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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(promisedStep1)
.then(promisedStep2)
.then(promisedStep3)
.then(promisedStep4)
.then(function (value4) {
// Do something with value4
})
.catch(function (error) {
// Handle any error from all above steps
})
.done();
With this approach, you also get implicit error propagation, just like try
,
catch
, and finally
. An error in promisedStep1
will flow all the way to
the catch
function, where it’s caught and handled. (Here promisedStepN
is
a version of stepN
that returns a promise.)
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.
Getting Started
The Q module can be loaded as:
- A
<script>
tag (creating aQ
global variable): ~2.5 KB minified and gzipped. - A Node.js and CommonJS module, available in npm as the q package
- An AMD module
- A component as
microjs/q
- Using bower as
q
- Using NuGet as Q
Q can exchange promises with jQuery, Dojo, When.js, WinJS, and more.
Resources
Our wiki contains a number of useful resources, including:
- A method-by-method Q API reference.
- A growing examples gallery, 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.
- 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.
- If you want materials that introduce the promise concept generally, and the below tutorial isn't doing it for you, check out our collection of presentations, blog posts, and podcasts.
- A guide for those coming from jQuery's
$.Deferred
.
We'd also love to have you join the Q-Continuum mailing list.
Tutorial
Promises have a then
method, which you can use to get the eventual
return value (fulfillment) or thrown exception (rejection).
promiseMeSomething()
.then(function (value) {
}, function (reason) {
});
If promiseMeSomething
returns a promise that gets fulfilled later
with a return value, the first function (the fulfillment handler) will be
called with the value. However, if the promiseMeSomething
function
gets rejected later by a thrown exception, the second function (the
rejection handler) will be called with the exception.
Note that resolution of a promise is always asynchronous: that is, the
fulfillment or rejection 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.
In this tutorial, we begin with how to consume and work with promises. We'll
talk about how to create them, and thus create functions like
promiseMeSomething
that return promises, below.
Propagation
The then
method returns a promise, which in this example, I’m
assigning to outputPromise
.
var outputPromise = getInputPromise()
.then(function (input) {
}, function (reason) {
});
The outputPromise
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 outputPromise
.
If you return a value in a handler,
outputPromise
will get fulfilled.If you throw an exception in a handler,
outputPromise
will get rejected.If you return a promise in a handler,
outputPromise
will “become” that promise. Being able to become a new promise is useful for managing delays, combining results, or recovering from errors.
If the getInputPromise()
promise gets rejected and you omit the
rejection handler, the error will go to outputPromise
:
var outputPromise = getInputPromise()
.then(function (value) {
});
If the input promise gets fulfilled and you omit the fulfillment handler, the
value will go to outputPromise
:
var outputPromise = getInputPromise()
.then(null, function (error) {
});
Q promises provide a fail
shorthand for then
when you are only
interested in handling the error:
var outputPromise = getInputPromise()
.fail(function (error) {
});
If you are writing JavaScript for modern engines only or using
CoffeeScript, you may use catch
instead of fail
.
Promises also have a fin
function that is like a finally
clause.
The final handler gets called, with no arguments, when the promise
returned by getInputPromise()
either returns a value or throws an
error. The value returned or error thrown by getInputPromise()
passes directly to outputPromise
unless the final handler fails, and
may be delayed if the final handler returns a promise.
var outputPromise = getInputPromise()
.fin(function () {
// close files, database connections, stop servers, conclude tests
});
- If the handler returns a value, the value is ignored
- If the handler throws an error, the error passes to
outputPromise
- If the handler returns a promise,
outputPromise
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.
If you are writing JavaScript for modern engines only or using
CoffeeScript, you may use finally
instead of fin
.
Chaining
There are two ways to chain promises. You can chain promises
- 1
- 2
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