Enforce that an array is exhaustive over a union type

by

TypeScript doesn’t really have direct support for an “exhaustive array”. You can guide the compiler into checking this, but it might be a bit messy for you. A stumbling block is the absence of partial type parameter inference (as requested in microsoft/TypeScript#26242). Here is my solution:

type Furniture="chair" | 'table' | 'lamp' | 'ottoman';

type AtLeastOne<T> = [T, ...T[]];

const exhaustiveStringTuple = <T extends string>() =>
  <L extends AtLeastOne<T>>(
    ...x: L extends any ? (
      Exclude<T, L[number]> extends never ? 
      L : 
      Exclude<T, L[number]>[]
    ) : never
  ) => x;


const missingFurniture = exhaustiveStringTuple<Furniture>()('chair', 'table', 'lamp');
// error, Argument of type '"chair"' is not assignable to parameter of type '"ottoman"'

const extraFurniture = exhaustiveStringTuple<Furniture>()(
  'chair', 'table', 'lamp', 'ottoman', 'bidet');
// error, "bidet" is not assignable to a parameter of type 'Furniture'

const furniture = exhaustiveStringTuple<Furniture>()('chair', 'table', 'lamp', 'ottoman');
// okay

As you can see, exhaustiveStringTuple is a curried function, whose sole purpose is to take a manually specified type parameter T and then return a new function which takes arguments whose types are constrained by T but inferred by the call. (The currying could be eliminated if we had proper partial type parameter inference.) In your case, T will be specified as Furniture. If all you care about is exhaustiveStringTuple<Furniture>(), then you can use that instead:

const furnitureTuple =
  <L extends AtLeastOne<Furniture>>(
    ...x: L extends any ? (
      Exclude<Furniture, L[number]> extends never ? L : Exclude<Furniture, L[number]>[]
    ) : never
  ) => x;

Playground link to code

More Related Contents:

  1. How to define Typescript type as a dictionary of strings but with one numeric “id” property
  2. Expression ___ has changed after it was checked
  3. Could not find a declaration file for module ‘module-name’. ‘/path/to/module-name.js’ implicitly has an ‘any’ type
  4. How to suppress “error TS2533: Object is possibly ‘null’ or ‘undefined'”?
  5. TypeScript conditional types – filter out readonly properties / pick only required properties
  6. Typescript: infer type of generic after optional first generic
  7. Typescript Discriminated Union allows invalid state
  8. Define global constants
  9. Create an enum with string values
  10. Typescript: TS7006: Parameter ‘xxx’ implicitly has an ‘any’ type
  11. What does ‘declare’ do in ‘export declare class Actions’?
  12. TypeScript 2: custom typings for untyped npm module
  13. “parserOptions.project” has been set for @typescript-eslint/parser
  14. What is target in tsconfig.json for?
  15. Extend (Update) third-party old type declaration interface with new one
  16. Derive union type from tuple/array values
  17. Is it possible to generate string literal combinations with template literal in TypeScript?
  18. Can’t narrow the type of an object property even after checking it with a type guard
  19. Why do subtypes need to be assigned to a variable before being used as a function argument?
  20. Typescript narrowing of keys in objects when passed to function
  21. What do square brackets mean where a field should be in typescript?
  22. Typescript type T or function () => T usage
  23. Flatten object with custom keys in Typescript
  24. In Typescript, How to check if a string is Numeric
  25. Get argument types for function / class constructor
  26. How to compile a specific file with tsc using the paths compiler option
  27. How to build a TypeScript class constructor with object defining class fields?
  28. Is it possible to define a non empty array type in Typescript?
  29. TypeScript: type for an object with only one key (no union type allowed as a key) [duplicate]
  30. Is this behavior of “any” a TypeScript Type System Bug?

Leave a Comment