback(null, 'folder');
    },
    write_file: ['get_data', 'make_folder', function(callback, results){
        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(callback, results){
        console.log('in email_link', JSON.stringify(results));
        // once the file is written let's email a link to it...
        // results.write_file contains the filename returned by write_file.
        callback(null, {'file':results.write_file, 'email':'user@example.com'});
    }]
}, function(err, results) {
    console.log('err = ', err);
    console.log('results = ', results);
});

This is a fairly trivial example, but to do this using the basic parallel and series functions would look like this:

async.parallel([
    function(callback){
        console.log('in get_data');
        // async code to get some data
        callback(null, 'data', 'converted to array');
    },
    function(callback){
        console.log('in make_folder');
        // async code to create a directory to store a file in
        // this is run at the same time as getting the data
        callback(null, 'folder');
    }
],
function(err, results){
    async.series([
        function(callback){
            console.log('in write_file', JSON.stringify(results));
            // once there is some data and the directory exists,
            // write the data to a file in the directory
            results.push('filename');
            callback(null);
        },
        function(callback){
            console.log('in email_link', JSON.stringify(results));
            // once the file is written let's email a link to it...
            callback(null, {'file':results.pop(), 'email':'user@example.com'});
        }
    ]);
});

For a complicated series of async tasks, using the auto function makes adding new tasks much easier (and the code more readable).

retry([opts = {times: 5, interval: 0}| 5], task, [callback])

Attempts to get a successful response from task no more than times times before returning an error. If the task is successful, the callback will be passed the result of the successful task. If all attempts fail, the callback will be passed the error and result (if any) of the final attempt.

Arguments

opts - Can be either an object with times and interval or a number.
- times - The number of attempts to make before giving up. The default is 5.
- interval - The time to wait between retries, in milliseconds. The default is 0.
- If opts is a number, the number specifies the number of times to retry, with the default interval of 0.
task(callback, results) - A function which receives two arguments: (1) a callback(err, result) which must be called when finished, passing err (which can be null) and the result of the function's execution, and (2) a results object, containing the results of the previously executed functions (if nested inside another control flow).
callback(err, results) - An optional callback which is called when the task has succeeded, or after the final failed attempt. It receives the err and result arguments of the last attempt at completing the task.

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 the default 5 times no delay between each retry 
async.retry(apiMethod, function(err, result) {
    // do something with the result
});

It can also be embedded within other control flow functions to retry individual methods that are not as reliable, like this:

async.auto({
    users: api.getUsers.bind(api),
    payments: async.retry(3, api.getPayments.bind(api))
}, function(err, results) {
  // do something with the results
});

iterator(tasks)

Creates an iterator function which calls the next function in the tasks array, returning a continuation to call the next one after that. It's also possible to “peek” at the next iterator with iterator.next().

This function is used internally by the async module, but can be useful when you want to manually control the flow of functions in series.

Arguments

tasks - An array of functions to run.

Example

var iterator = async.iterator([
    function(){ sys.p('one'); },
    function(){ sys.p('two'); },
    function(){ sys.p('three'); }
]);

node> var iterator2 = iterator();
'one'
node> var iterator3 = iterator2();
'two'
node> iterator3();
'three'
node> var nextfn = iterator2.next();
node> nextfn();
'three'

apply(function, arguments..)

Creates a continuation function with some arguments already applied.

Useful as a shorthand when combined with other control flow functions. Any arguments passed to the returned function are added to the arguments originally passed to apply.

Arguments

function - The function you want to eventually apply all arguments to.
arguments... - Any number of arguments to automatically apply when the continuation is called.

Example

// using apply

async.parallel([
    async.apply(fs.writeFile, 'testfile1', 'test1'),
    async.apply(fs.writeFile, 'testfile2', 'test2'),
]);


// the same process without using apply

async.parallel([
    function(callback){
        fs.writeFile('testfile1', 'test1', callback);
    },
    function(callback){
        fs.writeFile('testfile2', 'test2', callback);
    }
]);

It's possible to pass any number of additional arguments when calling the continuation:

node> var fn = async.apply(sys.puts, 'one');
node> fn('two', 'three');
one
two
three

nextTick(callback), setImmediate(callback)

Calls callback on a later loop around the event loop. In Node.js this just calls process.nextTick; in the browser it falls back to setImmediate(callback) if available, otherwise setTimeout(callback, 0), which means other higher priority events may precede the execution of callback.

This is used internally for browser-compatibility purposes.

Arguments

callback - The function to call on a later loop around the event loop.

Example

var call_order = [];
async.nextTick(function(){
    call_order.push('two');
    // call_order now equals ['one','two']
});
call_order.push('one')

times(n, iterator, [callback])

Calls the iterator function n times, and accumulates results in the same manner you would use with map.

Arguments

n - The number of times to run the function.
iterator - The function to call n times.
callback - see map

Example

// Pretend this is some complicated async factory
var createUser = function(id, callback) {
  callback(null, {
    id: 'user' + id
  })
}
// generate 5 users
async.times(5, function(n, next){
    createUser(n, function(err, user) {
      next(err, user)
    })
}, function(err, users) {
  // we should now have 5 users
});

Related

timesSeries(n, iterator, [callback])
timesLimit(n, limit, iterator, [callback])

Utils

memoize(fn, [hasher])

Caches the results of an async function. When creating a hash to store function results against, the callback is omitted from the hash and an optional hash function can be used.

If no hash function is specified, the first argument is used as a hash key, which may work reasonably if it is a string or a data type that converts to a distinct string. Note that objects and arrays will not behave reasonably. Neither will cases where the other arguments are significant. In such cases, specify your own hash function.

The cache of results is exposed as the memo property of the function returned by memoize.

Arguments

fn - The function to proxy and cache results from.
hasher - An optional function for generating a custom hash for storing results. It has all the arguments applied to it apart from the callback, and must be synchronous.

Example

var slow_fn = function (name, callback) {
    // do something
    callback(null, result);
};
var fn = async.memoize(slow_fn);

// fn can now be used as if it were slow_fn
fn('some name', function () {
    // callback
});

unmemoize(fn)

Undoes a memoized function, reverting it to the original, unmemoized form. Handy for testing.

Arguments

fn - the memoized function

ensureAsync(fn)

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.

Arguments

fn - 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.

Example

function sometimesAsync(arg, callback) {
  if (cache[arg]) {
    return callback(null, cache[arg]); // this would be synchronous!!
  } else {
    doSomeIO(arg, callback); // this IO would be asynchronous
  }
}

// this has a risk of stack overflows if many results are cached in a row
async.mapSeries(args, sometimesAsync, done);

// this will defer sometimesAsync's callback if necessary,
// preventing stack overflows
async.mapSeries(args, async.ensureAsync(sometimesAsync), done);

constant(values...)

Returns a function that when called, calls-back with the values provided. Useful as the first function in a waterfall, or for plugging values in to auto.

Example

async.waterfall([
  async.constant(42),
  function (value, next) {
    // value === 42
  },
  //...
], callback);

async.waterfall([
  async.constant(filename, "utf8"),
  fs.readFile,
  function (fileData, next) {
    //...
  }
  //...
], callback);

async.auto({
  hostname: async.constant("https://server.net/"),
  port: findFreePort,
  launchServer: ["hostname", "port", function (cb, options) {
    startServer(options, cb);
  }],
  //...
}, callback);

asyncify(func)

Alias: wrapSync

Take a sync function and make it async, passing its return value to a callback. This is useful for plugging sync functions into a waterfall, series, or other async functions. Any arguments passed to the generated function will be passed to the wrapped function (except for the final callback argument). Errors thrown will be passed to the callback.

Example

async.waterfall([
  async.apply(fs.readFile, filename, "utf8"),
  async.asyncify(JSON.parse),
  function (data, next) {
    // data is the result of parsing the text.
    // If there was a parsing error, it would have been caught.
  }
], callback)

If the function passed to asyncify returns a Promise, that promises's resolved/rejected state will be used to call the callback, rather than simply the synchronous return value. Example:

async.waterfall([
  async.apply(fs.readFile, filename, "utf8"),
  async.asyncify(function (contents) {
    return db.model.create(contents);
  }),
  function (model, next) {
    // `model` is the instantiated model object. 
    // If there was an error, this function would be skipped.
  }
], callback)

This also means you can asyncify ES2016 async functions.

var q = async.queue(async.asyncify(async function (file) {
  var intermediateStep = await processFile(file);
  return await somePromise(intermediateStep)
}));

q.push(files);

log(function, arguments)

Logs the result of an async function to the console. Only works in Node.js or in browsers that support console.log and console.error (such as FF and Chrome). If multiple arguments are returned from the async function, console.log is called on each argument in order.

Arguments

function - The function you want to eventually apply all arguments to.
arguments... - Any number of arguments to apply to the function.

Example

var hello = function(name, callback){
    setTimeout(function(){
        callback(null, 'hello ' + name);
    }, 1000);
};

node> async.log(hello, 'world');
'hello world'

dir(function, arguments)

Logs the result of an async function to the console using console.dir to display the properties of the resulting object. Only works in Node.js or in browsers that support console.dir and console.error (such as FF and Chrome). If multiple arguments are returned from the async function, console.dir is called on each argument in order.

Arguments

function - The function you want to eventually apply all arguments to.
arguments... - Any number of arguments to apply to the function.

Example

var hello = function(name, callback){
    setTimeout(function(){
        callback(null, {hello: name});
    }, 1000);
};

node> async.dir(hello, 'world');
{hello: 'world'}

noConflict()

Changes the value of async back to its original value, returning a reference to the async object.

README.md

Async.js

Quick Examples

Common Pitfalls (StackOverflow)

Synchronous iteration functions

Multiple callbacks

Binding a context to an iterator

Download

In the Browser

Documentation

Collections

Control Flow

Utils

Collections

each(arr, iterator, [callback])

forEachOf(obj, iterator, [callback])

map(arr, iterator, [callback])

filter(arr, iterator, [callback])

reject(arr, iterator, [callback])

reduce(arr, memo, iterator, [callback])

reduceRight(arr, memo, iterator, [callback])

detect(arr, iterator, [callback])

sortBy(arr, iterator, [callback])

some(arr, iterator, [callback])

every(arr, iterator, [callback])

concat(arr, iterator, [callback])

Control Flow

series(tasks, [callback])

parallel(tasks, [callback])

whilst(test, fn, callback)

doWhilst(fn, test, callback)

until(test, fn, callback)

doUntil(fn, test, callback)

during(test, fn, callback)

doDuring(fn, test, callback)

forever(fn, [errback])

waterfall(tasks, [callback])

compose(fn1, fn2...)

seq(fn1, fn2...)

applyEach(fns, args..., callback)

queue(worker, [concurrency])

priorityQueue(worker, concurrency)

cargo(worker, [payload])

auto(tasks, [concurrency], [callback])

retry([opts = {times: 5, interval: 0}| 5], task, [callback])

iterator(tasks)

apply(function, arguments..)

nextTick(callback), setImmediate(callback)

times(n, iterator, [callback])

Utils

memoize(fn, [hasher])

unmemoize(fn)

ensureAsync(fn)

constant(values...)

asyncify(func)

log(function, arguments)

dir(function, arguments)

noConflict()

Common Pitfalls _{(StackOverflow)}