CODEWITHSUNDEEP
typescript
WHITECOLOR
WHITECOLOR

Reputation: 26160

Typescript: why not strict return type?

There is a snipet with no error in return of funciton with decared return type.

type Human = {
  head: number
  body: number
}

const human: Human = {
  head: 1,
  body: 1,
}

const humanFn: (human: Human) => Human = (human) => {
  return {
    // ...human, 
    head: human.head,
    body: human.body,
    tail: 1, // allows illegal props
  }
}

humanFn({
  ...human,
  tail: 1, // here is and error
})

So the question is why TS allows adding additinal props to return object tail: 1, // allows illegal props? Is there a way to avoid it?

UPD:

so actually I found the exiting TS issue https://github.com/microsoft/TypeScript/issues/241

And "workaround" to wrap return object in something like R.identity<Human>(...) (from ramda)

Upvotes: 1

Views: 1117

Answers (1)

Naetmul
Naetmul

Reputation: 15562

Your code does not throw a compile error,
but the following code throws a compile error.

function humanFn(human: Human): Human {
  return {
    // ...human, 
    head: human.head,
    body: human.body,
    tail: 1, // Error
  }
}

and the following also does throw a compile error

const humanFn: (human: Human) => Human = (human): Human => {
  return {
    // ...human, 
    head: human.head,
    body: human.body,
    tail: 1, // Error
  }
}

Your question seems to be another case of this question.

Finally I found the issue in GitHub, exactly about the problem. In short:

Ideally this would be an error. Unfortunately it turns out to be very difficult to fix this without possibly having consequences in terms of runaway recursion and/or performance

Upvotes: 3

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