Skip to content

Cannot use matching indexes across objects with similar types #31445

Closed
@ftrimble

Description

@ftrimble

TypeScript Version: Reproduced in the playground - i assume this is running latest. before running in the playground was running on 3.4.5

Search Terms:
keys across objects cannot be compared
type values at same indexes do not honor that keys can only assume one value at a time

(I'm having a hard time describing this issue, so I may have failed to search for it effectively; forgive me if that's the case.)

Code

type A = { a: 'a' | 'b'; b: 'c' | 'd'; };
type B = { a: ('a' | 'b')[]; b: ('c' | 'd')[]; };
type C = { a: string; b: number };
type D = { a: string[]; b: number[]; }


const a: A = { a: 'a', b: 'c' };
const b: B = { a: [], b: [] };
const c: C = { a: 'asdf', b: 1234 };
const d: D = { a: [], b: [] };

(['a', 'b'] as const).forEach(key => {
    b[key].push(a[key]);
    b[key] = [...b[key], a[key]];
    d[key].push(c[key]);
    d[key] = [...d[key], c[key]];
});

Expected behavior:

All of the options for adding the keys to the array in the second half of the example above should compile and work correctly.

Actual behavior:

The LHS of the equation is considered an & type while the RHS is an | type, which makes them incompatible. These types are clearly correct and should work, but e.g. b[key] is consider a never for the purposes of push ('a' | 'b' & 'c' |'d'), while d[key] is considered a string & number for those purposes (not sure why this isn't also a never)

The important part of this issue is that none of the possible ways of using these types together compile. All possible variants create too strict of a typing for what needs to go into the array variant of the original type, even though the types of the objects at identical keys are compatible

Playground Link:

playground link

Related Issues:

Metadata

Metadata

Assignees

No one assigned

    Labels

    Design LimitationConstraints of the existing architecture prevent this from being fixed

    Type

    No type

    Projects

    No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions