This is breaking jQuery, which contains isFunction: function (obj) { return toString.call(obj) === "[object Function]"; }, Talking about what to do on es5-discuss@mozilla.org led to the proposal from Mark Miller that ES5 be changed by erratum (in time for ISO, barely) to specify "[object undefined]" for |this| bound to undefined, and "[object null]" for |this| bound to null. This smells better than retreating to "[object Window]" or whatever the global class calls itself. I propose trying this out in Firefox 4 beta 2 in order to see if we can get away with it. I hope Microsoft can try Mark's proposal out in an IE9 Developer Preview, as well. More test coverage, more eyeballs. /be

First comment on <http://blogs.msdn.com/b/ie/archive/2010/07/16/how-ie9-platform-preview-feedback-changed-the-javascript-standard.aspx>: I'm unsure why you are going to return [Object undefined] while the type of the 'undefined' value is 'Undefined', as stated in the ES5 spec. Same goes for 'null' whose type is 'Null'. To be consistant, you should return "[Object Undefined]" and "[Object Null]", or is there any reason why you should not ?

Question in comment 1 above raised on es5-discuss list at https://mail.mozilla.org/pipermail/es5-discuss/2010-July/003647.html

Dão, probably a nit, just for the record: in no case would "object" in "[object ...]" be capitalized. The other class names either correspond to constructors, whose names are consistently capitalized, or to internal [[Class]] values such as "Arguments". In the latter case you can't extract the name from Object.prototype.toString.call and construct a program that new's it, so there's no consistent requirement that the name spell any existing property name. But "null" and "undefined" do name reserved identifiers or (now in ES5) constant bindings. So "Null" and "Undefined" are a bit cleaner and more consistent. Thanks for bringing this up. /be

Need to get this fixed before release. /be

Patch 4 in bug 514570 fixes this, right? /be

Seems so: js> ({}).toString.call(undefined) "[object Undefined]" js>

Nice. With test coverage in that bug, this one could even be dup'ed IMHO. /be