q

What is q?

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.

What are q's main functionalities?

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));

Other packages similar to q

bluebird

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

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'.

promise

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'.

README

Provides a defer/when style promise API for JavaScript.

• usable as a CommonJS module, in Node,
• usable as a
• compatible with jQuery and Dojo promises,
• inspired by Tyler Close's Waterken ref_send promises, and
• compliant with
  • http://wiki.commonjs.org/wiki/Promises/A
  • http://wiki.commonjs.org/wiki/Promises/B
  • http://wiki.commonjs.org/wiki/Promises/D

For Node:

$ curl http://npmjs.org/install.sh | sh
$ npm install q
$ node examples/test.js

EXAMPLES

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);
        else
            result.resolve(list);
    });
    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 aValue.
• 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 bPromise.
• If the fulfillment callback throws an exception, bPromise will be rejected with the thrown error as the reason.

This example illustrates how the when primitive can be used to observe either the fulfillment or rejection of a promise.

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 bPromise.

Node File-system Examples

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);
        });
    });
});

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 rejected.

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 q/util.

var Q = require("q/util");
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 blocks.

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

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.

THE HALLOWED API

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 Q.when(a, function (a) {
        return Q.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 = Q.defer();
    Q.when(a, union.resolve);
    Q.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.

    Q.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.

ADVANCED API

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 UTIL MODULE

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 QUEUE MODULE

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)

Changelog

0.4.1

• Added an "end" method to the promise prototype, as a shorthand for waiting for the promise to be resolved gracefully, and failing to do so, to dump an error message.