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Provides a defer/when style promise API for JavaScript.
- usable as a CommonJS module, in Node,
- usable as a <script> in all web browsers,
- compatible with jQuery and Dojo promises,
- inspired by Tyler Close's Waterken ref_send promises, and
- compliant with
For Node:
$ curl | sh
$ npm install q qq
$ node examples/test.js
The "q/util" module has moved to its own package and
repository. It is now "qq". Since it is used in the
following examples, I've added it to the list of modules to
install. "qq" provides everything in "q" plus some derrived
utility functions.
### `defer`
This example provides a promise-oriented `delay` function
based on the callback-oriented `setTimeout` function.
function delay(ms) {
var deferred = Q.defer();
setTimeout(deferred.resolve, ms);
return deferred.promise;
This example takes a promise and returns a promise that will
be rejected if the given promise is not fulfilled in a
timely fashion.
function timeout(promise, ms) {
var deferred = Q.defer();
Q.when(promise, deferred.resolve);
Q.when(delay(ms), function () {
deferred.reject("Timed out");
return deferred.promise;
This example wraps Node's file listing function, returning a
promise instead of accepting a callback.
var FS = require("fs"); // from Node
function list(path) {
path = String(path);
var result = Q.defer();
FS.readdir(path, function (error, list) {
if (error)
return result.reject(error);
return result.promise;
### `when`
This example illustrates how the `when` primitive can be
used to observe the fulfillment of a promise.
var bPromise = Q.when(aPromise, function (aValue) {
return bValue;
* If `aPromise` is fulfilled, the callback is called in a
future turn of the even loop with the fulfilled value as
* If `aPromise` is rejected, `bPromise` will be resolved
with `aPromise` (the rejection will be forwarded).
* `bPromise` is eventually resolved with `bValue`.
* `aPromise` does not actually need to be a promise. It
can be any value, in which case it is treated as an
already fulfilled promise.
* `bValue` does not actually need to be a value. It can be
a promise, which would further defer the resolution of
* If the fulfillment callback throws an exception,
`bPromise` will be rejected with the thrown error as the
This example illustrates how the `when` primitive can be
used to observe either the fulfillment or rejection of a
var bPromise = Q.when(aPromise, function (aValue) {
return bValue;
}, function (aReason) {
return bValue; // or
throw bReason;
* If `aPromise` is rejected, the second callback, the
rejection callback, will be called with the reason for the
rejection as `aReason`.
* The value returned by the rejection callback will be used
to resolve `bPromise`.
* If the rejection callback throws an error, `bPromise` will
be rejected with the error as the reason.
* Unlike a `try` and `catch` block, the rejection callback
will not be called if the fulfillment callback throws an
error or returns a rejection. To observe an exception
thrown in either the fulfillment or the rejection
callback, another `when` block must be used to observe the
rejection of `bPromise`.
In general,
* If the rejection callback is falsy and `aPromise` is
rejected, the rejection will be forwarded to `bPromise`.
* If the fulfillment callback is falsy and `aPromise` is
fulfilled, the fulfilled value will be forwarded to
### Node File-system Examples
In Node, this example reads itself and writes itself out in
all capitals.
var Q = require("q");
var FS = require("q-fs");
var text =;
Q.when(text, function (text) {
You can also perform actions in parallel. This example
reads two files at the same time and returns an array of
promises for the results.
var Q = require("qq");
var FS = require("q-fs");
var self =;
var passwd ="/etc/passwd");
Q.join(self, passwd, function (self, passwd) {
console.log(__filename + ':', self.length);
console.log('/etc/passwd:', passwd.length);
This example reads all of the files in the same directory as
the program and notes the length of each, in the order in
which they are finished reading.
var Q = require("q");
var FS = require("q-fs");
var list = FS.list(__dirname);
var files = Q.when(list, function (list) {
list.forEach(function (fileName) {
var content =;
Q.when(content, function (content) {
console.log(fileName, content.length);
This example reads all of the files in the same directory as
the program and notes the length of each, in the order in
which they were listed.
var Q = require("qq");
var FS = require("q-fs");
var list = FS.list(__dirname);
var files = Q.when(list, function (list) {
return list.reduce(function (ready, fileName) {
var content =;
return Q.join(ready, content, function (ready, content) {
console.log(fileName, content.length);
### Parallel Join
Promises can be used to do work either in parallel or
serial, depending on whether you wait for one promise to be
fulfilled before beginning work on a second. To do a
parallel join, begin work and get promises and use nested
`when` blocks to create a single promise that will be
resolved when both inputs are resolved, or when the first is
var aPromise = aFunction();
var bPromise = bFunction();
var cPromise = Q.when(aPromise, function (aValue) {
return Q.when(bPromise, function (bValue) {
return cValue;
For short, you can use the `join` function in `qq`.
var Q = require("qq");
var aPromise = aFunction();
var bPromise = bFunction();
Q.join(aPromise, bPromise, function (aValue, bValue) {
return cValue;
If a piece of work can be done on each value in an array in
parallel, you can use either a `forEach` loop or a `reduce`
loop to create a `done` promise.
var done;
array.forEach(function (value) {
var work = doWork(value);
done = Q.when(done, function () {
return work;
return done;
It is a bit more concise with a `reduce` loop.
return array.reduce(function (done, value) {
var work = doWork(value);
return Q.when(done, function () {
return work;
}, undefined);
### Serial Join
If you have two pieces of work and the second cannot be done
until the first completes, you can also use nested `when`
var aPromise = aFunction();
var cPromise = Q.when(aPromise, function (aValue) {
var bPromise = bFunction(aValue);
return Q.when(bPromise, function bValue) {
return cValue;
If you can do work on each value in an array, but want to do
them in order and one at a time, you can use `forEach` or
`reduce` loop.
var done;
array.forEach(function (value) {
done = Q.when(done, function () {
return doWork(value);
return done;
It is more concise with `reduce`.
return array.reduce(function (done, value) {
return Q.when(done, function () {
return doWork(value);
### Recovery
You can use the rejection callback of `when` blocks to
recover from failure. Supposing that `doIt` will
intermittently fail (perhaps because of network conditions),
`justDoIt` will just keep trying indifinitely.
function justDoIt(value) {
var work = doIt(value);
work = timeout(1000, work);
return Q.when(work, function (work) {
return work;
}, function errback(reason) {
// just do it again
return justDoIt(value);
This will not blow out the stack because `when` blocks
guarantee that the fulfillment and rejection callbacks will
only be called on their own turn of the event loop.
### Conditional Array Serial Join
Consider the process of looking for the first directory in
an array of paths that contains a particular file. To do
this with a synchronous file API is very straight-forward.
function find(basePaths, soughtPath) {
for (var i = 0, ii = basePaths.length; i < ii; i++) {
var consideredPath = FS.join(basePaths[i], soughtPath);
if (FS.isFile(consideredPath))
return consideredPath;
throw new Error("Can't find.");
To do this with an asynchronous `FS.isFile` is more
elaborate. It is a serial iteration, but it halts at the
first success. This can be accomplished by creating a chain
of functions, each making progress on the returned promise
until the matching path is found, otherwise returning the
value returned by the next function in line, until all
options are exhausted and returning a rejection.
function find(basePaths, soughtPath) {
var find = basePaths.reduceRight(function (otherwise, basePath) {
return function () {
var consideredPath = FS.join(basePath, soughtPath);
var isFile = FS.isFile(consideredPath);
return Q.when(isFile, function (isFile) {
if (isFile) {
return consideredPath;
} else {
return otherwise();
}, function otherwise() {
throw new Error("Can't find");
return find();
The Q Ecosystem
derrived utilities
basic file system promises
alternate basic file system promises
http client and server promises
promise control flow and data structures
remote object communication
promising JSGI middleware
browser-side module promises
All available through NPM.
when(value, callback_opt, errback_opt)
Arranges for a callback to be called:
- with the value as its sole argument
- in a future turn of the event loop
- if and when the value is or becomes a fully resolved
Arranges for errback to be called:
- with a value respresenting the reason why the object will
never be resolved, typically a string.
- in a future turn of the event loop
- if the value is a promise and
- if and when the promise is rejected
Returns a promise:
- that will resolve to the value returned by either the callback
or errback, if either of those functions are called, or
- that will be rejected if the value is rejected and no errback
is provided, thus forwarding rejections by default.
The value may be truly _any_ value.
The callback and errback may be falsy, in which case they will not
be called.
- The callback will not be called before when returns.
- The errback will not be called before when returns.
- The callback will not be called more than once.
- The errback will not be called more than once.
- If the callback is called, the errback will never be called.
- If the errback is called, the callback will never be called.
- If a promise is never resolved, neither the callback or the
errback will ever be called.
- You can set up an entire chain of causes and effects in the
duration of a single event and be guaranteed that any
invariants in your lexical scope will not...vary.
- You can both receive a promise from a sketchy API and return a
promise to some other sketchy API and, as long as you trust
this module, all of these guarantees are still provided.
- You can use when to compose promises in a variety of ways:
function and(a, b) {
return Q.when(a, function (a) {
return Q.when(b, function (b) {
// ...
Returns a "Deferred" object with a:
- promise property
- resolve(value) function
- reject(reason) function
The promise is suitable for passing as a value to
the "when" function.
Calling resolve with a promise notifies all observers
that they must now wait for that promise to resolve.
Calling resolve with a rejected promise notifies all
observers that the promise will never be fully resolved
with the rejection reason. This forwards through the
the chain of "when" calls and their returned "promises"
until it reaches a "when" call that has an "errback".
Calling resolve with a fully resolved value notifies
all observers that they may proceed with that value
in a future turn. This forwards through the "callback"
chain of any pending "when" calls.
Calling reject with a reason is equivalent to
resolving with a rejection.
In all cases where the resolution of a promise is set,
(promise, rejection, value) the resolution is permanent
and cannot be reset. All future observers of the
resolution of the promise will be notified of the
resolved value, so it is safe to call "when" on
a promise regardless of whether it has been or will
be resolved.
The Deferred separates the promise part from the resolver
part. So:
- You can give the promise to any number of consumers
and all of them will observe the resolution independently.
Because the capability of observing a promise is separated
from the capability of resolving the promise, none of the
recipients of the promise have the ability to "trick"
other recipients with misinformation.
- You can give the resolver to any number of producers
and whoever resolves the promise first wins. Furthermore,
none of the producers can observe that they lost unless
you give them the promise part too.
function or(a, b) {
var union = Q.defer();
Q.when(a, union.resolve);
Q.when(b, union.resolve);
return union.promise;
If value is a promise, returns the promise.
If value is not a promise, returns a promise that has
already been resolved with the given value.
Annotates a value, wrapping it in a promise, such that
that it is a local promise object which cannot be
serialized and sent to resolve a remote promise.
A def'ed value will respond to the `isDef` message
without a rejection so remote promise communication
libraries can distinguish it from non-def values.
Returns a promise that has already been rejected
with the given reason.
This is useful for conditionally forwarding a rejection
through an errback.
Q.when(API.getPromise(), function (value) {
return doSomething(value);
}, function (reason) {
if (API.stillPossible())
return API.tryAgain();
return reject(reason);
Unconditionally forwarding a rejection is equivalent to
omitting an errback on a when call.
Returns whether the given value is a promise.
Returns whether the given value is fully resolved.
The given value may be any value, including
but not limited to promises returned by defer() and
ref(). Rejected promises are not considered
Returns whether the given value is a rejected
Promises override their valueOf method such that if the
promise is fully resolved, it will return the fully
resolved value.
Accepts a reason and throws an error. This is a
convenience for when calls where you want to trap the
error clause and throw it instead of attempting a
recovery or forwarding.
enqueue(callback Function)
Calls "callback" in a future turn.
The "ref" promise constructor establishes the basic API for
performing operations on objects: "get", "put", "post", and
"del". This set of "operators" can be extended by creating
promises that respond to messages with other operator names,
and by sending corresponding messages to those promises.
makePromise(descriptor, fallback_opt, valueOf_opt)
Creates a stand-alone promise that responds to messages.
These messages have an operator like "when", "get",
"put", and "post", corresponding to each of the above
methods for sending messages to promises.
The descriptor is an object with function properties
(methods) corresponding to operators. When the made
promise receives a message and a corresponding operator
exists in the descriptor, the method gets called with
the variadic arguments sent to the promise. If no
descriptor exists, the fallback method is called with
the operator, and the subsequent variadic arguments
instead. These functions return a promise for the
eventual resolution of the promise returned by the
message-sender. The default fallback returns a
The `valueOf` function, if provided, overrides the
`valueOf` method of the returned promise. This is
useful for providing information about the promise in
the same turn of the event loop. For example, resolved
promises return their resolution value and rejections
return an object that is recognized by `isRejected`.
send(value, operator, ...args)
Sends an arbitrary message to a promise.
Care should be taken not to introduce control-flow
hazards and secuirity holes when forwarding messages to
promises. The methods above, particularly "when", are
carefully crafted to prevent a poorly crafted or
malicious promise from breaking the invariants like not
applying callbacks multiple times or in the same turn of
the event loop.
Copyright 2009-2011 Kristopher Michael Kowal
MIT License (enclosed)
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