Typescript: Method shorthand syntax in interfaces is looser

[nstepien]

TypeScript Version: 3.7.5

Search Terms: function syntax shorthand

Code

interface MyInterface {
  fn1(num: number | undefined): void;
  fn2: (num: number | undefined) => void;
}

const badObject: MyInterface = {
  fn1(num: number) {}, // no error??
  fn2(num: number) {}
}

const shouldBeBadObject: MyInterface = {
  fn1(num: number) {}, // no error??
  fn2(num: number| undefined) {}
}

const goodObject: MyInterface = {
  fn1(num: number | undefined) {},
  fn2(num: number | undefined) {}
}

Expected behavior: num: number parameter type should not be allowed for fn1, just like fn2. I don't expect the shorthand syntax to make the type looser.

Actual behavior: num: number parameter passes as a valid type for fn1. Shorthand syntax is looser.

Playground Link: https://www.typescriptlang.org/play/?target=7&ssl=1&ssc=1&pln=19&pc=2#code/JYOwLgpgTgZghgYwgAgLIE8CS5rycgbwChlkYQBGAChAFcBbALmTvoCNpkAfZWkAEwgxQEfgEpmANwD2wfgG4SZEACZmNBs1Yco3XgKEjxyALwA+ZDLmKAvkSIJpIAM5hkbOPwDybAFYQEMGYMbEhYRBQTQiVyalYtBh0xQhsAGmQAegyWaWRoKGkoAH4imNUNJhZE6GSCOzsHJ1dkZwALaVoAG34AIQgezx9-QOCsHHD8KOJSWIqE9hqU9KycvKgC4tKZ8viqhagePkFhEFFa+vtHFzcAc2lpbz8AoLQxsLxI6O24zT2dPSOhlOxjqqTKKjmf04hwMJzOKSINiAA

[dragomirtitian]

This is a side effect of the way strictFunctionTypes work differently for signatures originating in methods vs other signatures. Methods behave bi-variantly in spite of strictFunctionTypes. From PR #18654 :

Methods are excluded specifically to ensure generic classes and interfaces (such as Array) continue to mostly relate covariantly. The impact of strictly checking methods would be a much bigger breaking change as a large number of generic types would become invariant (even so, we may continue to explore this stricter mode).

[nstepien]

Thank you for clarifying this.
That's certainly an unexpected behavior. Are there plans to address this, perhaps with a more explicit syntax?

[RyanCavanaugh]

We're generally OK with this behavior. Different syntax can do different things because that's how you're able to write different programs.

[nstepien]

I agree with the need for a syntax, I disagree with the choice of syntax though.
Maybe it's similar with other typed languages, I don't know, but coming from JS, I did not expect the shorthand to be anything more than a syntactic sugar.

[typescript-bot]

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