Exact iterator

[balmy lion]

Is there any way can type iterator:

const iterator = ([1, 2, 3] as const)[Symbol.iterator]()

const first = iterator.next()
const second = iterator.next() 
const third = iterator.next() 

So that first is 1 instead of 1 | 2 | 3?

[balmy lion]

Basically functions or methods that “increment” the return type on each call.

[zenith lagoon]

I really doubt it.

Having a value change state as methods are called on is a very difficult pattern in TS in the best of cases.

I think there's some tricks you can do with method(): asserts this is SomeType but these tend to be limited and fragile

[balmy lion]

Do you have any examples?

[gritty garden]

declare class Foo {
  val: string | number;
  doThing(): asserts this is { val: string }
}

const x: Foo = new Foo()
x.doThing();
x.val
// ^? - (property) val: string

[zenith lagoon]

But again, this isn't great - this is pretty fragile (e.g. the type annotation on const x: Foo is not optional)

And the whole idea really falls apart when you have to deal with stuff like passing the iterator/class to a function where you don't know what will happpen to it.

[balmy lion]

Thanks, why is the annotation needed?

[zenith lagoon]

Not sure exactly; I just know that it's a limitation on assertions that the things they operate on need to have known types

[balmy lion]

Is there any way to make the assertion function also have a return value?