Question about [[Enumerate]] and property decision timing

[Allen Wirfs-Brock]

On Aug 10, 2015, at 1:35 AM, Jordan Harband wrote:

> I interpreted this spec text (and the included generator code example) to mean that the first "next" is the thing that determines the keys to be enumerated - since the generator function body (including the Reflect.ownKeys call) is not executed until that point.

The generator-based definition was intended to only be an informative example of a conforming enumeration algorithm. Anything it does that goes beyond requirements stated in the  normative prose should not be considered a normative requirement. The deferral of all setup processing to the first  `next` call is simply an artifact of the how generators works.

> 
> The question was also raised on that linked bug, why does the spec leave behavior undefined for keys added during enumeration? It seems like the spec should either dictate that added keys are *not* enumerated, or, that they are *always* enumerated.

See my previous reply.  When decisions about for-in were made for ES6, TC39 was not yet ready to try to mandate a normative enumeration order or other historically unspecified for-in semantic details. This could be reconsider in a future edition.

Allen