The 'q' npm package is a library for creating and managing promises in JavaScript. It provides a robust set of tools for working with asynchronous operations, allowing developers to write cleaner, more maintainable code by avoiding the 'callback hell' that can occur with deeply nested callbacks.
Creating Promises
This feature allows the creation of a new promise using Q.defer(). The deferred object has a promise property and methods for resolving or rejecting the promise.
const Q = require('q');
const deferred = Q.defer();
function asyncOperation() {
// Perform some asynchronous operation
setTimeout(() => {
// Resolve the promise after 1 second
deferred.resolve('Operation completed');
}, 1000);
return deferred.promise;
}
asyncOperation().then(result => console.log(result));Promise Chaining
This feature demonstrates how promises can be chained together, with the output of one promise being passed as input to the next.
const Q = require('q');
function firstAsyncOperation() {
var deferred = Q.defer();
setTimeout(() => deferred.resolve(1), 1000);
return deferred.promise;
}
function secondAsyncOperation(result) {
var deferred = Q.defer();
setTimeout(() => deferred.resolve(result + 1), 1000);
return deferred.promise;
}
firstAsyncOperation()
.then(secondAsyncOperation)
.then(result => console.log('Final result:', result));Error Handling
This feature shows how to handle errors in promise-based workflows. The catch method is used to handle any errors that occur during the promise's execution.
const Q = require('q');
function mightFailOperation() {
var deferred = Q.defer();
setTimeout(() => {
if (Math.random() > 0.5) {
deferred.resolve('Success!');
} else {
deferred.reject(new Error('Failed!'));
}
}, 1000);
return deferred.promise;
}
mightFailOperation()
.then(result => console.log(result))
.catch(error => console.error(error.message));Bluebird is a fully-featured promise library with a focus on innovative features and performance. It is known for being one of the fastest promise libraries and includes utilities for concurrency, such as Promise.map and Promise.reduce, which are not present in 'q'.
When is another lightweight Promise library that offers similar functionality to 'q'. It provides a solid API for creating and managing promises but is generally considered to have a smaller footprint and to be more modular than 'q'.
This package is a simple implementation of Promises/A+. It is smaller and may be more straightforward than 'q' for those who only need basic promise functionality without the additional utilities provided by 'q'.
Provides a defer/when style promise API for JavaScript
For Node:
$ curl http://npmjs.org/install.sh | sh
$ npm install q
$ node examples/test.js
Promises from callbacks
function delay(ms) {
var deferred = Q.defer();
setTimeout(deferred.resolve, ms);
return deferred.promise;
}
Timeout
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;
}
Promises with rejection from Node callbacks
function wrap(wrapped) {
var deferred = Q.defer();
wrapped(function (error, result) {
if (error) {
deferred.reject(error);
} else {
deferred.resolve(result);
}
});
return deferred.promise;
}
When Blocks
var bPromise = Q.when(aPromise, function (aValue) {
return bValue;
});
var bPromise = Q.when(aPromise, function (aValue) {
return bValue;
}, function (aReason) {
throw bReason;
});
Parallel Join
var aPromise = aFunction();
var bPromise = bFunction();
var cPromise = Q.when(aPromise, function (aValue) {
return Q.when(bPromise, function bValue) {
return cValue;
});
});
Serial Join
var aPromise = aFunction();
var cPromise = Q.when(aPromise, function (aValue) {
// bFunction will not proceed unless aPromise
// is fulfilled
var bPromise = bFunction();
return Q.when(bPromise, function bValue) {
return cValue;
});
});
Recovering from Failure
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);
});
}
Array Parallel Join
var done;
array.forEach(function (value) {
var work = doWork(value);
done = Q.when(done, function () {
return work;
});
});
return done;
Or
return array.reduce(function (done, value) {
var work = doWork(value);
return Q.when(done, function () {
return work;
});
}, undefined);
Array Serial Join
var done;
array.forEach(function (value) {
done = Q.when(done, function () {
return doWork(value);
});
});
return done;
Or
return array.reduce(function (done, value) {
return Q.when(done, function () {
return doWork(value);
});
});
Conditional Array Serial Join
var otherwise = function () {
throw new Error("No dice");
};
array.forEach(function (value) {
otherwise = (function (otherwise) {
return function () {
var work = doWork(value);
return Q.when(work, function (work) {
if (keepGoing(work)) {
return work;
} else {
return otherwise();
}
});
};
})(otherwise);
});
return otherwise();
Or
array.reduce(function (otherwise, value) {
var work = doWork(value);
return Q.when(work, function (work) {
if (keepGoing(work)) {
return work;
} else {
return otherwise();
}
});
}, function otherwise() {
throw new Error("No dice");
});
In Node, this example reads itself and writes itself out in all capitals.
var Q = require("q/util");
var FS = require("q-fs");
var text = FS.read(__filename);
Q.when(text, function (text) {
console.log(text.toUpperCase());
});
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("q/util");
var FS = require("q-fs");
var self = FS.read(__filename);
var passwd = FS.read("/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/util");
var FS = require("q-fs");
var list = FS.list(__dirname);
var files = Q.when(list, function (list) {
list.forEach(function (fileName) {
var content = FS.read(fileName);
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 list = FS.list(__dirname);
var files = Q.when(list, function (list) {
return list.reduce(function (ready, fileName) {
var content = FS.read(fileName);
return Q.join(ready, content, function (ready, content) {
console.log(fileName, content.length);
});
});
});
q-fs https://github.com/kriskowal/q-fs
basic file system promises
q-http https://github.com/kriskowal/q-http
http client and server promises
q-util https://github.com/kriskowal/q-util
promise control flow and data structures
q-comm https://github.com/kriskowal/q-comm
remote object communication
teleport https://github.com/gozala/teleport
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.
Guarantees:
- 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.
THIS IS COOL
- 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:
INTERSECTION
function and(a, b) {
return when(a, function (a) {
return when(b, function (b) {
// ...
});
})
}
defer()
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.
THIS IS COOL
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.
UNION
function or(a, b) {
var union = defer();
when(a, union.resolve);
when(b, union.resolve);
return union.promise;
}
ref(value)
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.
def(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.
reject(reason)
Returns a promise that has already been rejected
with the given reason.
This is useful for conditionally forwarding a rejection
through an errback.
when(API.getPromise(), function (value) {
return doSomething(value);
}, function (reason) {
if (API.stillPossible())
return API.tryAgain();
else
return reject(reason);
})
Unconditionally forwarding a rejection is equivalent to
omitting an errback on a when call.
isPromise(value)
Returns whether the given value is a promise.
isResolved(value)
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
resolved.
isRejected(value)
Returns whether the given value is a rejected
promise.
promise.valueOf()
Promises override their valueOf method such that if the
promise is fully resolved, it will return the fully
resolved value.
error(reason)
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
rejection.
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.
The Q utility module exports all of the Q module's API but additionally provides the following functions.
var Q = require("q/util");
step(...functions)
Calls each step function serially, proceeding only when
the promise returned by the previous step is deeply
resolved (see: `deep`), and passes the resolution of the
previous step into the argument or arguments of the
subsequent step.
If a step accepts more than one argument, the resolution
of the previous step is treated as an array and expanded
into the step's respective arguments.
`step` returns a promise for the value eventually
returned by the last step.
delay(timeout, eventually_opt)
Returns a promise for the eventual value after `timeout`
miliseconds have elapsed. `eventually` may be omitted,
in which case the promise will be resolved to
`undefined`. If `eventually` is a function, progress
will be made by calling that function and resolving to
the returned value. Otherwise, `eventually` is treated
as a literal value and resolves the returned promise
directly.
shallow(object)
Takes any value and returns a promise for the
corresponding value after all of its properties have
been resolved. For arrays, this means that the
resolution is a new array with the corresponding values
for each respective promise of the original array, and
for objects, a new object with the corresponding values
for each property.
deep(object)
Takes any value and returns a promise for the
corresponding value after all of its properties have
been deeply resolved. Any array or object in the
transitive properties of the given value will be
replaced with a new array or object where all of the
owned properties have been replaced with their
resolution.
reduceLeft(values, callback, basis, this) reduceRight(values, callback, basis, this) reduce(values, callback, basis, this)
The reduce methods all have the signature of `reduce` on
an ECMAScript 5 `Array`, but handle the cases where a
value is a promise and when the return value of the
accumulator is a promise. In these cases, each reducer
guarantees that progress will be made in a particular
order.
`reduceLeft` guarantees that the callback will be called
on each value and accumulation from left to right after
all previous values and accumulations are fully
resolved.
`reduceRight` works similarly from right to left.
`reduce` is opportunistic and will attempt to accumulate
the resolution of any previous resolutions. This is
useful when the accumulation function is associative.
The q/queue module provides a Queue object where
infinite promises for values can be dequeued before they are
enqueued.
put(value)
Places a value on the queue, resolving the next gotten
promise in order.
get()
Returns a promise for the next value from the queue. If
more values have been enqueued than dequeued, this value
will already be resolved.
close(reason_opt)
Causes all promises dequeued after all already enqueued
values have been depleted will be rejected for the given
reason.
closed
A promise that, when resolved, indicates that all
enqueued values from before the call to `close` have
been dequeued.
Copyright 2009, 2010 Kristopher Michael Kowal MIT License (enclosed)
0.2.10
join to "q/util" for variadically joining
multiple promises.FAQs
A library for promises (CommonJS/Promises/A,B,D)
The npm package q receives a total of 4,273,323 weekly downloads. As such, q popularity was classified as popular.
We found that q demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 2 open source maintainers collaborating on the project.
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