Following non-null assertion of optional field, it is still considered nullable

[orgads]

🔎 Search Terms

"undefined is not assignable" assertion

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about common bugs

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.1.6#code/JYOwLgpgTgZghgYwgAgGIHt3IN4FgBQAkALYCeAbnFAFzIDOYUoA5gNwEC+BoksiKAISo4CJClQD8tBkxBtOBAjACuIBGGDoQyABZwQAEwA2EDOgAUAfRiZaZgJQ4u+JavWbtewyaFRzAIypaX0c8ImAYZHMAQkCoADoySih7UUIoCDBlKBB2Ii9jU0wAqns85yA

💻 Code

interface Foo {
	myvar: string;
}
interface Bar {
	myvar?: string;
}

function handleFoo(_foo: Foo) {}

function handleBar(bar: Bar) {
	if (!bar.myvar)
		return;
	handleFoo(bar);
}

🙁 Actual behavior

Argument of type 'Bar' is not assignable to parameter of type 'Foo'.
  Types of property 'myvar' are incompatible.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.

🙂 Expected behavior

It should work, as I verified myvar is defined.

Additional information about the issue

No response

[MartinJohns]

The field is not considered nullable, but the check does not change the parent type.

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

Duplicate of #42384.

[orgads]

Thank you.