bug in type narrowing

[ARZarkesh]

Bug Report

I wrote a helper type to making some fields of object optional. But I got the following error in type narrowing.

🔎 Search Terms

type narrowing, optionalize

🕗 Version & Regression Information

• 4.4.4

⏯ Playground Link

Playground link with relevant code

💻 Code

type Optionalize<T, K extends keyof T> = Omit<T, K> & { [Key in K]: T[K] | undefined }

type API = { x: number, y: string }

function send(input: Optionalize<API, 'y'>): API {
  if (input.y === undefined) throw new Error('invalid input')

  return input
}

🙁 Actual behavior

input.y is string | undefined

🙂 Expected behavior

I expected input.y to be string

[MartinJohns]

Your described actual behaviour is wrong. The actual behaviour is what you describe in expected behaviour. The type of input.y is narrowed to string, it is not string | undefined anymore.

---

You expect the type of input to be narrowed to API, but that's not supported.

Quoting @jack-williams:

Type guards do not propagate type narrowings to parent objects. The narrowing is only applied upon access of the narrowed property which is why the destructing function works, but the reference function does not. Narrowing the parent would involve synthesizing new types which would be expensive. More detail in this comment.

Used search terms: type guard property parent

[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.