Return NaN after iteration

[parzhitsky]

In definition of XRange type, change void to undefined, since void doesn't work for some reason.

- type XRange = Generator<number, void>;
+ type XRange = Generator<number, undefined>;

declare const range: XRange;

const nextValue: number | void = range.next().value;

const value: number = nextValue!;
- //          ^ Error!
- // Type 'number | void' is not assignable to type 'number'.
- //  Type 'void' is not assignable to type 'number'
+ // no errors

[parzhitsky]

It turns out that undefined also brings some problems:

- type XRange = Generator<number, void>;
+ type XRange = Generator<number, undefined>;

declare const range: XRange;

if (range.next().value < SOME_VALUE) act();
- // no errors
+ //            ^ Error!
+ // Object is possibly 'undefined'.

One way to solve it (although I don't really like it) is to return NaN after iteration, instead of undefined (both implicit and explicit), as in:

for (const number of numbers)
  yield number;

+ return NaN;

This would allow defining XRange as Generator<number, number>, and have no silly errors.

The reason I don't like this approach is because conceptually outside of any numeric range there are not only numbers, but no values at all, of any type. This idea would be properly conveyed by producing a void (undefined or null), while NaN means that there is something outside of a numeric range, it is just not a number, — which is silly, but here we are.