Bug 144283 – ES6 classes: super() should throw when extending null

• Status: RESOLVED
• Resolution: INVALID
• Component: JavaScriptCore

[Erik Arvidsson]

http://trac.webkit.org/browser/trunk/LayoutTests/js/script-tests/class-syntax-super.js#L59

shouldThrow('new (class extends null { constructor() { super(); return undefined } })', '"TypeError: undefined is not an object (evaluating \'super()\')"');


and

http://trac.webkit.org/browser/trunk/LayoutTests/js/script-tests/class-syntax-super.js#L64

shouldThrow('new (class extends null { constructor() { super() } })', '"TypeError: undefined is not an object (evaluating \'super()\')"');


The reason super() throws is because Function.prototype is not constructable. Your test seems to be testing the return value. It should never get there.

[Ryosuke Niwa]

Oh, this is because of the way I'm retriecving Function.prototype. It is in fact throwing up before calling super() so our behavior is corect.

Thanks for filing all these bugs :)

[Erik Arvidsson]

It is not clear to me that you are throwing at the right spot:

http://people.mozilla.org/~jorendorff/es6-draft.html#sec-super-keyword-runtime-semantics-evaluation

You should throw in step 7/8 in SuperCall : super Arguments

In this case there is no undefined value.

[Ryosuke Niwa]

Yes, that's what's happening (throw in step 7 of 8) You're right that the error message isn't exactly descriptive/informative but I don't feel like I need to improve it since it's such an edge case.

Or do you feel that web devs will end up hitting this error often?

[Erik Arvidsson]

> Or do you feel that web devs will end up hitting this error often?

I don't think this is a common case but the current message is pretty confusing. I assume it is hard coded so you can easily change it?

[Ryosuke Niwa]

No, the problem is that I'm relying on accessing undefined to throw an exception in this case right now, and I'd need to add a runtime branch if I wanted to throw a nicer error message here.