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'.
If a function cannot return a value or throw an exception without blocking, it can return a promise instead. A promise is an object that represents the return value or the thrown exception that the function may eventually provide. A promise can also be used as a proxy for a remote object to overcome latency.
On the first pass, promises can mitigate the “Pyramid of Doom”: the situation where code marches to the right faster than it marches forward.
step1(function (value1) {
step2(value1, function(value2) {
step3(value2, function(value3) {
step4(value3, function(value4) {
// Do something with value4
});
});
});
});
With a promise library, you can flatten the pyramid.
Q.fcall(step1)
.then(step2)
.then(step3)
.then(step4)
.then(function (value4) {
// Do something with value4
}, function (error) {
// Handle any error from step1 through step4
})
.done();
With this approach, you also get implicit error propagation,
just like try, catch, and finally. An error in
step1 will flow all the way to step5, where it’s
caught and handled.
The callback approach is called an “inversion of control”. A function that accepts a callback instead of a return value is saying, “Don’t call me, I’ll call you.”. Promises un-invert the inversion, cleanly separating the input arguments from control flow arguments. This simplifies the use and creation of API’s, particularly variadic, rest and spread arguments.
The Q module can be loaded as:
<script> tag (creating a Q global variable):
~3.5 KB minified and gzipped.q
packageQ can exchange promises with jQuery, Dojo, When.js, WinJS, and more. Additionally, there are many libraries that produce and consume Q promises for everything from file system/database access or RPC to templating. For a list of some of the more popular ones, see Libraries.
Please join the Q-Continuum mailing list.
Promises have a then method, which you can use to get the eventual
return value (fulfillment) or thrown exception (rejection).
foo()
.then(function (value) {
}, function (reason) {
})
If foo returns a promise that gets fulfilled later with a return
value, the first function (the value handler) will be called with the
value. However, if the foo function gets rejected later by a
thrown exception, the second function (the error handler) will be
called with the error.
Note that resolution of a promise is always asynchronous: that is, the
value or error handler will always be called in the next turn of the
event loop (i.e. process.nextTick in Node). This gives you a nice
guarantee when mentally tracing the flow of your code, namely that
then will always return before either handler is executed.
The then method returns a promise, which in this example, I’m
assigning to bar.
var bar = foo()
.then(function (value) {
}, function (reason) {
})
The bar variable becomes a new promise for the return value of
either handler. Since a function can only either return a value or
throw an exception, only one handler will ever be called and it will
be responsible for resolving bar.
If you return a value in a handler, bar will get fulfilled.
If you throw an exception in a handler bar will get rejected.
If you return a promise in a handler, bar will “become”
that promise. Being able to become a new promise is useful for
managing delays, combining results, or recovering from errors.
If the foo() promise gets rejected and you omit the error handler,
the error will go to bar:
var bar = foo()
.then(function (value) {
})
If the foo() promise gets fulfilled and you omit the value
handler, the value will go to bar:
var bar = foo()
.then(null, function (error) {
})
Q promises provide a fail shorthand for then when you are only
interested in handling the error:
var bar = foo()
.fail(function (error) {
})
They also have a fin function that is like a finally clause.
The final handler gets called, with no arguments, when the promise
returned by foo() either returns a value or throws an error. The
value returned or error thrown by foo() passes directly to bar.
var bar = foo()
.fin(function () {
// close files, database connections, stop servers, conclude tests
})
barbar gets postponed. The
eventual value or error has the same effect as an immediate return
value or thrown error: a value would be ignored, an error would be
forwarded.There are two ways to chain promises. You can chain promises either inside or outside handlers. The next two examples are equivalent.
return foo()
.then(function (fooValue) {
return bar(fooValue)
.then(function (barValue) {
// if we get here without an error,
// the value returned here
// or the exception thrown here
// resolves the promise returned
// by the first line
})
})
return foo()
.then(function (fooValue) {
return bar(fooValue);
})
.then(function (barValue) {
// if we get here without an error,
// the value returned here
// or the exception thrown here
// resolves the promise returned
// by the first line
})
The only difference is nesting. It’s useful to nest handlers if you
need to capture both fooValue and barValue in the last
handler.
function eventualAdd(a, b) {
return a.then(function (a) {
return b.then(function (b) {
return a + b;
});
});
}
You can turn an array of promises into a promise for the whole,
fulfilled array using all.
return Q.all([
eventualAdd(2, 2),
eventualAdd(10, 20)
])
If you have a promise for an array, you can use spread as a
replacement for then. The spread function “spreads” the
values over the arguments of the value handler. The error handler
will get called at the first sign of failure. That is, whichever of
the recived promises fails first gets handled by the error handler.
function eventualAdd(a, b) {
return Q.spread([a, b], function (a, b) {
return a + b;
})
}
But spread calls all initially, so you can skip it in chains.
return foo()
.then(function (info) {
return [info.name, FS.read(info.location, "utf-8")];
// FS.read returns a promise, so this array
// mixes values and promises
})
.spread(function (name, text) {
})
The all function returns a promise for an array of values. If one
of the given promise fails, the whole returned promise fails, not
waiting for the rest of the batch. If you want to wait for all of the
promises to either be fulfilled or rejected, you can use
allResolved.
Q.allResolved(promises)
.then(function (promises) {
promises.forEach(function (promise) {
if (promise.isFulfilled()) {
var value = promise.valueOf();
} else {
var exception = promise.valueOf().exception;
}
})
})
If you have a number of promise-producing functions that need to be run sequentially, you can of course do so manually:
return foo(initialVal).then(bar).then(baz).then(quux);
However, if you want to run a dynamically constructed sequence of functions, you'll want something like this:
var funcs = [foo, bar, baz, quux];
var result = Q.resolve(initialVal);
funcs.forEach(function (f) {
result = result.then(f);
});
return result;
You can make this slightly more compact using reduce:
return funcs.reduce(function (soFar, f) {
return soFar.then(f);
}, Q.resolve(initialVal));
One sometimes-unintuive aspect of promises is that if you throw an exception in the value handler, it will not be be caught by the error handler.
foo()
.then(function (value) {
throw new Error("Can't bar.");
}, function (error) {
// We only get here if "foo" fails
})
To see why this is, consider the parallel between promises and
try/catch. We are try-ing to execute foo(): the error
handler represents a catch for foo(), while the value handler
represents code that happens after the try/catch block.
That code then needs its own try/catch block.
In terms of promises, this means chaining your error handler:
foo()
.then(function (value) {
throw new Error("Can't bar.");
})
.fail(function (error) {
// We get here with either foo's error or bar's error
})
When you get to the end of a chain of promises, you should either return the last promise or end the chain. Since handlers catch errors, it’s an unfortunate pattern that the exceptions can go unobserved.
So, either return it,
return foo()
.then(function () {
return "bar";
})
Or, end it.
foo()
.then(function () {
return "bar";
})
.done()
Ending a promise chain makes sure that, if an error doesn’t get handled before the end, it will get rethrown and reported.
This is a stopgap. We are exploring ways to make unhandled errors visible without any explicit handling.
Everything above assumes you get a promise from somewhere else. This is the common case. Every once in a while, you will need to create a promise from scratch.
Q.fcallYou can create a promise from a value using Q.fcall. This returns a
promise for 10.
return Q.fcall(function () {
return 10;
});
You can also use fcall to get a promise for an exception.
return Q.fcall(function () {
throw new Error("Can't do it");
})
As the name implies, fcall can call functions, or even promised
functions. This uses the eventualAdd function above to add two
numbers.
return Q.fcall(eventualAdd, 2, 2);
If you have to interface with asynchronous functions that are callback-based
instead of promise-based, Q provides a few shortcuts (like Q.nfcall and
friends). But much of the time, the solution will be to use deferreds.
var deferred = Q.defer();
FS.readFile("foo.txt", "utf-8", function (error, text) {
if (error) {
deferred.reject(new Error(error));
} else {
deferred.resolve(text);
}
});
return deferred.promise;
Note that a deferred can be resolved with a value or a promise. The
reject function is a shorthand for resolving with a rejected
promise.
// this:
deferred.reject(new Error("Can't do it"));
// is shorthand for:
var rejection = Q.fcall(function () {
throw new Error("Can't do it");
});
deferred.resolve(rejection);
This is a simplified implementation of Q.delay.
function delay(ms) {
var deferred = Q.defer();
setTimeout(deferred.resolve, ms);
return deferred.promise;
}
This is a simplified implementation of Q.timeout
function timeout(promise, ms) {
var deferred = Q.defer();
Q.when(promise, deferred.resolve);
Q.when(delay(ms), function () {
deferred.reject(new Error("Timed out"));
});
return deferred.promise;
}
If you are using a function that may return a promise, but just might return a value if it doesn’t need to defer, you can use the “static” methods of the Q library.
The when function is the static equivalent for then.
return Q.when(valueOrPromise, function (value) {
}, function (error) {
});
All of the other methods on a promise have static analogs with the same name.
The following are equivalent:
return Q.all([a, b]);
return Q.fcall(function () {
return [a, b];
})
.all();
When working with promises provided by other libraries, you should
convert it to a Q promise. Not all promise libraries make the same
guarantees as Q and certainly don’t provide all of the same methods.
Most libraries only provide a partially functional then method.
This thankfully is all we need to turn them into vibrant Q promises.
return Q.when($.ajax(...))
.then(function () {
})
If there is any chance that the promise you receive is not a Q promise
as provided by your library, you should wrap it using a Q function.
You can even use Q.invoke as a shorthand.
return Q.invoke($, 'ajax', ...)
.then(function () {
})
A promise can serve as a proxy for another object, even a remote object. There are methods that allow you to optimistically manipulate properties or call functions. All of these interactions return promises, so they can be chained.
direct manipulation using a promise as a proxy
-------------------------- -------------------------------
value.foo promise.get("foo")
value.foo = value promise.put("foo", value)
delete value.foo promise.del("foo")
value.foo(...args) promise.post("foo", [args])
value.foo(...args) promise.invoke("foo", ...args)
value(...args) promise.fapply([args])
value(...args) promise.fcall(...args)
If the promise is a proxy for a remote object, you can shave
round-trips by using these functions instead of then. To take
advantage of promises for remote objects, check out Q-Comm.
Even in the case of non-remote objects, these methods can be used as shorthand for particularly-simple value handlers. For example, you can replace
return Q.fcall(function () {
return [{ foo: "bar" }, { foo: "baz" }];
})
.then(function (value) {
return value[0].foo;
})
with
return Q.fcall(function () {
return [{ foo: "bar" }, { foo: "baz" }];
})
.get(0)
.get("foo")
There is a makeNodeResolver method on deferreds that is handy for
the NodeJS callback pattern.
var deferred = Q.defer();
FS.readFile("foo.txt", "utf-8", deferred.makeNodeResolver());
return deferred.promise;
And there are Q.nfcall and Q.ninvoke for even shorter
expression.
return Q.nfcall(FS.readFile, "foo.txt", "utf-8");
return Q.ninvoke(FS, "readFile", "foo.txt", "utf-8");
There is also a Q.nfbind function that that creates a reusable
wrapper.
var readFile = Q.nfbind(FS.readFile);
return readFile("foo.txt", "utf-8");
Note that, since promises are always resolved in the next turn of the
event loop, working with streams can be tricky. The
essential problem is that, since Node does not buffer input, it is
necessary to attach your "data" event listeners immediately,
before this next turn comes around. There are a variety of solutions
to this problem, and even some hope that in future versions of Node it
will be ameliorated.
A method-by-method Q API reference is available on the wiki.
A growing examples gallery is available on the wiki, showing how Q can be used to make everything better. From XHR to database access to accessing the Flickr API, Q is there for you.
Copyright 2009-2012 Kristopher Michael Kowal MIT License (enclosed)
0.8.12
Q.isFulfilled; this lets Q.all
work on arrays containing foreign promises. #154FAQs
A library for promises (CommonJS/Promises/A,B,D)
The npm package q receives a total of 10,849,216 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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