> On May 31, 2016, at 3:25 AM, Logan Smyth <loganfsm...@gmail.com> wrote: > > The correct spec behavior is to return an instance of `MyError`. Step 7 in > your example is not equivalent to calling `Error` without `new`, which seems > to be your assumption. The `newTarget` parameter passed to `Construct` is > used to determine the prototype of the final object, and in this context > `newTarget` would be `MyError`. > > Babel does not support extending builtins by default because it is difficult > to transpile properly and would add more boilerplate in all class cases, even > those which do not require logic to handle native extension. If you need > builtin extension support, the only way at the moment is to explicitly enable > it via a plugin like > https://www.npmjs.com/package/babel-plugin-transform-builtin-extend > <https://www.npmjs.com/package/babel-plugin-transform-builtin-extend> > > On Mon, May 30, 2016 at 11:56 PM, Gray Zhang <otakus...@gmail.com > <mailto:otakus...@gmail.com>> wrote: > I know NodeJS outputs `true` in this case, but my problem is from the spec it > seems `false` is the correct result, or am I missing some chapters in spec? > > Jordan Harband <ljh...@gmail.com <mailto:ljh...@gmail.com>>于2016年5月31日周二 > 下午2:47写道： > In which engine did you try this? Please refer to > http://kangax.github.io/compat-table/es6/ > <http://kangax.github.io/compat-table/es6/> under "Subclassing" to see if > your browser supports subclassing builtins yet. > > On Mon, May 30, 2016 at 11:32 PM, Gray Zhang <otakus...@gmail.com > <mailto:otakus...@gmail.com>> wrote: > Recently I encountered an issue about subclassing Error and instance > operator, my simplified code is: > > ``` > class MyError extends Error { > constructor() { > super('my error’); > } > } > > let error = new MyError(); > console.log(error instanceof MyError); > console.log(error.constructor); > ``` > > Surprisingly the output is `false` and `function Error() { [native code] }` > > I dived into the ECMAScript 2015 spec and find the behavior is correct, > chapter 12.3.5.1 says: > SuperCall : super Arguments > Let newTarget be GetNewTarget > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-getnewtarget>(). > If newTarget is undefined, throw a ReferenceError exception. > Let func be GetSuperConstructor > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-getsuperconstructor>(). > ReturnIfAbrupt > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-returnifabrupt>(func). > Let argList be ArgumentListEvaluation of Arguments. > ReturnIfAbrupt > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-returnifabrupt>(argList). > Let result be Construct > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-construct>(func, > argList, newTarget). > ReturnIfAbrupt > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-returnifabrupt>(result). > Let thisER be GetThisEnvironment > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-getthisenvironment>( > ). > Return thisER.BindThisValue > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-bindthisvalue>(result). > Since Error can be called as a function without new operator, the result of > step 7 is an error instance rather than undefined, so this error instance > becomes `this` value of `thisER` and then returned as the result of new > MyError constructor.
Huh?? Step 7 isn’t a [[Call]] it is a [[Construct]]. Syntactic rules such as “must use new to invoke a constructor” can’t apply at the level of pseudo-code. > > The problem is, the spec also said "The Error constructor is designed to be > subclassable." so I think instanceof should work correctly on subclasses, > however it fails now. It is specified to work. Your implementation must be wrong or imp complete. > > This also comes to Map, Set, Array and Object constructors. Also should work Allen
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