Promises/A+

This proposal clarifies the behavioral clauses of the Promises/A proposal, extending it to cover de facto behaviors and omitting parts that are underspecified or problematic.

As with Promises/A, this proposal does not deal with how to create, fulfill, or reject promises.

For a full description of the differences between Promises/A+ and Promises/A, see Differences from Promises/A.

A promise represents a value that may not be available yet. The primary method for interacting with a promise is its then method.

1. "promise" is an object or function with a then method whose behavior conforms to this specification.
2. "thenable" is an object or function that defines a then method.
3. "value" is any legal JavaScript value (including undefined, a thenable, or a promise).
4. "exception" is a value that is thrown using the throw statement.
5. "reason" is a value that indicates why a promise was rejected.

A promise must be in one of three states: pending, fulfilled, or rejected.

1. When pending, a promise:

   1. may transition to either the fulfilled or rejected state.

2. When fulfilled, a promise:

   1. must not transition to any other state.
   2. must have a fulfillment value, which must not change.

3. When rejected, a promise:

   1. must not transition to any other state.
   2. must have a rejection reason, which must not change.

Here, "must not change" means immutable identity (i.e. ===), but does not imply deep immutability.

A promise must provide a then method to access its current or eventual fulfillment value or rejection reason.

A promise's then method accepts two arguments:

promise.then(onFulfilled, onRejected)

1. Both onFulfilled and onRejected are optional arguments:

   1. If onFulfilled is not a function, it must be ignored.
   2. If onRejected is not a function, it must be ignored.

2. If onFulfilled is a function:

   1. it must be called after promise is fulfilled, with promise's fulfillment value as its first argument.
   2. it must not be called more than once.
   3. it must not be called if onRejected has been called.

3. If onRejected is a function,

   1. it must be called after promise is rejected, with promise's rejection reason as its first argument.
   2. it must not be called more than once.
   3. it must not be called if onFulfilled has been called.

4. then must return before onFulfilled or onRejected is called [4.1].

5. onFulfilled and onRejected must be called as functions (i.e. with no this value). [4.2]

6. then may be called multiple times on the same promise.

   1. If/when promise is fulfilled, all respective onFulfilled callbacks must execute in the order of their originating calls to then.
   2. If/when promise is rejected, all respective onRejected callbacks must execute in the order of their originating calls to then.

7. then must return a promise [4.3].

   promise2 = promise1.then(onFulfilled, onRejected);

   1. If either onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x).
   2. If either onFulfilled or onRejected throws an exception e, promise2 must be rejected with e as the reason.
   3. If onFulfilled is not a function and promise1 is fulfilled, promise2 must be fulfilled with the same value.
   4. If onRejected is not a function and promise1 is rejected, promise2 must be rejected with the same reason.

The promise resolution procedure is an abstract operation taking as input a promise and a value, which we denote as [[Resolve]](promise, x). If x is a thenable, it attempts to make promise adopt the state of x, under the assumption that x behaves at least somewhat like a promise. Otherwise, it fulfills promise with the value x.

This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to "assimilate" nonconformant implementations with reasonable then methods.

To run [[Resolve]](promise, x), perform the following steps:

1. If x is a promise, adopt its state [4.4]:
   1. If x is pending, promise must remain pending until x is fulfilled or rejected.
   2. If/when x is fulfilled, fulfill promise with the same value.
   3. If/when x is rejected, reject promise with the same reason.
2. Otherwise, if x is an object or function,
   1. Let then be x.then. [4.5]
   2. If retrieving the property x.then results in a thrown exception e, reject promise with e as the reason.
   3. If then is a function, call it with x as this, first argument resolvePromise, and second argument rejectPromise, where:
      1. If/when resolvePromise is called with a value y, run [[Resolve]](promise, y).
      2. If/when rejectPromise is called with a reason r, reject promise with r.
      3. If both resolvePromise and rejectPromise are called, or multiple calls to the same argument are made, the first call takes precedence, and any further calls are ignored.
      4. If calling then throws an exception e,
         1. If resolvePromise or rejectPromise have been called, ignore it.
         2. Otherwise, reject promise with e as the reason.
   4. If then is not a function, fulfill promise with x.
3. If x is not an object or function, fulfill promise with x.

1. In practical terms, an implementation must use a mechanism such as setTimeout, setImmediate, or process.nextTick to ensure that onFulfilled and onRejected are not invoked in the same turn of the event loop as the call to then to which they are passed.

2. That is, in strict mode this will be undefined inside of them; in sloppy mode, it will be the global object.

3. Implementations may allow promise2 === promise1, provided the implementation meets all requirements. Each implementation should document whether it can produce promise2 === promise1 and under what conditions.

4. Generally, it will only be known that x is a true promise if it comes from the current implementation. This clause allows the use of implementation-specific means to adopt the state of known-conformant promises.

5. This procedure of first storing a reference to x.then, then testing that reference, and then calling that reference, avoids multiple accesses to the x.then property. Such precautions are important for ensuring consistency in the face of an accessor property, whose value could change between retrievals.

To the extent possible under law, the Promises/A+ organization has waived all copyright and related or neighboring rights to Promises/A+ Promise Specification. This work is published from: United States.