Rework Promise::always() to eliminate unintended side-effects.

[johnyanarella]

promise.always(fn) is akin to a finally in a try..catch..finally block (although it can occur anywhere a Promise chain).

Originally envisioned as a shorthand for promise.then(fn, fn), promise.always(fn) has a couple odd quirks:

• it can be called with either a result or an error; and
• when used in a chain, it can cause an accidental recovery from an error simply by returning a value (or no value at all) instead of rethrowing the error, hiding it from promises further down the chain.

The typical use case for promise.always() is to add handler logic that should be triggered regardless of whether the originating promise is fulfilled or rejected. In this scenario, the result or error parameter is ignored and the developer typically wouldn't think to inspect it and potentially rethrow it in the case of an Error.

Consequently, it makes sense for promise.always() to be changed to eliminate these quirks:

• it should pass no parameters to its callback; and
• when used in a chain, the fulfillment or rejection of its originating promise should be passed through transparently.

In the rare scenarios where the original behavior was actually desired, promise.then(fn, fn) can be used instead.

[johnyanarella]

This feature and its associated unit tests have been implemented in the promises_aplus branch and will be part of the Deft JS 0.9 release.

[johnyanarella]

The reimplementation of Deft.Promise::always() is now available as part of the new Promises/A+ compliant rewrite of Deft JS's Promises.

See also: #82