>-----Original Message----- >From: [email protected] [mailto:es5-discuss- >[email protected]] On Behalf Of Maciej Stachowiak >Sent: Tuesday, June 23, 2009 11:19 AM >To: Mark S. Miller ... >> The documentation of .call(), .apply(), and .bind() are in terms of >> invoking an internal [[Call]] (or for .bind(), also a [[Construct]]) >> property). However, the Ch15 constructor documentation doesn't >> actually mention these internal property names. It explains instead >> what happens when a constructor is called as a constructor (with >> "new"), which is probably adequate to infer the behavior of the >> [[Construct]] property. And it explains what happens when a >> constructor is called as a function. But it is not clear from the >> latter what, if anything, one should infer about the constructor's >> [[Call]]. In particular, what should happen when .call(), .apply(), >> or .bind() causes a Ch15 constructor's [[Call]] to be invoked with a >> thisArg that either 1) does not inherit from that >> constructor's .prototype, or 2) does not have the [[Class]] that the >> constructor would normally place on the newly constructed object? > >It could be made more clear that the "when called as a function" and >"when called as a constructor" descriptions correspond to the [[Call]] >and [[Construct]] internal properties respectively. Perhaps a single >sentence at the start of the built-in object section could take care >of this.
Because built-in constructors are not (necessarily) implemented in ECMAScript code they are not required to use the definitions of [[Call]] and [[Construct]] given in 13.2.2 and 13.2.3. Arguably the "Constructor Called as a Function" and "called as part of a new expression" sections of chapter 15 are specifying the behavior of custom [[Call]] and [[Construct]] internal methods for each built-in constructor. I'll see if can find a way to clarify this in the introduction to chapter 15. ... >> >> One interpretation of "called as a function" in the Ch15 constructor >> documentation is that the thisArg (since it is never mentioned) is >> always ignored. This seems consistent with the behavior I just >> observed on FF3.0.10, Safari 3.2.3, and Chrome 3.0.189 on the Mac. >> This seems fine, except that it means one cannot use the normal >> "subclassing"ish pattern for defining new error constructors that >> extend Error, since Error will ignore the normal constructor >> chaining. Is this intended? > >I think this should be the behavior. Built-in or host constructors >always make a brand-new object of a specific [[Class]], they ignore >the object that would be passed in by "new" for a JS-implemented >constructor function. The spec for Error seems unclear on this since >it refers to "the newly constructed object" without clearly explaining >how it is created. > Actually, I think the spec. language is pretty clear on this for Error in both ES5 and ES5 (same basic language). 15.11.1 says "When Error is called as a function rather than as a constructor, it creates and initialises a new Error object. " The phrase "the newly constructed object" in 15.11.1.1 seems to clearly be a reference to that new Error object. Allen _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
