getter & setter for private objects

[Mike Samuel]

2009/11/2 Brendan Eich <brendan at mozilla.com>:
> On Nov 2, 2009, at 10:47 AM, Mike Samuel wrote:
>
>> What do getters and setters do around with?
>>
>>  var flipFlop;
>>  with ({ get test: function () { return (flipFlop = !flipFlop) ?
>> 'flip' : 'flop'; } }) {
>>    for (var i = 0; i < 10; ++i) { alert(test); }
>>  }
>>
>> The with block invokes NewObjectEnvironment with O being the binding
>> object.
>> But I'm unclear on whether line 4 in GetBindingValue below would
>> invoke a getter.
>
> It ought to, because ES3 with anything like getters, including a native or
> host object cooperating [[HasProperty]] and [[Get]] implementations -- but
> also Mozilla's getter and setter implementation of ten years' standing,
> would invoke the getter.
>
> ES3 spec sections are 12.10, 10.1.4, and 11.1.2 which feeds into GetValue
> when the Identifier is used as a rvalue; GetValue does call [[Get]].
>
> Indeed ES5 is correctly compatible with ES3 + getters or internal
> [[HasProperty]]/[[Get]] implementations.
>
>> 10.2.1.1.4 GetBindingValue(N,S)
>> The concrete Environment Record method GetBindingValue for declarative
>> environment records simply
>> returns the value of its bound identifier whose name is the value of
>> the argument N. The binding must already
>> exist. If S is true and the binding is an uninitialized immutable
>> binding throw a ReferenceError exception.
>> 1. Let envRec be the declarative environment record for which the
>> method was invoked.
>> 2. Assert: envRec has a binding for N.
>> 3. If the binding for N in envRec is an uninitialized immutable binding,
>> then
>> a. If S is false, return the value undefined, otherwise throw a
>> ReferenceError exception.
>> 4. Else, return the value currently bound to N in envRec.
>> === end quote ===
>
> Wrong section. The  concrete type of envRec for a 'with' statement is an
> object environment record, so you want:
>
> 10.2.1.2.4 GetBindingValue(N,S)

Ah thanks.

> The concrete Environment Record method GetBindingValue for object
> environment records returns the value of its associated binding object‘s
> property whose name is the String value of the argument identifier N. The
> property should already exist but if it does not the result depends upon the
> value of the S argument:
>
> 1. Let envRec be the object environment record for which the method was
> invoked.
> 2. Let bindings be the binding object for envRec.
> 3. Let value be the result of calling the [[HasProperty]] internal method of
> bindings, passing N as the property name.
> 4. If value is false, then
> a. If S is false, return the value undefined, otherwise throw a
> ReferenceError exception.
> 5. Return the result of calling the [[Get]] internal method of bindings,
> passing N for the argument.
>
> /be