In which engine did you try this? Please refer to 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 <[email protected]> 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 > > 1. Let *newTarget* be GetNewTarget > > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-getnewtarget> > (). > 2. If *newTarget* is *undefined*, throw a *ReferenceError* exception. > 3. Let *func* be GetSuperConstructor > > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-getsuperconstructor> > (). > 4. ReturnIfAbrupt > > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-returnifabrupt> > (*func*). > 5. Let *argList* be ArgumentListEvaluation of *Arguments*. > 6. ReturnIfAbrupt > > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-returnifabrupt> > (*argList*). > 7. Let *result* be Construct > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-construct> > (*func*, *argList*, *newTarget*). > 8. ReturnIfAbrupt > > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-returnifabrupt> > (*result*). > 9. Let *thisER* be GetThisEnvironment > > <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-getthisenvironment>( > ). > 10. 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. > > 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. > > This also comes to Map, Set, Array and Object constructors. > > Babel 6.7.7 has the correct behavior which fails on instanceof operator > but it really introduces some troubles, is this by design and how could I > avoid such issues? > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > >
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