ES6 doesn't need opt-in

Allen Wirfs-Brock allen at wirfs-brock.com
Mon Jan 9 12:41:20 PST 2012


On Jan 8, 2012, at 10:32 AM, Brendan Eich wrote:

> On Jan 8, 2012, at 8:28 AM, Mark S. Miller wrote:
> ...
> 
>> The other change I hope fits into the same bucket is <http://wiki.ecmascript.org/doku.php?id=strawman:fixing_override_mistake>. Right now, because of pressure from test262, we are in danger of having all browsers conform to this mistake, at which point it may be too late to fix it. Today, the diversity of actual browser behaviors means it is still possible to fix this mistake, much as the diversity of ways ES3 implementations were broken made it possible for ES5 to fix many mistakes.
> 
> The [[CanPut]] check goes back to ES1, though. Recent-ish deviations in JSC and (because V8 was drafting off JSC) V8 don't nullify all that history.
> 
> On the other hand, JSC and V8 are doing fine AFAIK. It's hard to make a real-world case where this matters, even with Object.create. And I see the ocap (not just SES) appeal of the fix.
> 

Just to be even clearer.  This was not a mistake in ES5/5.1 and it is not a bug.  It is a semantics, which as Brendan points out goes all the way back to ES1.  It is also a behavior which makes complete sense from a prototypal inheritance perspective and can be found in the Self language. 

The basic idea is that the properties prototype object are shared parts of all of inheriting child object.  Modifying such a shared part by a child, introduces a local change that is visible to that child (and its children) so this requires creation of a "own" property on the child. However, read-only properties can not modified (by normal means, eg assignment) so there is no need to create a "own" copy.  Assigning to an inherited read-only property or a "own" read-only property should have the same affect (whether it is ignoring the assignment, throwing, etc.).  Allowing assignment to an inherited read-only property would break the invariant that that a prototype's readonly property is an immutable value that is  shared among all children of the prototype.

If there was a mistake in designing ES5, it was allowing Object.defineOwnProperty to create child properties that over-ride inherited read-only data properties.  This broke an invariant that previously existed in the language but this invariant  was already violated by some pre-ES5 clause 15 objects, (eg the writability of the prototype property of some children of Function.prototype).  However, I think the ES5 decision was probably the right one given the legacy clause 15 usages and the overall reflective nature of defineOwnProperty).

Allen





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