Number check functions such as isNaN shouldn't constrain their parameter type

[falsandtru]

TypeScript Version: master

Code

isNaN(<any>null); // should be able to pass any value without type casting

Expected behavior:

declare function isNaN(target: any): boolean;

Actual behavior:

declare function isNaN(number: number): boolean;

[ghost]

While code like Number.isNaN("foo") technically won't crash, it won't accomplish anything either.
We also have errors for new Set<string>().has(0); and /rgx/.test(0);.

[falsandtru]

For example, when we write compare function also working with NaN, we need the exact function detecting NaN.

var a: number | string = '';
[''].findIndex(b => a === b || Number.isNaN(a) && Number.isNaN(b));

I believe this code should be accepted.

[falsandtru]

Sorry, I found global isNaN also constrains the parameter type. So I fix the code to:

var a: number | string = '';
[''].findIndex(b => a === b || isNaN(a) && isNaN(b));

[DanielRosenwasser]

Duplicate of #4002

Also see #15173 (comment)

[falsandtru]

I searched it but probably I had some typo. Thanks.

[DanielRosenwasser]

No worries! Sorry if that came off the wrong way, didn't mean to be abrasive.