Unscopeable

Peter van der Zee ecma at qfox.nl
Wed Aug 21 06:18:34 PDT 2013


Can somebody (private mail is okay) please clarify this for me? It looks me
like there's a proposal to introduce `@@` syntax to support something
that's forbidden in most es6 code anyways (`with`)? Or does it have other
use cases? Where can I read more about this?

Cheers

- peter


On Wed, Aug 21, 2013 at 1:58 PM, Andreas Rossberg <rossberg at google.com>wrote:

> I'm still trying to wrap my head around the idea of introducing an
> @@unscopeable property, as discussed at the last meeting. There seem
> to be plenty of edge cases:
>
> * How would it interact with inheritance?
>
>   let p = {a: 1}
>   let o = {__proto__: p, [@@unscopeable]: ['a']}
>   let a = 2
>   with (o) {
>     a  // 1 or 2?
>   }
>
>   let p = {[@@unscopeable]: ['a']}
>   let o = {__proto__: p, a: 1}
>   let a = 2
>   with (o) {
>     a  // 1 or 2?
>   }
>
> * How would it affect assignment?
>
>   let o = {[@@unscopeable]: ['a']}
>   let a = 2
>   with (o) {
>     a = 1
>     a  // 1 or 2?
>     o.a  // 1 or undefined?
>   }
>   a  // 1 or 2?
>
>   let p = {a: 1, [@@unscopeable]: ['a']}
>   let o = {__proto__: p}
>   let a = 2
>   with (o) {
>     a = 3
>     a  // 1 or 2 or 3?
>     o.a  // 1 or 3?
>   }
>   a  // 2 or 3?
>
> * How would it interact with mutation?
>
>   let o = {a: 1}
>   let a = 2
>   with (o) {
>     o[@@unscopeable] = ['a']
>     a  // 1 or 2?
>   }
>
> * How would it interact with compound assignment?
>
>   let o = {a: 1}
>   let a = 2
>   with (o) {
>     a += (o[@@unscopeable] = ['a'], 2)
>     a  // 2 or 3 or 4?
>     o.a  // 1 or 3?
>   }
>   a  // 2 or 3 or 4?
>
> * How would it affect the global object? (At least currently, with
> scopes and the global scope share the same object environment
> mechanism.)
>
>   var a = 1
>   this[@@unscopeable] = ['a']
>   a  // 1 or undefined?
>
>   var a
>   this[@@unscopeable] = ['a']
>   a = 1
>   a  // 1 or undefined?
>
>   this[@@unscopeable] = ['a']
>   </script><script>
>   var a = 1
>   a  // 1 or undefined?
>
> * How would it expose side effects?
>
>   let s = ""
>   let o = {a: 1}
>   let u = {get '0'() { s+="0"; return 'a' }, get '1'() { s+="1"; return
> 'b' }}
>   Object.defineProperty(o, @@unscopeable, {get: function() { s += "u";
> return u }})
>
>   with (o) {}
>   s  // "" or "u" or "u01" or ...?
>
>   with (o) { c; c }
>   s  // "u01" or "u01u01" or ...?
>
>   with (o) { a }
>   s  // "u01" or "u0" or ...?
>
>   with (o) { a+=1 }
>   s  // "u0" or "u00" or "u01" or "u0101" or ...?
>
> Some of this looks rather whacky, if not unpleasant.
>
> /Andreas
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