A few arrow function specification issues

Domenic Denicola domenic at domenicdenicola.com
Mon Apr 23 16:59:48 PDT 2012

As a day-to-day user who was not using strict until recently (stubborn other team member for the loss), I can say that moving to strict was much more a "cleanup" experience than a "mode" experience, with only a few small exceptions:

1) Poison-pilling arguments.callee: some code on the internet (notably [1], but elsewhere too) uses this property.

2) Throw on `delete`ing non-configurables: unlike the other throw-on-doing-something-bad, `delete`ing non-configurable properties was something we actually did on purpose. E.g. using it to clear an "entry" in a "map", whether or not that entry was ever filled. (Cf. [2].) We never tried writing to a non-writable property, because that would cause bugs, but `delete`ing a property that doesn't exist was a common pattern.

[1]: http://www.devthought.com/2011/12/22/a-string-is-not-an-error/
[2]: https://github.com/cjohansen/Sinon.JS/commit/80c38bd3e4b8813ab74ef27a4db3646e4778e31c


Otherwise, it was just cleanup: we got to get rid of our global detector (`setInterval(compareWindowPropertiesToLastTimeYouChecked(), 1000)`), never missed `with` or evil-`eval`, and due to a V8 bug never had boxed `this` in the first place. We're using it transparently throughout the codebase, in fact, due to using an XHR+`eval` module loader that can insert the `"use strict"` pragma before `eval`ing.

> -----Original Message-----
> From: es-discuss-bounces at mozilla.org [mailto:es-discuss-
> bounces at mozilla.org] On Behalf Of David Herman
> Sent: Monday, April 23, 2012 19:11
> To: Brendan Eich
> Cc: es-discuss
> Subject: Re: A few arrow function specification issues
> On Apr 23, 2012, at 2:44 PM, Brendan Eich wrote:
> > Brendan Eich wrote:
> >> Without arguments runtime semantic shifts, and with |this| lexical,
> there aren't many strict mode changes left IIRC, and they are pretty edgy
> edge cases.
> >
> > True enough, but I hang tough on wanting arrows to imply strictness. I
> may be wrong but the edge cases cited so far (global variable creation by
> assignment, 'with', direct eval injecting 'var' into its dynamic scope)
> along with this |this|-boxing one are underwhelming.
> IMO, this decision hangs on whether we think strict mode can be thought
> of as a "cleaning up" that will mostly just catch bugs, resulting in
> failing faster but otherwise not changing program behavior significantly.
> POSITION "CLEANUP": If it's just a cleanup, then it makes sense to
> introduce strict mode in a bunch of places in the language. It'll catch
> bugs, and for the most part people won't have to think to themselves "is
> this strict code?"
> POSITION "MODE": If it's something that is likely to change behavior in
> meaningful ways other than failing faster, then it's something
> programmers will have to be mindful of. In that case, having multiple
> constructs that implicitly introduce strict mode imposes a burden on
> programmers: they have to know which features are strict and which ones
> aren't.
> Every fiber of my being screams MODE. Brendan argues CLEANUP based on the
> rationale that the non-error semantic changes of strict mode are unlikely
> edge cases. This is the strongest argument against MODE. I can't prove it
> wrong, but it's a risky bet. If it turns out to be the case that in
> practice you need to keep track of whether you're in strict mode, then
> weak programmers who don't know better will end up with a confusing mess,
> and strong programmers will prophylactically make sure to put their
> entire programs in strict mode rather than memorize which subset of
> syntaxes opt in. Since strong programmers have the option to do that
> anyway even without multiple implicit opt-ins, why take the risk?
> Pascal^H^H^H^H^H^H Dave
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