isNaN() should accept any value (in type definition)

[KostyaTretyak]

TypeScript Version: 2.1.4

Code

isNaN('222');

Expected behavior:
Works without error

Actual behavior:
Throw error:

[ts] Argument of type '"222"' is not assignable to parameter of type 'number'.

[KostyaTretyak]

This also applies to Number.isInteger(), Number.isNaN() methods.

For example, I have the following code:

function some(one: string | number)
{
  if( Number.isInteger(one) )
  {
    // Code...
  }
}

Why TypeScript throwing error?

Argument of type 'string | number' is not assignable to parameter of type 'number'.
Type 'string' is not assignable to type 'number'.

[DickvdBrink]

@RyanCavanaugh explained it pretty clear in this issue why it is the way it is
#4002 (comment)

So it is by design