Return types of arrow functions allow additional properties.

[ChristophGr]

Returned objects of arrow functions can have more properties than the type specified in the variable declaration .

TypeScript Version: 4.1.2 / 4.2.0-dev.20201207

Search Terms:
arrow function return type covariant additional properties

Code

type MyResult = { foo: string };

const arrowProvider: () => MyResult = () => ({ foo: 'bar', bar: 'foo' });
// no error


function funcProvider(): MyResult {
  return { foo: 'bar', bar: 'foo' }; // error
}

Expected behavior:
Both funcProvider and arrowProvider should not compile because of the superfluous property bar in there.

Actual behavior:
Only funcProvider triggers a compile-error

Playground Link:
https://www.typescriptlang.org/play?#code/C4TwDgpgBAsiBKEDOBXANsKBeKBvKAZgPZEBcUSwATgJYB2A5lAL4DcAUOwMZF2VQBDKlSIB3AAoiAbjQAmEKuQAUASmwA+WAmTpMOVRqhL8xMlADkAIyHmANFGuKLp8yxUd2BFHS7AavQm8uSSIZeSpVcjhEVAw8digoKghgFCo6PEIScisbe0cclxYOZiA

Related Issues:

[MartinJohns]

Duplicate of #41834 and many others I can't find right now.

You don't specify a return-type for your function, so the return type is inferred. Then, when assigning your function to the variable, the compiler checks if the return-types are compatible, which they are.

[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.