Default values of object destructuring of Generic type

[Pomar81]

Hi, TypeScript Team

Version: 3.3.4

I have a problem with default values of destructed variables inside a generic function. If I don't specify type for destructuring construction I could set anything as default values.

type HeaderTypes = 'h1' | 'h2' | 'h3'

type HeaderProps<H extends HeaderTypes> = {
    abc?: H,
    className?: string,
}

const Header = <H extends HeaderTypes>(props: HeaderProps<H>) => {
    const { abc = 'h1' }: HeaderProps<H> = props;
   // ......
}  

Expected behavior:
abc can be assigned to 'h1'
Actual behavior:
TS2322:Type 'h1' is not assignable to type 'H'

Playground Link: example

[jack-williams]

Essentially the same problem as: #29528 and #30157

[Pomar81]

Hi @jack-williams
Yes. #29528 is very close. But is there any solution on how to fix my problem?

[jack-williams]

I don't understand your use-case fully but I would write something like:

const Header = <H extends HeaderTypes>(props: HeaderProps<H>) => {
    const { abc = 'h1' }: HeaderProps<H | "h1"> = props;
   // ......
}  

[Pomar81]

In my current context, it is ok. Thank you.
But in general, it will be great to somehow eliminate this problem with Generic

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.