I was alluding to this behavior for interface objects (e.g., "URL"):
4.4.1.1. Interface object [[Call]] method ยง<http://dev.w3.org/2006/webapi/WebIDL/#es-interface-call> If the interface<http://dev.w3.org/2006/webapi/WebIDL/#dfn-interface> is declared with a [Constructor]<http://dev.w3.org/2006/webapi/WebIDL/#Constructor> extended attribute<http://dev.w3.org/2006/webapi/WebIDL/#dfn-extended-attribute>, then the interface object<http://dev.w3.org/2006/webapi/WebIDL/#dfn-interface-object> can be called as a function to create an object that implements that interface. Interfaces that do not have a constructor will throw an exception when called as a function. ..."not supported" just means "URL is not a constructor/won't parse a URL string parameter". As Cameron noted, it's not an idea test-in the instance you note in Chome, that's due to the subtle difference in implementation of WebIDL per the above requirement. From: Rick Waldron [mailto:waldron.r...@gmail.com] Sent: Friday, September 28, 2012 4:24 PM To: Cameron McCormack Cc: Travis Leithead; Boris Zbarsky; public-script-co...@w3.org; public-webapps Subject: Re: In WebIDL, should having a .prototype on interface objects be optional? On Fri, Sep 28, 2012 at 4:14 PM, Cameron McCormack <c...@mcc.id.au<mailto:c...@mcc.id.au>> wrote: Travis Leithead: I guess you'd check for URL.href then? Or try { new URL("/test"); } catch (ex) { console.log("not supported"); } I agree with Travis, you should be checking the particular features you want to use, rather than checking the existence of the prototype as a proxy for that test. If URL.prototype was required to exist, you could just do ("href" in URL.prototype). Since it currently doesn't, you could do ("href" in (URL.prototype || {})). I'm interested in knowing more about what would throw. If URL is not a constructor function, it will throw, but if it is what aspect of the above would be "not supported". I ask because currently Chrome's URL (webkitURL) can construct, but it constructs a fairly useless instance and throws nothing (new webkitURL("/test");) Thanks in advance Rick
