Typescript: How to use just one of interfaces with different properties

[sorryformystupidquestions]

TypeScript Version: 3.3.0-dev.20181206

Search Terms: interface, union

Code

type WithId = {
  id: number
}

type WithNumber = {
  number: number
}

type User = WithId | WithNumber

const user: User = {
    id: 1,
    number: 1
};

Expected behavior: user cannot have both id and number properties

Actual behavior: user can have both id and number properties. How can I achieve this?

Playground Link: http://www.typescriptlang.org/play/#src=type%20WithId%20%3D%20%7B%0D%0A%20%20%20%20id%3A%20number%0D%0A%7D%0D%0A%0D%0Atype%20WithNumber%20%3D%20%7B%0D%0A%20%20%20%20number%3A%20number%0D%0A%7D%0D%0A%0D%0Atype%20User%20%3D%20WithId%20%7C%20WithNumber%0D%0A%20%20%0D%0Aconst%20user%3A%20User%20%3D%20%7B%0D%0A%20%20%20%20id%3A%201%2C%0D%0A%20%20%20%20number%3A%201%0D%0A%7D%3B

Related Issues: https://github.com/Microsoft/TypeScript/issues/12745, https://github.com/Microsoft/TypeScript/issues/20060, https://github.com/Microsoft/TypeScript/issues/23535

[jack-williams]

I think this is a duplicate of #20863.

Currently a property is excess in a union type if the property is excess in all branches. In this instance no one property is excess in both branch.

I would be interested to get the opinion from the team as to whether it would be reasonable to have a partial solution that works for 1 or 2 levels deep (but correctly for intersection and unions). I think it would handle most cases people have, but would be tractable to implement and would hopefully retain the performance benefits EPC gives currently.

I will also take this shameless opportunity to plug my experimental work on exact types, which would solve this issue (#28749).

type Exact<T> = {| [K in keyof T]: T[K]; |}

type WithId = {
  id: number
}

type WithNumber = {
  number: number
}

type User = Exact<WithId> | Exact<WithNumber>;

const user: User = {
    id: 1,
    number: 1
};
/*
Type '{ id: number; number: number; }' is not assignable to type 'User'.
  Type '{ id: number; number: number; }' is not assignable to type 'Exact<WithNumber>'.
    Object literal may only specify known properties, and 'id' does not exist in type 'Exact<WithNumber>'. [2322]
*/

[weswigham]

Probably worth noting that explicitly listing out all the properties you care about not being in excess with an optional undefined type will make the types work as you'd expect:

type WithId = {
    id: number
    number?: undefined,
}

type WithNumber = {
  id?: undefined,
  number: number
}

type User = WithId | WithNumber

const user: User = {
    id: 1,
    number: 1
};

these are the kinds of types we infer when you're working with fresh literals to accomplish the same thing.

But yes, mostly a duplicate of #20863

[jack-williams]

Here is a convoluted way to implement @weswigham's suggestion without changing the constituent types. (Disclaimer: I haven't really debugged these).

type WithId = {
    id: number;
}

type WithNumber = {
    number: number;
}

const user: UserExact = { // error
    id: 1,
    number: 1
};

type User = WithId | WithNumber;
type UserExact = Exactify<User>;

type Exactify<T> = Exactify2<T, T>;
type Exactify2<T, U> = T extends unknown ? ExactifyWorker<T, U> : never;
type ExactifyWorker<T, U> = OptionalK<T, MinusKeys<T, U> & string>;
type KeyMap<U> = U extends unknown ? keyof U : never;
type MinusKeys<T, U> = Exclude<KeyMap<U>, keyof T>;
type OptionalK<T, K extends string> = T & { [P in K]?: undefined };

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.