On Nov 15, 2011, at 7:17 AM, Andy Wingo wrote:
> Hello,
>
> Do I have it right that the intention with `let' and `const' is to allow
> them in strict mode?
>
> SpiderMonkey appears to allow them, as does V8 with --harmony, but V8
> without --harmony and JSC both abort on `const' in strict mode.
This is a gray area. To quote Waldemar from
https://mail.mozilla.org/pipermail/es-discuss/2010-October/011972.html
"It's a judgment call, and I'd take these on a case-by-case basis. For const
and let, I see little harm in browsers allowing them now in strict mode with
the purely non-hoisting (C++-like) scoping rules, particularly if they
complained when you accidentally declared a variable twice directly in the same
block."
In order to clear the decks, some engines ban let and const in strict mode.
Others (at least SpiderMonkey) allow them because (a) the engine in question
supported 'let' with nearly the Harmony semantics; (b) users generally use them
in a future-friendly way (no use before initialization, pretend-block-scope for
const).
This doesn't matter a whole lot yet on the web, since strict mode is not being
adopted aggressively, and where it has been adopted, interoperation constraints
mean JS hackers have not used let at all, and (AFAIK) they've not used const
either. No 'let' support apart from Firefox, and 'const' isn't in IE (and in
Opera it's a 'var' synonym).
For Firefox, or really SpiderMonkey, we have had a 'let' implementation dating
to the ES4 proposals era, and it's "close" but of course imperfect. We have
many XUL apps and add-ons using 'let' in JS. We would like them to "use
strict". So we made the call the other way from V8 and JSC.
> Likewise, none of the major engines abort on `const' in non-strict mode.
> In non-strict mode in browsers, which semantics should `const' have?
> The old one, with hoisting?
There is no well-specified "old one" -- no 'const' in ECMA-262 yet. The
SpiderMonkey implementation dates back to before my hair was gray. It's goofy
(have you played with it? var scope, use-before-init ok, you can re-init if in
a loop) but with some discipline, programmers can use it carefully, per the
Crockford rules for var (manually hoisted, no use before init).
SpiderMonkey maintainers will take the heat of changing 'let' semantics
shortly, and making 'const' truly block scoped. The temporal dead zone (no use
before initialization) should not break much code. We'll find out how bad a
mess we've made, but again it's confined to Mozilla-only apps and add-ons. I'll
report to the list on how it's going as this happens.
/be
_______________________________________________
es-discuss mailing list
[email protected]
https://mail.mozilla.org/listinfo/es-discuss
