Object with optional properties is not type narrowed

[bartenra]

Bug Report

🔎 Search Terms

Type narrowing optional property

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type narrowing.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Slot = { id?: string }

function needsId(slot: { id: string }) {
}

function test(slot: Slot) {
  if (slot.id) {
    needsId(slot)
  }
}

🙁 Actual behavior

The compiler complains that on the line where needsId is called, string | undefined cannot be assigned to string.

🙂 Expected behavior

Inside the if-statement, the Typescript compiler knows that slot.id is a string. I expect slot to be narrowed to Slot & { id: string } inside the if-statement, so that needsId(slot) is a valid call.

[MartinJohns]

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.

Duplicate of #42384.