This is interesting issue. There is a subtle difference between "prototype
chain is the shared part" Self mindset and the "prototype chain is fallback
delegation" mindset. Though I knew of Self and knew it had an impact on
Javascript creation, I had always an impression that in Javascript (having
become ECMAScript) it was the latter, that is the philosophy is that child
can override the default from prototype chain.
So what is the actual philosophy of ES prototype chain?
Herby
-----Pôvodná správa-----
From: Allen Wirfs-Brock
Sent: Monday, January 09, 2012 9:41 PM
To: Brendan Eich
Cc: Mark S. Miller ; es-discuss Steen
Subject: Re: ES6 doesn't need opt-in
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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