On Nov 22, 2010, at 11:49 AM, Brendan Eich wrote:
> On Nov 22, 2010, at 11:14 AM, Oliver Hunt wrote:
>> And all existing standards compliant code can no longer rely on for in doing
>> what it has done for years. Suddently for in _behaviour_ may change based
>> on the prototype chain.
>
> That's true in Harmony,
I think I misread your "based on the prototype chain" words.
I was making the general case for runtime semantic fixes -- few and worth
making -- in Harmony as in ES5 strict.
But I think you were supposing a proxy on a non-proxy's prototype chain could
start returning non-string non-keys via its iterate trap. Not so. Here's a
session with a fresh js shell (Firefox 4 latest code):
js> var proxy = Proxy.create({
has: function () { return false; },
enumerate: function () { return ["a", "b", "c"]; },
iterate: function () { for (let i = 0; i < 10; i++) yield i; }
});
js>
js> var obj = Object.create(proxy);
js>
js> for (var i in proxy)
print(i);
0
1
2
3
4
5
6
7
8
9
js>
js> for (var i in obj)
print(i);
a
b
c
Once for-in starts up a non-proxy object's prototype chain, only enumerate
traps -- never iterate. And enumerate is fundamental, so if missing, the for in
fails:
js> var proxy = Proxy.create({
has: function () { return false; },
iterate: function () { for (let i = 0; i < 10; i++) yield i; }
});
js>
js> var obj = Object.create(proxy);
js>
js> for (var i in proxy)
print(i);
0
1
2
3
4
5
6
7
8
9
js>
js> for (var i in obj)
print(i);
typein:11: TypeError: enumerate is not a function
/be
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