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>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/ 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> 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 >> es-discuss@mozilla.org >> https://mail.mozilla.org/listinfo/es-discuss >> >> >
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