A simple, powerful library for managing asynchrous control flow and synchronizing asynchronous calls.
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Latest commit 312d873 Jun 23, 2015 fresheneesz m


DEPRECATED - I'm deprecating async-future in favor of bluebird promises, which is 10 times faster in simple cases, and probably moreso in some complex cases, despite this module's respectable performance in comparison to other futures modules. Bluebird is painstakingly optimized, including its powerful set of utilities. It also elegantly does away with done, has a great cancellation api, a super useful bind method, and isn't bloated.


A simple, powerful library for managing asynchronous control flow and saner handling of asynchronous exceptions in node.js and the browser.

Why use async-future?

  • If you want to avoid callback-hell
  • If you want exception bubbling with familiar and well understood try-catch-finally semantics for asynchronous code
  • If you ever need to wait for two or more asynchronous calls to complete before executing some code

Read more about how promises simplify asyncronous programming.


Simple return with error handling

var Future = require('async-future')  // node.js and webpack

var f = new Future
asynchronousFunction(function(result) {
    if(error) f.throw(error)
    else      f.return(result)

f.then(function(result) {

}).catch(function(e) {
   console.error("Oh no! "+error)


Immediates, familar try-catch-finally semantics, and Future.wrap

var f2 = Future(45) // you can create immediately-resolved futures for convenience
var f3 = Future(undefined) // note that undefined has to be explicitly passed in
                           // so "Future()" won't work the same way
var futureFn2 = Future.wrap(asyncFn2) // turns an asynchronous function into a function
                                      // that returns a future
f2.then(function(result) {
    console.log(result) // prints 45
    return Future(true) // returning a future passes the result of the future (in this
                        // case 45) to the next `then` statement
}).then(function(result2) {
    console.log(result2) // prints true
    return futureFn2(result2) // using the wrapped function is just like using the
                              // unwrapped function, but without the callback parameter
}).finally(function() {
    // this is ran regardless of whether an exception was thrown above ^

}).catch(function(e){  // unlike try-catch semantics, `catch` should come after `finally`

}).done() // ensures that any exception still in the pipes here is asynchronously thrown
         // (instead of being lost). This is equivalent to 'detatch' in fibers/future

function asyncFn2(parameter, /*more parameters if you want, */ callback) {
    try {
       callback(undefined, doStuff(parameter))
    } catch(e) {


npm install async-future


var Future = require('async-future')


The most important part of async-future is future-chains. These are chains that have try-catch-finally style semantics, but are asynchronous. Future-chains consist of chains of then, catch, and finally calls and must always eventually end in a done call. When an exception is thrown (or an error-resolved future is returned) from a previous link in the chain, the error propagates down the chain to the first catch it comes across, skipping any thens and running any finallys.

The methods then, catch, and finally return a new Future that is resolved when their callback completes. The future returned by all three of these methods will resolve with an error if an exception is thrown from the callbacks.

The future returned by the methods then and catch will resolve to the following return values (non-errors):

  • undefined, if the callback returns undefined (ie nothing), or
  • if the callback returns a Future object, it resolves to the value that future resolves to

Other than undefined, you may not return a value that isn't a Future object from these callbacks.

f.then(<callback>) - executes <callback> if f is returned from. <callback> gets the return value of f as its only parameter. Returns a future that resolves when then completes or if f is resolved with an error. Does not execute the callback if f resolves to an error, but instead resolves its return value with the error from f.

f.catch(<callback>) - executes <callback> if f is thrown from. <callback> gets the error value of f as its only parameter. Returns a future that resolves when its callback completes or when f is returned from. Does not execute the callback if f resolves to a return value, but instead resolves its return value with the return value of f.

f.finally(<callback>) - executes a callback when f resolves, regardless of whether f was resolved with a return value or an error. The callback takes no arguments. Returns a future that resolves when <callback> completes or when the future it returns completes. If the future <callback> returns resolves to an exception, the future returned from finally will too. If the future <callback> returns resolves to a return value, or if <callback> returns undefined, the future finally returns will resolves to the same result as f.

f.done() - marks a future chain as done, which means that if any subsequent exceptions happen, it will be thrown asynchronously (and likely caught by a domain if its node.js, or by window.onerror in browsers). Every future or future-chain that won't have one of the chain-methods (or the resolver method) called on it, should call .done(), so that thrown exceptions won't get lost.


. then catch finally
parameter value of calling future exception from calling future none
returned future resolves to returned future's value returned future's value thrown exception or calling future's value
exceptions thrown in callback propogate propogate propogate

Other Instance properties

f.return(<value>) - resolves a future with a return value (undefined if a value isn't passed)

f.throw(<exception>) - resolves a future with an exception

f.resolver() - returns an errback from a future. This is useful for using functions that require an errback-style callback (a function that takes two parameters, (error, result))

f.resolved() - returns true if the future has already been resolved, false otherwise.

Static properties and methods

Future.all(<futures> - returns a future that resolves when all futures inside resolve (or throws an error when one of the futures returns an error).

Future.wrap(<fn>) - wraps a function that takes an errback so that it returns a future instead of calling an errback.

Future.wrap(<object>, <method>) - wraps a method that takes an errback so that it returns a future instead of calling an errback. Example:

  • Example: var wrappedMethod = Future.wrap(object, 'methodName')

Future.wrapSingleParameter(<fn>) - Like Future.wrap but for functions who's callback only takes one parameter - the return value (no error is available).

Future.wrapSingleParameter(<object>, <method>) - wrapSingleParameter for object methods who's callback only takes one parameter.

Future.isLikeAFuture(<fn>) - Returns true if the object looks like a future (duck typing). You might want to use this instead of instanceof because its possible that you might have different versions of async-future floating around (in which case instanceof might not work like you expect).


function a(x, errback) {
    if(x === false)
        errback(Error('x isnt true : ('))
        errback(undefined, x)

var aFuture = Future.wrap(a)

// prints 5
aFuture(5).then(function(result) {

// prints an exception
aFuture(false).then(function(result) {
    console.log(result) // never gets here

Future.error(<handler>) - sets up a function that is called when an unhandled error happens. <handler> gets one parameter, the unhandled exception. Unhandled errors happen when done is called and an exception is thrown from the future.

Future.debug - if true, gives each future a unique id (default is false) and enables long-stack-traces in exceptions (where the stack trace for the context in which then, catch, or finally is called in is printed below the original exception's stack trace).

Usage in IE<9

Catch is a reserved word in IE<9, meaning afuture.catch(func) throws a syntax error. To work around this, you can use a string to access the property:

aFuture['catch'](function(err) {
  // ...

Note that most common minifiers employ this technique, making the minified code safe for old browsers and production without using this explicitly in your code.


Why use async-future over ...

  • jQuery deferred doesn't bubble exceptions
  • Q promises doesn't give you the flexibility to resolve your promises, rather it splits things into promises and deffereds. This makes things both more complicated and more restrictive.
  • fibers/future has a nicer interface, but only works on node.js
  • when.js has no way easy to chain futures together.
  • parallel.js is more complicated and doesn't help you bubble exceptions
  • substack's node-seq is also much more complicated, and doesn't have sane semantics for passing futures/promises out of functions


  • Consider adding a way to close over a whole function (to ensure errors are all handled through the future
    • Future.closure method? `return Future.closure(function() {...})
    • Function.task() like with fibers/future?
  • Maybe allow then and catch callbacks to return arrays of future, which would allow you to do something like this: future.then(function() { return [Future(one), future2] }).then(function(one, two) { // use one and two })
  • Think about how to handle domains - right now done throws into whatever domain the beginning of the chain was called in, instead of the context done was called in
    • This might be ok since done is intended to be a safety net, and not generally used to catch and report errors
  • Standalone bundle (via ) build-modules
  • timeout or cancellation (probably cancellation is more general)


  • 1.0.6 - Future.throw now returns the future, so you can chain and/or return using it ( https://github.com/fresheneesz/asyncFuture/issues/2 )
  • 1.0.5 - adding the wrapSingleParameter function
  • 1.0.4
    • changing a future's initial 'n' so that a setTimeout doesn't happen for the first few hundred chains
    • adding some info to "callback returned an object that isn't a future" error
    • fixing browser tests (they need testServer.js to be running)
  • 1.0.3 - adding long traces (when Future.debug is set to true)
  • 1.0.2 - adding code to prevent "too much recursion" RangeErrors from being caused by code with a ridiculous number of chained thens using a setTimeout every 400th chain
  • 1.0.1 - fixing obscure bug in catch
  • 1.0.0 - making a simple change that hugely improves performance at the cost of making it a little more impportant when exactly you call return or throw (though that also has the upside of allowing more control)
  • 0.1.18 - fixing bug in finally where exceptions and results weren't being correctly passed through
  • 0.1.16 - fixing behavior when exceptions happen in a finally block
  • 0.1.10 - fixing another (i think the last) related bug where finally was being called before a future returned by a previous then callback was resolved
  • 0.1.9 - fixing related bug with futures returned from a then callback, where exceptions weren't being properly propogated
  • 0.1.8 - fixing bug with the done method when a future with an error is returned from a then callback

How to Contribute!

Anything helps:

  • Creating issues (aka tickets/bugs/etc). Please feel free to use issues to report bugs, request features, and discuss changes
  • Updating the documentation: ie this readme file. Be bold! Help create amazing documentation!
  • Submitting pull requests.

How to submit pull requests:

  1. Please create an issue and get my input before spending too much time creating a feature. Work with me to ensure your feature or addition is optimal and fits with the purpose of the project.
  2. Fork the repository
  3. clone your forked repo onto your machine and run npm install at its root
  4. If you're gonna work on multiple separate things, its best to create a separate branch for each of them
  5. edit!
  6. If it's a code change, please add to the unit tests (at test/asyncFuturesTest.js) to verify that your change
  7. When you're done, run the unit tests and ensure they all pass
  8. Commit and push your changes
  9. Submit a pull request: https://help.github.com/articles/creating-a-pull-request



Released under the MIT license: http://opensource.org/licenses/MIT