Using Omit on an Object Union applies on the merged type instead of the union members

[james-yeoman]

🔎 Search Terms

"omit", "omit union"

#54371 is related to this issue, but is specific to tuples. This issue applies to objects in general

🕗 Version & Regression Information

• This is the behavior in every version I tried, and there are no applicable FAQ pages

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAMhBmwoF4oG8BQUqkgLigCIAbBYQgbiygGNiB7AZwgIAoBKFAPigDd6AlgBMq2RgFcARgFsBwNmACGIBoqEFGwAE4CAdgHNOyHgAUt9WcwA8-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-uUWjuv4cAygqs5Csuymo7FyhA8kcfJQIkjwEEKby0qYorFD8fwAkCILsOCUKwvKQA

💻 Code

type Left = {
  type: "left";
  close: () => void;
  submit: (payload: string) => Promise<void>;
}

type Right = {
  type: "right";
  close: () => void;
  submit: (payload: number) => Promise<void>;
  original: string;
}

type Union = Left | Right;

const foo: Union = {
  type: "right",
  original: "foobar",
  close: () => {},
  submit: payload => Promise.resolve()
  //^? - payload is typed as `number`
};

type Omitted = Omit<Union, "close">;

const bar: Omitted = {
  type: "right",
  original: "foobar",
  submit: payload => Promise.resolve()
  //^? - a union of the two submit types
}

// https://github.com/microsoft/TypeScript/issues/54371#issuecomment-1568870836
type HomomorphicOmit<T, K> = { [P in keyof T as P extends K ? never : P]: T[P] };

type NewOmitted = HomomorphicOmit<Union, "close">;

const baz: NewOmitted = {
  type: "right",
  original: "foobar",
  submit: payload => Promise.resolve()
  //^? - payload is typed as `number`
}

🙁 Actual behavior

Omit consolidates the union into a single type

🙂 Expected behavior

I expected Omit to operate on all members of a union

Additional information about the issue

In #54371 (comment) Ryan mentions a modified version of Omit, that I've verified in my Playground link works for object unions.

type MappedOmit<T, K> = { [P in keyof T as P extends K ? never : P]: T[P] };

[MartinJohns]

This is working as intended. See also #54451, and many many many other issues (search for Omit in:title).

[james-yeoman]

Ah, my bad. Guess I should've been more diligent with my issue searching :/

Still not too keen on the push into userland, but #53169 was mentioned in #54451 and the points outlined make the MappedOmit userland definition an easier pill to swallow.

Would it be possible to either add to the JSDoc of Omit to mention the incompatibility with union types, add an entry to the FAQ about the incompatibility with union types, or both? At least until a solution can be found for the problems with Omit?

[fatcerberus]

Yeah, I don't think the TS team really wanted to push it into userland either, but it was kind of a catch-22: Omit already exists but with an inconvenient definition, we can't change it without breaking code in the wild, and since there are multiple competing userland definitions of Omit, adding a second variant to the compiler builtins is just opening the floodgates for people to ask for built-in support for other variants. Especially since (in this hypothetical situation) they've already tacitly admitted that the "canonical" Omit is inadequate by implementing MappedOmit.

[typescript-bot]

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

[james-yeoman]

we can't change it without breaking code in the wild

@fatcerberus Is this not what Semver Major versions are for? At the very least, it'd be great to get some sort of an official commitment to fix Omit in Typescript v6

[MartinJohns]

TypeScript does not follow SemVer (for good reasons). And Omit is such a widely used type, I think even announced ahead and with a new major version it would be a very very bad idea to break it.