Excess properties allowed in function weak return types

[michaldudak]

Bug Report

🔎 Search Terms

excess, extra

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

interface EmptyInterface {}

interface NonemptyInterface { _?: unknown }

interface SomeOtherInterface {
    x?: string;
}

type ObjectOrCallback<T> =
  | (SomeOtherInterface & T)
  | (() => (SomeOtherInterface & T));

// @ts-expect-error
const a: ObjectOrCallback<EmptyInterface> = { excessProp: 1 };

const fa: ObjectOrCallback<EmptyInterface> = () => ({ excessProp: 1 }); // why doesn't this cause an error too?

// @ts-expect-error
const b: ObjectOrCallback<NonemptyInterface> = { excessProp: 1 };

// @ts-expect-error
const fb: ObjectOrCallback<NonemptyInterface> = () => ({ excessProp: 1 });

🙁 Actual behavior

The function fa, which is defined as having the return type { x?: string } & {} can return an object with extra properties, whereas function fb, having the return type { x?: string } & { _?: unknown} cannot. This seems to be inconsistent.

🙂 Expected behavior

Since both EmptyInterface & SomeOtherInterface and NonemptyInterface & SomeOtherInterface are types with optional properties only, both fa and fb should not allow excess properties on their return type.

[MartinJohns]

// why doesn't this cause an error too?

Because your function has no return type annotation, so the return type is inferred. Then the type of your function is checked for assignability, in which case excess property checks are not occurring anymore. This is working as intended, and there are numerous issues about this.

[michaldudak]

Why the behavior of the fb function is different, though?

[RyanCavanaugh]

Duplicate #241

[typescript-bot]

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