TypeScript: Why can’t I assign a valid field of an object with type { a: “a”, b: “b” }

by

I believe this is because objects are contravariant in their key types.

For more information see this answer.

Likewise, multiple candidates for the same type variable in contra-variant positions causes an intersection type to be inferred.

const paths = ['a', 'b'] as const

type Path = typeof paths[number]

type PathMap = {
    [path in Path]: path
}

type a="a"
type b = 'b'

type c = a & b // never

{
    const BASE_PATHS = paths.reduce((map: PathMap, p: Path) => {
        let x = map[p]
        map[p] = p // same here
        return map
    }, {} as PathMap)

Intersection of a and b produces never.

If you remove as const from paths it will compile, because string & string = string

Btw, since you are using functional approach try to avoid object mutations.

Here, in my blog, you can find more information about mutations in TS

Credits to @aleksxor

Here you can find official explanation

More Related Contents:

  1. How to fix TS2322: “could be instantiated with a different subtype of constraint ‘object'”?
  2. In TypeScript, how to get the keys of an object type whose values are of a given type?
  3. Types for function that applys name of function and arguments
  4. Typescript: How can I make entries in an ES6 Map based on an object key/value type
  5. Why this CustomExtract returns an different result from default Extract?
  6. Typescript Function with Generic Return Type
  7. Typescript Key-Value relation preserving Object.entries type
  8. Why can’t the generic interface in TS infer the type correctly?
  9. Mapping tuple-typed value to different tuple-typed value without casts
  10. Eliminate nevers to make union possible
  11. extend and only specify known properties?
  12. Generic type extending union is not narrowed by type guard
  13. Getting the return type of a function which uses generics
  14. Expression ___ has changed after it was checked
  15. Could not find a declaration file for module ‘module-name’. ‘/path/to/module-name.js’ implicitly has an ‘any’ type
  16. How to suppress “error TS2533: Object is possibly ‘null’ or ‘undefined'”?
  17. TypeScript conditional types – filter out readonly properties / pick only required properties
  18. Define global constants
  19. What does ‘declare’ do in ‘export declare class Actions’?
  20. “parserOptions.project” has been set for @typescript-eslint/parser
  21. angular2 bootstrap with data from ajax call(s)
  22. Interface is allowing extra property when it is used as return type of a function
  23. Typescript: What’s the difference between an optional field and a union with undefined?
  24. Typescript: what is a “naked type parameter”
  25. Why do subtypes need to be assigned to a variable before being used as a function argument?
  26. Typescript narrowing of keys in objects when passed to function
  27. Flatten object with custom keys in Typescript
  28. Typescript and filter Boolean
  29. Angular2 – root relative imports
  30. A way to mark arbitrary strings in Typescript Template Literals

Leave a Comment