On May 13, 2013, at 2:16 PM, Erik Arvidsson wrote: > On Mon, May 13, 2013 at 5:01 PM, Allen Wirfs-Brock <[email protected]> > wrote: > > On May 13, 2013, at 1:50 PM, Erik Arvidsson wrote: > >> The way that WebIDL require Object.prototype.toString to return "[object >> TypePrototype]" for the interface prototype object and "[object Type]" for >> the instances seems to imply that every instance needs to have an own >> @@toStringTag. >> >> http://people.mozilla.org/~jorendorff/es6-draft.html#sec-15.2.4.2 >> http://dev.w3.org/2006/webapi/WebIDL/#es-environment >> >> If an instance does not have its own @@toStringTag, >> Object.prototype.toString will read through to the [[Prototype]] which would >> return the wrong string. > > Well, toString just does a [[Get]] for @@toStringTag. You are perfectly free > to implement it as a get accessor that takes into account whether the this > value is an instance or a prototype object. Not sure whether the complexity > is really worth it in most cases. I considered building something like that > into Object.prototype.toString but it seemed hard to justify and there was no > (ES) legacy reason for doing so. > > OK. Something like this might work: > > Object.defineProperty(Type.prototype, @@toStringTag, { > get: function() { > return 'Type' + (this === Type.prototype ? 'Prototype' : ''); > } > }); > > This is of course observable if we expose @@toStringTag.
@@toStringTag is just a ES code accessible extension mechanism for Object.prototype.toString. Nobody ever claimed it wasn't observable. > > The preferred way to over-ride toString should be via a toString method, not > via @@toStringTag. > > Agreed, but WebIDL currently mandates the result of Object.prototype.toString. Maybe it shouldn't. Part of the motivation here is to start migrating people way from thinking that O.p.toString is a reliable brand test, particularly WRT to anything new that is created. Allen
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