What is the best way to restrict the key of an object type to only be within a specific union in TypeScript?

Question

I need to create a set of server types in a union like this:

type Union = 'A' | 'B' | 'C';

After that, I want to define an object type where the keys are limited to only certain options from this Union:

// Use only 'A' and 'B' from the union
const myObject: { [K in Union]: number } = {
  A: 1,
  B: 2,
} as const;

I also want to ensure that myObject remains constant so that I can expect the following behavior:

console.log(myObject.A); // Should work fine
console.log(myObject.C); // Should result in an error

However, when I use [K in Union], the compiler throws an error stating:

Property 'C' is missing in type '{ readonly A: 1; readonly B: 2; }' but required in type '{ A: number; B: number; C: number; }'.(2741)

Is it possible to select only specific types from the union for the const object?

The goal here is to declare an object that will exclusively have keys corresponding to a subset of specified union types.

Answer 1

Answer №1

type League = 'X' | 'Y' | 'Z';

const yourItem = {
  X: 10,
  Y: 20,
} as const fulfills { [L in League]?: number | unknown };

This solution seems promising.

Answer 2

type League = 'X' | 'Y' | 'Z';

const yourItem = {
  X: 10,
  Y: 20,
} as const fulfills { [L in League]?: number | unknown };

This solution seems promising.