Array.prototype[Symbol.unscopables] is not a function

[kazatsuyu]

TypeScript Version: 3.7.0-dev.20191018

Search Terms:

• Array
• @@unscopables

Code

const x = [][Symbol.unscopables];
console.log(x.copyWithin);

const y = [][Symbol.unscopables]();
console.log(y.copyWithin);

Expected behavior:
x.copyWithin occurs no error, and [][Symbol.unscopables]() occurs type error.

Actual behavior:
x.copyWithin occurs type error, and [][Symbol.unscopables]() occurs runtime exception.

Playground Link:
http://www.typescriptlang.org/play/?ts=Nightly&ssl=6&ssc=1&pln=1&pc=1#code/MYewdgzgLgBAHjAvDA2gXRQZQJ4FsBGIANgHQCukoADgIb5ECmEaA3ALABQokxDJRIAOYAKOCWrYA6gEsoAC2lgAlOw6du0GNiSoMOAsXKUQtekzTCV68BF78hw7OJNTZC5aqA

Related Issues:

[NicholasLYang]

Is this because [Symbol.unscopables] is not a function but marked as one in the lib declaration file? According to MDN, object[Symbol.unscopables] should be an object. I removed the parens on this line and it appeared to fix the problem.

Building works correctly with the change but my IDE complains: TS2717: Subsequent property declarations must have the same type. I'm not really sure where the other property declarations of Symbol.unscopables are. Could I get a point in the right direction? Also should I change all the tests that use [Symbol.unscopables]() to [Symbol.unscopables]?

[ExE-Boss]

@rbuckton This is still broken, so I’m fixing it in #42566.