Allow explicit declaration of an object property to be omitted when the property is typed as `undefined`

[C0RR1T]

Allow explicit declaration of an object property to be omitted when the property is typed as `undefined`

[whzx5byb]

What about

type TestType =
  | {
      as: "link";
      id: string;
    }
  | {
      as: "div";
      id?: undefined
    };

[C0RR1T]

Strictly speaking, this could be a breaking change when not implemented correctly. Based on this question on StackOverflow, JS discrimates between implicitly setting a property to undefined vs. setting it implicitly as undefined (by not defining it). So there is the possibility that this could be breaking, for example in this code:

type TestType = {
  as: 'div';
  id?: undefined;
}

const test = (props: TestType) => "id" in props;


// true
console.log(test({as: 'div', id: undefined}));

// false
console.log(test({as: 'div'}));

I don't have an idea to on how this could be implemented without breaking prior code. Im also not sure if the code above has actually been written before since I don't see how the code above could be useful.

[C0RR1T]

What about

type TestType =
  | {
      as: "link";
      id: string;
    }
  | {
      as: "div";
      id?: undefined
    };

Lol, haven't thought of that, thanks for your comment. Maybe id?: never would be even better.

Imo, this code doesn't express the intent as good as id: undefined would, but I mean, it works. What do you guys think?

[whzx5byb]

Maybe id?: never would be even better.

Yeah, if you want ts report error on test({as: 'div', id: undefined}) you can replace id?: undefined with id?: never and please make sure to turn on exactOptionalPropertyTypes: true

[C0RR1T]

Alright, thanks alot @whzx5byb! Im gonna go ahead and close this feature-request since this would be a breaking change for a small (and arguable) improvement. Consider the solutions provided by whzx or me :) .