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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.

General

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

Terminology

  1. "promise" is an object or function that defines a then method.
  2. "value" is any legal JavaScript value (including undefined or a promise).
  3. "reason" is a value that indicates why a promise was rejected.
  4. "must not change" means immutable identity (i.e. ===), but does not imply deep immutability.

Requirements

Promise States

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

  1. When in pending, a promise:

    1. may transition to either the fulfilled or rejected state.
  2. When in fulfilled, a promise:

    1. must not transition to any other state.
    2. must have a value, which must not change.
  3. When in rejected, a promise:

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

The then Method

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. then may be called multiple times on the same promise.
    1. If/when promise is fulfilled, respective onFulfilled callbacks must execute in the order of their originating calls to then.
    2. If/when promise is rejected, respective onRejected callbacks must execute in the order of their originating calls to then.
  6. then must return a promise [4.2].

    promise2 = promise1.then(onFulfilled, onRejected);
    
    1. If either onFulfilled or onRejected returns a value that is not a promise, promise2 must be fulfilled with that value.
    2. If either onFulfilled or onRejected throws an exception, promise2 must be rejected with the thrown exception as the reason.
    3. If either onFulfilled or onRejected returns a promise (call it returnedPromise), promise2 must assume the state of returnedPromise [4.3]:
      1. If returnedPromise is pending, promise2 must remain pending until returnedPromise is fulfilled or rejected.
      2. If/when returnedPromise is fulfilled, promise2 must be fulfilled with the same value.
      3. If/when returnedPromise is rejected, promise2 must be rejected with the same reason.
    4. If onFulfilled is not a function and promise1 is fulfilled, promise2 must be fulfilled with the same value.
    5. If onRejected is not a function and promise1 is rejected, promise2 must be rejected with the same reason.

Notes

  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. 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.

  3. The mechanism by which promise2 assumes the state of returnedPromise is not specified. One reasonable approach is to call returnedPromise.then(fulfillPromise2, rejectPromise2), where:

    1. fulfillPromise2 is a function which fulfills promise2 with its first parameter.
    2. rejectPromise2 is a function which rejects promise2 with its first parameter.

    Given that returnedPromise may not be Promises/A+-compliant, but could instead be any object with a then method, it isn't always possible to satisfy the requirement of promise2 assuming the same state as returnedPromise. Thus, the procedure here represents a best-faith effort.


CC0
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.

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