On 2/20/17 10:00 AM, Michał Wadas wrote:
http://stackoverflow.com/questions/40595389/why-does-browser-get-symbol-unscopables-twice
Which implementation is bugged? Can someone familiar specification
details fill bug in proper engine?
var a, b, flag = true
with (a = { x: 7 })
with (b = { x: 4, get [Symbol.unscopables]() { return { x: flag=!flag } } })
x++
// Chrome FF
console.log(a) // {x:5} {x:7}
console.log(b) // {x:4} {x:8}
TL;DR: both are buggy. The correct output is {x:7}, {x:5}. I have
filed https://bugzilla.mozilla.org/show_bug.cgi?id=1341061 and
https://bugs.chromium.org/p/v8/issues/detail?id=5992
I think we come into
https://tc39.github.io/ecma262/#sec-postfix-increment-operator-runtime-semantics-evaluation
here. The first question is what "lhs" ends up being.
I'm pretty sure that evaluating "x" will call
https://tc39.github.io/ecma262/#sec-getidentifierreference which will
start with envRc being the object Environment Record created by the
"with (b)" and start looking for the binding for "x". Specifically, it
will call HasBinding() on envRec, which will call
https://tc39.github.io/ecma262/#sec-object-environment-records-hasbinding-n
which will check @@unscopables. It will get x: false, I think (assuming
that none of the bareword lookups used in defining "b" end up looking up
things in the "with (b)" environment. So we will get back a Reference
whose base is the object Environment Record for "with (b)". This claim
could use double-checking.
Anyway, if that's correct, then
https://tc39.github.io/ecma262/#sec-postfix-increment-operator-runtime-semantics-evaluation
step 2 does https://tc39.github.io/ecma262/#sec-getvalue which in step
6a goes to
https://tc39.github.io/ecma262/#sec-object-environment-records-getbindingvalue-n-s
which will return the value "4" afaict, without ever checking @@unscopables.
And then
https://tc39.github.io/ecma262/#sec-postfix-increment-operator-runtime-semantics-evaluation
step 4 will call https://tc39.github.io/ecma262/#sec-putvalue which in
step 7a will call
https://tc39.github.io/ecma262/#sec-object-environment-records-setmutablebinding-n-v-s
which will set b.x to 5.
So as far as I can tell from the spec, the output should be: {x:7},
{x:5}, and there should be one single call to the getter. And both of
the browsers tested are buggy.
In Safari, for what it's worth, I get the output I expect: {x:7}, {x:5}.
-Boris
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