Object returned from a function can have keys not present in its return type

[Dwlad90]

Bug Report

When a function is typed to return an object of a certain type, the properties of the object can be expanded, meaning extra properties can be added to the ones described in the type or interface.

🔎 Search Terms

function return object type

🕗 Version & Regression Information

This is observable on the playground in all available versions.

Couldn't find anything I understood to be relevant in https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs

• This is a crash: No
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about interfaces and types of function return values

⏯ Playground Link

Buggy example on @next

💻 Code

interface Result {
    foo: string;
}

type Func = () => Result

const noError:Func = () => ({
  foo: 'string1',
  // @ts-expect-error
  bar: 2,
});

const error:Func = (): Result => ({
  foo: 'string1',
  // @ts-expect-error
  bar: 2,
});

const error2:Func = (): ReturnType<Func> => ({
  foo: 'string1',
  // @ts-expect-error
  bar: 2,
});

🙁 Actual behavior

Extra properties in the returned object, which aren't defined in the interface or type, do not raise an error.

🙂 Expected behavior

Extra properties in the returned object, which aren't defined in the interface or type, should raise an error.

[MartinJohns]

This is not a bug. The return type is not specified, so it's inferred. The inferred return type is then checked for compatibility, and it's compatible. This comes up pretty often, e.g. #41850.

[RyanCavanaugh]

See also #241

[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.