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yap - Yet Another Promise module

An implementation of the Promise/A specification from Common/JS (


yap is used to implement asynchronous methods and provides a consistent mechanism to handle the results and report errors.



Returns a promise for an array of values promised by elements of the array a. The promise will be fulfilled when all of the promises in the array are fulfilled. If any of the promises are broken then the returned promise is broken and is rejected with the first error reported. If any of the values of array a is not a promise the value is copied to the result.

Assuming the existence of a function waitForIt(v,m), that creates a promise that resolves to v after m milliseconds, the following,

yap.all([waitForIt(1, 10), waitForIt(2, 100), waitForIt(3, 20)]).then(function (p) {;

will print [1, 2, 3] to the console after 100ms, which is the time it takes to resolve the longest promise. All the resolve asynchronously.


Returns a promise for the work performed by f. f is called with three parameters, resolve, reject and progress which are the resolve, reject and progress functions of a deferral. If an exception is thrown in f then reject is automatically called.

The following is an example of using defer to generate a promise that will be resolved after a second with the value 1.

 var waitOneSec = yap.defer(function (resolve) {
     setTimeout(function() {
     }, 1000);

The resolve function can be passed directly as node standard async callback function. It recognizes the common callback patterns. The following example takes advantage of this to create a promise for buffer of 1024 random bytes.

var somethingRandom = yap.defer(function (resolve) {
    crypto.randomBytes(1024, resolve);


Create a deferral. A deferral is the core of yap. An asynchronous operation first creates a deferral and then either reports it successfully delivered on a promise by calling resolve(), or the failure to deliver on the promise by calling reject(). Long running asynchronous tasks can report their progress by calling progress(). A promise can be extracted by calling promise() which will only support then() and related helpers. Although a deferral follows the promise pattern, it should not be visuble outside the asynchronous task.

The following example creates a promise for a 1 in one second.

var d = yap.deferral();
setTimout(function () {
}, 1000);
var waitOneSec = d.promise();

Returns true if p is a promise created by yap, false otherwise.

Returns true if p supports the promise pattern (e.g. has a then method), false otherwise. This should return true for any promise implementation, not just yap. yap uses like() to decide whether a value is a promise. This enables other promise implementations to be usable with yap helpers such as all().


Returns a promise that is already resolved to the value v.


Returns a promise for the result of calling f. If an exception is raised in f then the promise is rejected. Turns a synchronouse function into a promise.

yap.timeout(delay, [value])

Returns a promise that will resolve to value (or undefined if missing) after delay milliseconds. This returns a promise that contains a cancel() method as well that allows the callback to be cancelled. If it is cancelled the promise will never be resolved.


Resolves a deferral with value v. Once a deferral is resolved subsequent calls to resolve(), progress() and reject() are ignored. This will cause the first parameters of the then methods to be called.

deferral.resolve(e, v)

An alternate way to call resolve where e is either an exception object or null and v is the value with which to resolve the deferral. This allows resolve to be passed in as the callback function for a standard node callback.


Rejects a deferral with the error e. e should be an Error object such as would be received in a catch clause. Once a deferral is rejected subsequent calls to resolve(), progress() and reject() are ignored. This causes the functions passed as the second parameter of the then methods to be invoked.


Reports progress of a deferral. This causes the functions passed as the third parameter of the then methods to be invoked.


Returns the promise that for the work that communicates it result with a deferral. Even though the deferral is itself a promise, it should not be available outside the asynchronouse task. Use the promise returned by promise() instead.

promise.then(fulfilledHandler, [errorHandler], [progressHandler])

Supply callback to be called when the promise is either fulfilled, rejected or is reporting progress. This is the central method of the promise pattern. The then() method returns the promise itself which allows the promise to be chained. If the fulfilledHandler returns a result then that result will be the value passed to any fulfilledHandlers added after it. If it returns a promise the fulfilledHandlers that are added after it are resolved with the result of the promise. For example,

waitForIt(1, 100).then(function (v) {
    return v + 1;
}).then(function (v) {
	return waitForIt(v + 1, 100);
}).then(function (v) {;

will report 3 to the console after 200ms.


A helper method to call then() with only a fulfilledHandler. This differs from then() in that it doesn't return the promise. This can be used indicate that no more then() methods are expected.

A helper method to call then() with only a errorHandler.


A helper method to call then() with only a errorHandler.