> Le 4 mars 2016 à 00:57, Allen Wirfs-Brock <al...@wirfs-brock.com> a écrit :
>
>>
>> On Mar 3, 2016, at 10:57 AM, Claude Pache <claude.pa...@gmail.com
>> <mailto:claude.pa...@gmail.com>> wrote:
>>
>> Hi,
>>
>> Consider the following test case:
>>
>> ```html
>> <script>
>>
>> function returnThisValue() { "use strict"; return this }
>>
>> Object.defineProperties(Object.prototype, {
>> getThisValue: { value: returnThisValue, configurable: true }
>> , thisValue: { get: returnThisValue, configurable: true }
>> })
>>
>> window.alert(getThisValue() + "\n" + thisValue)
>>
>> </script>
>> ```
>>
>> What should be displayed? According to my tests:
>>
>> * Firefox [1], Chrome and Edge:
>>
>> undefined
>> [object Window]
>>
>> [1]: (For Firefox, use v.46+ in order to avoid interference with this
>> recently-solved bug: https://bugzilla.mozilla.org/show_bug.cgi?id=603201
>> <https://bugzilla.mozilla.org/show_bug.cgi?id=603201>)
>>
>> * Safari (although I get contradictory results with further tests involving
>> `__proto__`):
>>
>> undefined
>> undefined
>>
>> * ECMA-262, if I read correctly:
>>
>> undefined
>> [object Window]
>>
>> The relevant steps in the spec are:
>>
>> * for `getThisArg()` : [12.3.1.4.1] step 4.b.ii, where `refEnv` is an Object
>> Environment Record associated to the global object;
>> `refEnv.WithBaseObject()` will be `undefined`.
>> * for `thisArg`: [8.1.1.2.6] step 5, where `bindings` is the global object
>> and `N` is `"thisArg"`.
>>
>> [12.3.1.4.1]:
>> https://tc39.github.io/ecma262/#sec-function-calls-runtime-semantics-evaluation
>>
>> <https://tc39.github.io/ecma262/#sec-function-calls-runtime-semantics-evaluation>
>> [8.1.1.2.6]:
>> https://tc39.github.io/ecma262/#sec-object-environment-records-getbindingvalue-n-s
>>
>> <https://tc39.github.io/ecma262/#sec-object-environment-records-getbindingvalue-n-s>
>>
>> Is it an intentional and/or desired behavior?
>
> Get accessors on the global object have been defined with this behavior since
> ES5, when accessor properties were introduced. It was intentional that
> global variable access has the semantics of [[Get]]/[[Set]] accesses to the
> corresponding property of the global object.
>
>>
>> In practice it has an influence on what a bare `__proto__` should evaluate
>> to.
>
>
> How so? As currently specified bare `__proto__` does a get/setPrototypeOf
> the global object that is in scope.
>
One could want (or, at least, find logical) that the corresponding
getter/setter receives `undefined` as its this-value, so that evaluating
`__proto__` would throw a TypeError ("cannot convert undefined to object").
Just like calling `valueOf()` does.
—Claude
>
> Allen
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