Thanks to both of you, I wasn't aware that this was already documented in an ES6 draft - my apologies.
Rick On Fri, Jan 27, 2012 at 1:23 PM, Allen Wirfs-Brock <al...@wirfs-brock.com>wrote: > > On Jan 27, 2012, at 9:59 AM, Rick Waldron wrote: > > > On Fri, Jan 27, 2012 at 12:08 PM, Brendan Eich <bren...@mozilla.org>wrote: > >> [[Class]] begone in ES6. >> > > Assuming the public facing API has no intention of breaking back > compatibility, what internal property will be read to determine the return > value of Object.prototype.toString (15.2.4.2)? > > > The current ES6 draft defines its like this: > > > *15.2.4.2 Object.prototype.toString ( )* > > When the *toString* method is called, the following steps are taken: > > 1. If the *this* value is *undefined*, return *"[object Undefined]"*. > 2. If the *this* value is *null*, return *"[object Null]"*. > 3. Let *O *be the result of calling ToObject passing the *this *value > as the argument. > 4. If *O* has a [[NativeBrand]] internal property, let *tag* be the > corresponding value from the Table 23. > 5. Else, let *tag* be the string value *"Object"*. > 6. Return the String value that is the result of concatenating the > three Strings *"[object "*, *tag*, and *"]"*. > > *Table 23 — Tags for Classified Native Objects* > *[[NativeBrand]] Value* > *tag Value* > NativeFunction > *"Function"* > NativeArray > *"Array"* > StringWrapper > *"String"* > BooleanWrapper > *"Boolean"* > NumberWrapper > *"Number"* > NativeMath > *"Math"* > NativeDate > *"Date"* > NativeRegExp > *"RegExp"* > NativeError > *"Error"* > NativeJSON > *"JSON"* > NativeArguments > *"Arguments"* > > > Basically, there is a fixed set of hardwired (to the spec.) object kinds > that toString knows about. There is no implication that all objects must > carry around an additional string value that is used to parameterize > toString. If you want to extend an implementation with new built-in object > kinds that toString recognizes then you must also extend your > implementation of toString. > > [[Class]] has been treated by some as a mandatory implementation level > extension point for adding new internal types, but it wasn't. > > Note that I have considered extending the above definition of toString > with an explicit ES level extension point based using private names: > > If *O* has as well known private name property (think of it as: > privateToStringTag) then *tag* is set to ToString of the value of that > property. This would happen between steps 4 and 5. > > Allen >
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