Differences from Promises/A

Promises/A+ is based on the concepts and then API presented in the CommonJS Promises/A proposal. However, it differs from Promises/A in several important ways.

Omissions

The following parts of Promises/A have been intentionally omitted:

1. Progress handling: in practice, it has proven to be underspecified and currently does not have an agreed-upon or de facto behavior within the promise implementor community.
2. Interactive promises: this is deemed out of scope of the minimal API necessary for interoperable promises.
3. promise1 !== promise2 is not a requirement for var promise2 = promise1.then(onFulfilled, onRejected).

Clarifications

Promises/A+ uses different terminology from Promises/A, reflecting what has become the de facto vocabulary among promise implementations. Specifically:

1. The promise states are given as “pending”, “fulfilled”, and “rejected”.
2. When promises are fulfilled, they have a “value”; when they are rejected, they have a “reason”.
3. It introduces the term “thenable” as distinct from “promise”, so as to more precisely talk about the duck-typing tests necessary for implementation interoperation.

Additions

Promises/A+ additionally specifies:

1. the behavior in the case where onFulfilled or onRejected returns a thenable, including the details of the resolution procedure;
2. the reason passed to onRejected must be the thrown exception in the case where a handler throws;
3. onFulfilled and onRejected must be called asynchronously;
4. onFulfilled and onRejected must be called as functions;
5. strict ordering of calls to onFulfilled and onRejected for subsequent calls to then on the same promise.