Proposal: strict built-in functions

Claude Pache claude.pache at gmail.com
Mon Dec 9 08:37:42 UTC 2019



> Le 8 déc. 2019 à 20:49, Jack Works <zjwpeter at gmail.com> a écrit :
> 
> I thought the "caller" has been removed from the spec, so there isn't much to do with the "caller" since it is not standard. It's implementation's own extension.
> But maybe we can also extend The forbidden extensions section.

The `caller` property of functions (not to be confused with the `caller` property of the `arguments` object, which is indeed dead) has never been in the spec, except some restrictions in the [Forbidden Extensions] section introduced at the same time as strict-mode functions. And the problem is, precisely, that it is underspecified.

Now, it is indeed almost trivial to resolve spec-wise *the particular* issue you noted, by amending the [Forbidden Extensions] section: Just find the bullet that begins with “If an implementation extends any function object with an own property named "caller"...”, and replace therein all occurrences of “strict function” with “strict function or built-in function” (or maybe with “anything but a non-strict function”). That, however, is not the most implementation-friendly way, because that leaves up to them to guess what to do instead (return null, throw an error, etc.), that will not break the web for their users. 

Moreover, the `caller` property has most probably some other gotchas; one of them I recall to have noted some time ago, is that, in at least one implementation, it is publicised as a non-writable, non-configurable own data property, but it may change its value.

I think that it is better, at this point, to specify Function#caller and Function#arguments, as proposed in [gh-issue 562].

[Forbidden Extensions]: https://tc39.es/ecma262/#sec-forbidden-extensions <https://tc39.es/ecma262/#sec-forbidden-extensions>
[gh-issue 562]: https://github.com/tc39/ecma262/issues/562 <https://github.com/tc39/ecma262/issues/562>

—Claude


> 
> On Mon, Dec 9, 2019, 1:12 AM Claude Pache <claude.pache at gmail.com <mailto:claude.pache at gmail.com>> wrote:
> 
> 
>> Le 8 déc. 2019 à 14:43, Jack Works <zjwpeter at gmail.com <mailto:zjwpeter at gmail.com>> a écrit :
>> 
>> 
>> In the current spec, strictness of the built-in functions are implementation-dependent behaviors. This proposal is going to fix this problem.
>> https://github.com/Jack-Works/proposal-strict-built-in-functions <https://github.com/Jack-Works/proposal-strict-built-in-functions>
>> 
> 
> Hi,
> 
> See https://github.com/tc39/ecma262/issues/562#issuecomment-218531285 <https://github.com/tc39/ecma262/issues/562#issuecomment-218531285> for another  testcase. 
> 
> The real problem is that the semantics of `f.caller` is left to implementations. There are in fact some restrictions in the spec, see https://tc39.es/ecma262/#sec-forbidden-extensions <https://tc39.es/ecma262/#sec-forbidden-extensions>, but they are insufficient.
> 
> Note that it doesn’t really make sense to mandate that builtin functions be “strict”: The notion of strictness is defined only for so-called ECMAScript functions, which are functions whose implementation is written in ECMAScript code. That excludes builtin functions (unless the implementation choose to  implement them in ECMAScript), but also bound functions, proxies, and probably some other cases. 
> 
> —Claude

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