At the TC39 meeting on Thursday, I joked to Mark (deadpan) that the
typeof registry is "as good as the global object". That got big laughs,
but seriously, it's better. Apart from the null special-case, it's like
a global object where only const is allowed. You can't redefine typeof
-- first to define it for a user-defined value object wins.
I forgot to say that the maps are pre-seeded with the built-in strings
and exemplars, of course, to prevent user-defined value objects from
usurping "function", "string", etc.
/be
Brendan Eich wrote:
Two Map instances, internal to the specification:
ExemplarToTypeofMap: object -> string
TypeofToExemplarMap: string -> object
New Function.setTypeOf static, a peer of Function.defineOperator:
Function.setTypeOf(V, U)
1. Let S = ToString(U)
2. Let T = V
3. If T is null then
3a. If S is not "null" and S is not "object", throw
4. Else
4a. If T is not a value object constructor, throw
4b. T = T.prototype
4c. If TypeofToExemplarMap.has(S), throw
5. Call TypeofToExemplarMap.set(S, T)
6. Call ExemplarToTypeofMap.set(T, S)
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