On 4/13/13 12:12 AM, Brandon Benvie wrote:
Based on recent discussions, it seems that there's some confusion
about what exactly Symbols are. So far I've seen three different
alternatives:
1.) Exotic objects that conform to the most recent ES6 spec, but with
a special typeof .That is, they have special operations for all the
internal methods. These internal methods make them act as stateless,
immutable, prototypeless objects such that they can be treated like
primitives. The problem here is introducing new typeof semantics,
where something that otherwise acts like an object is not typeof ===
"object".
2.) A new type of primitive that have no object wrapper version, thus
`ToObject(symbol)` throws. This means either they have to be falsey or
a new situation arises where `if (val != null) val.prop` throws
(ignoring accessors and proxies).
3.) A new type of primitive along with a new type of wrapper. In this
case we use the String/Number/Boolean precedent where `Symbol()` and
`new Symbol()` produce different kinds of results. The problem here is
the confusion that comes with ToString/ToPropertyKey when called on a
Symbol wrapper. What does `obj[new Symbol] = 5` do, for example? It
allows footguns like `obj[key + '_ext']` to silently do the wrong thing.
A relevant gist here demonstrates test cases for option #1. I'll see
if I can add comparable sets of tests for the other two options. [1]
https://gist.github.com/Benvie/5375078
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I don't have much to add to this discussion, but I would like to point
out that option 1 would make it much easier to implement symbols in
SpiderMonkey, so as an implementer I have a vested interested in option
1 winning out ;-)
In case anybody is interested, I've summarized why implementing symbols
as primitive types in SpiderMonkey is hard here:
https://bugzilla.mozilla.org/show_bug.cgi?id=645416#c14
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