Interface prototype objects and ES6 @@toStringTag

Erik Arvidsson erik.arvidsson at
Mon May 13 14:16:17 PDT 2013

On Mon, May 13, 2013 at 5:01 PM, Allen Wirfs-Brock <allen at>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.
> 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.

The preferred way to over-ride toString should be via a toString method,
> not via @@toStringTag.

Agreed, but WebIDL currently mandates the result of

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