Module Namespace Objects - "writable"

Mark Miller erights at gmail.com
Wed Oct 24 18:03:03 UTC 2018


Ah. Crossed in the mail. Yes, Alan raises the same issues regarding the TDZ
vs non-writable worry.

Thanks for the pointer.

On Wed, Oct 24, 2018 at 11:01 AM Logan Smyth <loganfsmyth at gmail.com> wrote:

> Here's one other post about this from Allen:
> https://github.com/tc39/ecma262/issues/749#issuecomment-265637923
>
> On Wed, Oct 24, 2018 at 10:42 AM T.J. Crowder <
> tj.crowder at farsightsoftware.com> wrote:
>
>> I'm curious if I've inferred the rationale for something correctly.
>>
>> The module namespace object properties for exports have `writable: true`
>> in their property descriptors, but of course, they aren't writable (the
>> module namespace object has its own [[Set]] implementation that returns
>> false). I wondered why they weren't `writable: false`, so I went looking.
>>
>> I found discussion in the March 24 2015 meeting notes about whether to
>> even have `getOwnPropertyDescriptor` work. The consensus was yes, it should
>> work (mixins!), and that it should report a basic data property that isn't
>> configurable -- but is writable. Yahuda Katz points out that:
>>
>> > it is writable, but it's not writable by you
>>
>> though that's not always true (it may be declared `const`; that
>> information isn't leaked from the module, though).
>>
>> In the May 25 2017 notes I found a comment from Mark S. Miller in
>> relation to `writable: true`:
>>
>> > Non-writeable provides guarantee. Writable provides no guarantee.
>>
>> And this exchange between Yahuda Katz, Allen Wirfs-Brock, and Adam Klein:
>>
>> > YK: There is a a new property that we define as writable that is not
>> writable.
>> >
>> > AWB: Not new.
>> >
>> > AK: Since... Proxys!
>>
>> There was some discussion of considering some flag basically saying what
>> YK said, e.g., it's writable, but not by you :-) -- but that was more a
>> brief digression that didn't go anywhere.
>>
>> So, then, what I infer is: They're marked writable because:
>>
>> 1. They may be writable by the exporting module, so code can't assume the
>> value won't change; `writable: false` would make that assumption valid
>> 2. Whether or not they're writable by the exporting module isn't
>> information that should leak out of it
>> 3. Non-writable `writable: true` data properties were already a thing
>> (Proxies)
>>
>> So the most sensible thing was `writable: true` rather than `writable:
>> false`.
>>
>> How'd I do? :-)
>>
>> -- T.J. Crowder
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-- 
  Cheers,
  --MarkM
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