Promises/A+ Changelog

Version 1.1.1 / 2014-05-05

Informative Changes

These changes help clarify and improve the specification, but should not affect implementation conformance.

• Clarified fulfillment value and rejection reason propagation when onFulfilled and onRejected passed to then are not functions.

Version 1.1 / 2013-09-29

Informative Changes

These changes help clarify and improve the specification, but should not affect implementation conformance.

• Improved the introductory text to give a better idea of what promises are, and what the purpose of Promises/A+ is.
• Separated out the concept of a possibly-nonconformant “thenable” from the concept of a conformant “promise.”
• Added a definition for “exception.”

Normative Changes

These changes impose new requirements on implementations, either to specify a previously-undefined behavior, or to fix something incorrect that the spec allowed in version 1.0.

• Specified that onFulfilled and onRejected must be called as functions, with no this value.
• Changed the way in which asynchronicity was mandated for onFulfilled and onRejected, to enforce the important invariant that the stack be clear. In particular, the new wording prevents fulfilling or rejecting a promise from ever synchronously calling the handlers.
• Prohibited implementations from calling onFulfilled or onRejected before the corresponding promise was respectively fulfilled or rejected.
• Specified the Promise Resolution Procedure, instead of leaving the mechanism for adopting a thenable’s state unspecified.
  • The recursive nature of the now-specified procedure improves upon the naïve non-recursive suggestion given in version 1.0.
  • Edge cases, such as how to deal with a getter for then, or synchronous exceptions that could be thrown during the process, are nailed down.
  • Self-resolution now causes rejection with a TypeError.
  • Infinite recursion from circular thenable chains are discussed, optionally allowing this to result in a TypeError.

Version 1.0 / 2012-12-06

• Initial release. For differences from its predecessor, the Promises/A specification, see Differences from Promises/A.