Crashes with some polyfills: @@toStringTag is not writable.

[amangeot]

Hello,
I receive this error in console on Android Kitkat webviews:

Uncaught TypeError: Cannot assign to read only property '@@toStringTag' of [object Object]", source: http://192.168.1.23:5050/vendor.19.js

Troubles arise from this line: Gp[toStringTagSymbol] = "Generator";
because toStringTagSymbol is not writable.

Some polyfills made it writable polyfillpolyfill/polyfill-service#1295 to make sure it works with regenerator.

So I wrote about the issue here: medikoo/es6-symbol#24, but apparently the spec wants @@toStringTag to not be writable.
EDIT: I thought it was related to this polyfill I use

Is it possible to not overwrite @@toStringTag ?

[zloirock]

You refer to Symbol.prototype[@@toStringTag], but it's absolutely unrelated this problem and Generator.prototype[@@toStringTag].

[amangeot]

Oh sorry you're right! I updated the issue, I'm quite confused with these tags.

Do you have any suggestion on how to tackle this issue? Is there something to polyfill, or a polyfill to fix?

[zloirock]

Without digging the problem, I can say only that Symbol polyfill from core-js should work as expected -)

[amangeot]

Yes that should be safe!

Considering that @@toStringTag is set to writable: false for GeneratorFunction, according to this spec,

is this line ok: Gp[toStringTagSymbol] = "Generator"?

---

Gp is defined as is:

  var Gp = GeneratorFunctionPrototype.prototype =
    Generator.prototype = Object.create(IteratorPrototype);
  GeneratorFunction.prototype = Gp.constructor = GeneratorFunctionPrototype;
  GeneratorFunctionPrototype.constructor = GeneratorFunction;
  GeneratorFunctionPrototype[toStringTagSymbol] =
    GeneratorFunction.displayName = "GeneratorFunction";

[zloirock]

It should be non-writable, but regenerator not so strict. var Gp = Object.create(IteratorPrototype), so the problem in %IteratorPrototype%. regenerator uses %IteratorPrototype% from your environment where it's available, so that means that you have %IteratorPrototype% with non-writable @@toStringTag. It was fixed as writable in polyfill-service, but by the spec %IteratorPrototype% should not have @@toStringTag property.

[medikoo]

Fixed in es6-iterator, new version published at v2.0.3

[ExE-Boss]

This has been fixed by #402.

See also:

• Web compat: The %Symbol.toStringTag% property on %Iterator.prototype% needs to be writable tc39/proposal-iterator-helpers#115