Andreas Rossberg <mailto:[email protected]>
January 24, 2012 2:37 AM
On 23 January 2012 19:25, Brendan Eich<[email protected]> wrote:
Andreas Rossberg<mailto:[email protected]>:
V8 currently allows
var w = 1; w = 2; print(w); const w = 3
which will output 2. The idea most likely was that const should behave
like var. This, and other, similar examples clearly have to break if
should const become official in classic mode, so the compatibility
argument may not carry far.
SpiderMonkey:
js> var w = 1; w = 2; print(w); const w = 3
typein:19: TypeError: redeclaration of var w:
typein:19: var w = 1; w = 2; print(w); const w = 3
typein:19: ....................................^
js> var w = 1
js> const w = 3
typein:22: TypeError: redeclaration of var w
However:
js> const w = 3; var w = 1; print(w);
3
This follows from var not disturbing a pre-existing binding. Or so we
thought long ago when adding const, but that was years before const as
initialize-only-and-at-most-once let.
Why does V8 do something else, do you know the impetus for diverging?
No, unfortunately I don't know the full history there. I can try to find it out.
But just to be obnoxious, with Firefox:
print(c);
Object.defineProperty(this, 'c', {value: 1});
print(c);
const c = 2;
print(c);
=> undefined 1 2
You bet -- const does redefine the global property, unlike var. This is
not something we propose for standardization, of course!
Or even:
print(c);
Object.defineProperty(this, 'c', {value: 1, writable: true});
print(c);
c = 2;
print(c);
const c = 3;
print(c);
=> undefined 1 2 3
Apparently, FF avoids breaking the object model only for the price of
keeping the non-writable const property configurable until the actual
initialization -- effectively breaking const completely.
More fun:
js> function f(k) {
for (var i = 0; i < k; i++) {
const j = i*i;
a.push(j);
}
}
js> a = []
[]
js> f(10)
js> a
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
SpiderMonkey const has never been initialize-at-most-once. Some people
claim to like this, but we are going to try breaking it in favor of ES6.
Consequently, I maintain my claim that it is impossible to reconcile
sane let/const semantics with the idea of having toplevel bindings
represented as data properties on the global object.
I agree. For ES4 we equated let to var at top-level, a mistake. For ES6
IIRC you have proposed an implicit block scope at top level (of
functions at least, I don't see why not programs as well), whereby let
and const bind lexically. Only var and function make global object
properties.
If there is some
trick for "hiding" accessor semantics that doesn't break the object
model then I'd be really interested in seeing it. :)
Me too.
/be
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