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API Reference
Most promise methods have "static" counterparts on the main Q object, which
will accept either a promise or a non-promise, and in the latter case create
a fulfilled promise first. For example, Q.when(5, onFulfilled) is equivalent
to Q(5).then(onFulfilled). All others have static counterparts that
are named the same as the promise method.
Some methods are named the same as JavaScript reserved words, like try,
catch, and finally. This helps show the very clear parallel between standard
synchronous language constructs and asynchronous promise operations. However,
such use of words as property names is only supported as of the ECMAScript 5
edition of the JavaScript language, which isn't implemented in certain older
browsers like IE8, Safari 5, Android 2.2, or PhantomJS 1.8. If you're targeting
those browsers, and aren't using a language like CoffeeScript that takes care of
this for you, use their aliases instead, or escape them like Q["try"](...) or
promise["catch"](...).
The then method from the Promises/A+ specification, with an additional
progress handler.
Alias: promise.fail (for non-ES5 browsers)
A sugar method, equivalent to promise.then(undefined, onRejected).
Deprecated: promise.observeEstimate or a similar interface is due to replace this method in version 2. Progress does not compose well. https://github.com/kriskowal/gtor#progress-and-estimated-time-to-completion
A sugar method, equivalent to promise.then(undefined, undefined, onProgress).
The onProgress handler receives values that were sent to this promise either from the notify method of the corresponding deferred, or from the promise that this promise became by virtue of being returned from a handler.
Alias: promise.fin (for non-ES5 browsers)
Like a finally clause, allows you to observe either the fulfillment or
rejection of a promise, but to do so without modifying the final value. This is
useful for collecting resources regardless of whether a job succeeded, like
closing a database connection, shutting a server down, or deleting an unneeded
key from an object.
finally returns a promise, which will become resolved with the same
fulfillment value or rejection reason as promise. However, if callback
returns a promise, the resolution of the returned promise will be delayed until
the promise returned from callback is finished. Furthermore, if the returned
promise rejects, that rejection will be passed down the chain instead of the
previous result.
Much like then, but with different behavior around unhandled rejection. If
there is an unhandled rejection, either because promise is rejected and no
onRejected callback was provided, or because onFulfilled or onRejected
threw an error or returned a rejected promise, the resulting rejection reason
is thrown as an exception in a future turn of the event loop.
This method should be used to terminate chains of promises that will not be
passed elsewhere. Since exceptions thrown in then callbacks are consumed and
transformed into rejections, exceptions at the end of the chain are easy to
accidentally, silently ignore. By arranging for the exception to be thrown in a
future turn of the event loop, so that it won't be caught, it causes an
onerror event on the browser window, or an uncaughtException event on
Node.js's process object.
Exceptions thrown by done will have long stack traces, if
Q.longStackSupport is set to true. If Q.onerror is set,
exceptions will be delivered there instead of thrown in a future turn.
The Golden Rule of done vs. then usage is: either return your promise to
someone else, or if the chain ends with you, call done to terminate it. Terminating
with catch is not sufficient because the catch handler may itself throw an error.
Returns a promise to get the named property of an object. Essentially equivalent to
promise.then(function (o) {
return o[propertyName];
});Experimental Alias: promise.mapply
Returns a promise for the result of calling the named method of an object with
the given array of arguments. The object itself is this in the function, just
like a synchronous method call. Essentially equivalent to
promise.then(function (o) {
return o[methodName].apply(o, args);
});Alias: promise.send
Experimental Alias: promise.mcall
Returns a promise for the result of calling the named method of an object with
the given variadic arguments. The object itself is this in the function, just
like a synchronous method call.
Returns a promise for an array of the property names of an object. Essentially equivalent to
promise.then(function (o) {
return Object.keys(o);
});Returns a new function that calls a function asynchronously with the given variadic arguments, and returns a promise. Notably, any synchronous return values or thrown exceptions are transformed, respectively, into fulfillment values or rejection reasons for the promise returned by this new function.
This method is especially useful in its static form for wrapping functions to ensure that they are always asynchronous, and that any thrown exceptions (intentional or accidental) are appropriately transformed into a returned rejected promise. For example:
var getUserData = Q.fbind(function (userName) {
if (!userName) {
throw new Error("userName must be truthy!");
}
if (localCache.has(userName)) {
return localCache.get(userName);
}
return getUserFromCloud(userName);
});Returns a promise for the result of calling a function, with the given array of arguments. Essentially equivalent to
promise.then(function (f) {
return f.apply(undefined, args);
});Note that this will result in the same return value/thrown exception translation
as explained above for fbind.
Static Alias: Q.try (ES5 browsers only)
Returns a promise for the result of calling a function, with the given variadic
arguments. Has the same return value/thrown exception translation as explained
above for fbind.
In its static form, it is aliased as Q.try, since it has semantics similar to
a try block (but handling both synchronous exceptions and asynchronous
rejections). This allows code like
Q
.try(function () {
if (!isConnectedToCloud()) {
throw new Error("The cloud is down!");
}
return syncToCloud();
})
.catch(function (error) {
console.error("Couldn't sync to the cloud", error);
});Returns a promise that is fulfilled with an array containing the fulfillment value of each promise, or is rejected with the same rejection reason as the first promise to be rejected.
This method is often used in its static form on arrays of promises, in order to execute a number of operations concurrently and be notified when they all succeed. For example:
Q.all([getFromDisk(), getFromCloud()]).done(function (values) {
assert(values[0] === values[1]); // values[0] is fromDisk and values[1] is fromCloud
});Returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected.
This method is often used in its static form on arrays of promises, in order to execute a number of operations concurrently and be notified when they all finish, regardless of success or failure. For example:
Q.allSettled([saveToDisk(), saveToCloud()]).spread(function (disk, cloud) {
console.log("saved to disk:", disk.state === "fulfilled");
console.log("saved to cloud:", cloud.state === "fulfilled");
}).done();The state snapshots will be in the same form as those retrieved via
promise.inspect, i.e. either
{ state: "fulfilled", value: v } or { state: "rejected", reason: r }.
Like then, but "spreads" the array into a variadic fulfillment handler. If any
of the promises in the array are rejected, instead calls onRejected with the
first rejected promise's rejection reason.
This is especially useful in conjunction with all, for example:
Q.all([getFromDisk(), getFromCloud()]).spread(function (diskVal, cloudVal) {
assert(diskVal === cloudVal);
}).done();No Static Counterpart
A sugar method, equivalent to promise.then(function () { return value; }).
No Static Counterpart
A sugar method, equivalent to promise.then(function () { throw reason; }).
Introduced in version 1.1.0 (November 2014)
Attaches a handler that will observe the value of the promise when it becomes fulfilled, returning a promise for that same value, perhaps deferred but not replaced by the promise returned by the onFulfilled handler.
Q("Hello, World!")
.delay(1000)
.tap(console.log)
.then(function (message) {
expect(message).toBe("Hello, World!");
})Returns a promise that will have the same result as promise, except that if
promise is not fulfilled or rejected before ms milliseconds, the returned
promise will be rejected with an Error with the given message. If message
is not supplied, the message will be "Timed out after " + ms + " ms".
promise.timeout(10000).then(
function (result) {
// will be called if the promise resolves normally
console.log(result);
},
function (err) {
// will be called if the promise is rejected, or the 10 second timeout occurs
console.log(err);
}
);Returns a promise that will have the same result as promise, but will only be
fulfilled after at least ms milliseconds have passed. If promise is rejected, the resulting promise will be rejected immediately.
If the static version of Q.delay is passed only a single argument, it returns
a promise that will be fulfilled with undefined after at least ms
milliseconds have passed. (If it's called with two arguments, it uses the usual
static-counterpart translation, i.e. Q.delay(value, ms) is equivalent to
Q(value).delay(ms).)
This is a convenient way to insert a delay into a promise chain, or even simply
to get a nicer syntax for setTimeout:
Q.delay(150).then(doSomething);Returns whether a given promise is in the fulfilled state. When the static
version is used on non-promises, the result is always true.
Returns whether a given promise is in the rejected state. When the static
version is used on non-promises, the result is always false.
Returns whether a given promise is in the pending state. When the static version
is used on non-promises, the result is always false.
Returns a "state snapshot" object, which will be in one of three forms:
{ state: "pending" }{ state: "fulfilled", value: <fulfllment value> }{ state: "rejected", reason: <rejection reason> }
Returns a "deferred" object with a:
-
promiseproperty -
resolve(value)method -
reject(reason)method -
notify(value)method -
makeNodeResolver()method
The resolve and reject methods control the state of the promise property,
which you can hand out to others while keeping the authority to modify its state
to yourself. The notify method is for progress notification, and the
makeNodeResolver method is for interfacing with Node.js (see below).
In all cases where a promise is resolved (i.e. either fulfilled or rejected),
the resolution is permanent and cannot be reset. Attempting to call resolve,
reject, or notify if promise is already resolved will be a no-op.
Deferreds are cool because they separate 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 (or indeed interfere with them in any way).
-
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.
Calling resolve with a pending promise causes promise to wait on the passed
promise, becoming fulfilled with its fulfillment value or rejected with its
rejection reason (or staying pending forever, if the passed promise does).
Calling resolve with a rejected promise causes promise to be rejected with
the passed promise's rejection reason.
Calling resolve with a fulfilled promise causes promise to be fulfilled with
the passed promise's fulfillment value.
Calling resolve with a non-promise value causes promise to be fulfilled with
that value.
Calling reject with a reason causes promise to be rejected with that reason.
Calling notify with a value causes promise to be notified of
progress with that value. That is, any onProgress handlers registered with
promise or promises derived from promise will be called with the progress
value.
If value is a Q promise, returns the promise.
If value is a promise from another library it is coerced into a Q promise (where possible).
If value is not a promise, returns a promise that is fulfilled with value.
Returns a promise that is rejected with reason.
Synchronously calls resolver(resolve, reject, notify) and returns a promise
whose state is controlled by the functions passed to resolver. This is an
alternative promise-creation API that has the same power as the deferred
concept, but without introducing another conceptual entity.
If resolver throws an exception, the returned promise will be rejected with
that thrown exception as the rejection reason.
note: In the latest github, this method is called Q.Promise, but if you are using the npm package version 0.9.7 or below, the method is called Q.promise (lowercase vs uppercase p).
Q provides a number of functions for interfacing with Node.js style
(err, result) callback APIs.
Several of these are usually used in their static form, and thus listed here as such. Nevertheless, they also exist on each Q promise, in case you somehow have a promise for a Node.js-style function or for an object with Node.js-style methods.
Note that if a Node.js-style API calls back with more than one non-error
parameter (e.g. child_process.execFile), Q packages these into an
array as the promise's fulfillment value when doing the translation.
Alias: Q.denodeify
Creates a promise-returning function from a Node.js-style function, optionally binding it with the given variadic arguments. An example:
var readFile = Q.nfbind(FS.readFile);
readFile("foo.txt", "utf-8").done(function (text) {
});Note that if you have a method that uses the Node.js callback pattern, as
opposed to just a function, you will need to bind its this value before
passing it to nfbind, like so:
var Kitty = mongoose.model("Kitty");
var findKitties = Q.nfbind(Kitty.find.bind(Kitty));The better strategy for methods would be to use Q.nbind, as shown below.
Creates a promise-returning function from a Node.js-style method, optionally binding it with the given variadic arguments. An example:
var Kitty = mongoose.model("Kitty");
var findKitties = Q.nbind(Kitty.find, Kitty);
findKitties({ cute: true }).done(function (theKitties) {
});Calls a Node.js-style function with the given array of arguments, returning a promise that is fulfilled if the Node.js function calls back with a result, or rejected if it calls back with an error (or throws one synchronously). An example:
Q.nfapply(FS.readFile, ["foo.txt", "utf-8"]).done(function (text) {
});Note that this example only works because FS.readFile is a function exported
from a module, not a method on an object. For methods, e.g. redisClient.get,
you must bind the method to an instance before passing it to Q.nfapply (or,
generally, as an argument to any function call):
Q.nfapply(redisClient.get.bind(redisClient), ["user:1:id"]).done(function (user) {
});The better strategy for methods would be to use Q.npost, as shown below.
Calls a Node.js-style function with the given variadic arguments, returning a promise that is fulfilled if the Node.js function calls back with a result, or rejected if it calls back with an error (or throws one synchronously). An example:
Q.nfcall(FS.readFile, "foo.txt", "utf-8").done(function (text) {
});The same warning about functions vs. methods applies for nfcall as it does for
nfapply. In this case, the better strategy would be to use Q.ninvoke.
Deprecated Alias: Q.nmapply
Calls a Node.js-style method with the given arguments array, returning a promise that is fulfilled if the method calls back with a result, or rejected if it calls back with an error (or throws one synchronously). An example:
Q.npost(redisClient, "get", ["user:1:id"]).done(function (user) {
});Alias: Q.nsend
Deprecated Alias: Q.nmcall
Calls a Node.js-style method with the given variadic arguments, returning a promise that is fulfilled if the method calls back with a result, or rejected if it calls back with an error (or throws one synchronously). An example:
Q.ninvoke(redisClient, "get", "user:1:id").done(function (user) {
});If callback is a function, assumes it's a Node.js-style callback, and calls it
as either callback(rejectionReason) when/if promise becomes rejected, or
as callback(null, fulfillmentValue) when/if promise becomes fulfilled. If
callback is not a function, simply returns promise.
This method is useful for creating dual promise/callback APIs, i.e. APIs that return promises but also accept Node.js-style callbacks. For example:
function createUser(userName, userData, callback) {
return database.ensureUserNameNotTaken(userName)
.then(function () {
return database.saveUserData(userName, userData);
})
.nodeify(callback);
}Returns a function suitable for passing to a Node.js API. That is, it has a
signature (err, result) and will reject deferred.promise with err if
err is given, or fulfill it with result if that is given.
This functionality is experimental.
This is an experimental tool for converting a generator function into a deferred
function. This has the potential of reducing nested callbacks in engines that
support yield. See the generators example for further information.
This immediately runs a generator function, and forwards any uncaught errors to
Q.onerror. An uncaught error is deemed to occur if the function returns a
rejected promise. Note that this automatically occurs if the generator function
throws an error, e.g. by yielding on a promise that becomes rejected without surrounding that code with a try/catch:
Q.spawn(function* () {
// If `createUser` returns a rejected promise, the rejection reason will
// reach `Q.onerror`.
var user = yield createUser();
showUserInUI(user);
});A settable property that will intercept any uncaught errors that would otherwise
be thrown in the next tick of the event loop, usually as a result of done.
Can be useful for getting the full stack trace of an error in browsers, which is
not usually possible with window.onerror.
Gets an array of reasons belonging to rejected promises that currently have not
been handled, i.e. no onRejected callbacks have been called for them, they
haven't been chained off of, etc. Generally these represent potentially-"lost"
errors, so this array should be empty except possibly at times when you are
passing a rejected promise around asynchronously so that someone can handle the
rejection later.
Turns off unhandled rejection tracking, which provides a slight efficiency boost if you don't find that debug information helpful. It also prevents Q from printing any unhandled rejection reasons upon process exit in Node.js.
Resets Q's internal tracking of unhandled rejections, but keeps unhandled rejection tracking on. This method is exposed mainly for testing and diagnostic purposes, where you may have accumulated some unhandled rejections but want to re-start with a clean slate.
Returns whether the given value is a Q promise.
Returns whether the given value is a promise (i.e. it's an object with a then function).
Creates a new version of func that accepts any combination of promise and
non-promise values, converting them to their fulfillment values before calling
the original func. The returned version also always returns a promise: if
func does a return or throw, then Q.promised(func) will return
fulfilled or rejected promise, respectively.
This can be useful for creating functions that accept either promises or non-promise values, and for ensuring that the function always returns a promise even in the face of unintentional thrown exceptions.
A settable property that lets you turn on long stack trace support. If turned
on, "stack jumps" will be tracked across asynchronous promise operations, so
that if an uncaught error is thrown by done or a rejection reason's stack
property is inspected in a rejection callback, a long stack trace is produced.
The Q promise constructor establishes the basic API for performing operations
on objects: "get", "put", "del", "post", "apply", and "keys". 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.
Sends an arbitrary message to a promise, with the given array of arguments.
Care should be taken not to introduce control-flow hazards and security holes
when forwarding messages to promises. The functions above, particularly then,
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.