How to declare an array of exactly one alias declared in a type alias

Question

If I have a TypeScript type alias defined like this:

export type PlatformType = 'em' | 'ea' | 'hi';

how can I declare a property in an interface so that that property is an array of exactly one type of each PlatformType alias? For example, I want this:

export interface Constants {
  platformTypes: PlatformType[]; // how to declare a type for platformTypes?
}

so that when I have an object of Constants type, I need to have this:

export const constants: Constants = {
  platformTypes: ['em', 'ea', 'hi']
}

Basically, I would like to get an error if I have this:

export const constants: Constants = {
  platformTypes: ['em', 'ea'] // should error, missing 'hi'
}

or

export const constants: Constants = {
  // this will error anyway because 'extra' doesn't belong to PlatformType alias
  platformTypes: ['em', 'ea', 'hi', 'extra']
}

or

export const constants: Constants = {
  platformTypes: ['em', 'em', 'ea', 'hi'] // should error, duplicate 'em'
}

Does this answer your question? [How to transform union type to tuple type](https://stackoverflow.com/questions/55127004/how-to-transform-union-type-to-tuple-type) — lawrence-witt, Sep 08 '21 at 10:11

captain-yossarian from Ukraine · Accepted Answer · 2021-09-08T12:53:59.577

I think you should define a permutation of all allowed tuples.

For instance, if you have a union of three elements you need to create at least 6 tuples:

["em", "ea", "hi"] | ["em", "hi", "ea"] | ["ea", "em", "hi"] | ["ea", "hi", "em"] | ["hi", "em", "ea"] | ["hi", "ea", "em"]

This is because union is unordered data structure.

export type PlatformType = 'em' | 'ea' | 'hi';


// credits goes to https://twitter.com/WrocTypeScript/status/1306296710407352321
type TupleUnion<U extends string, R extends any[] = []> = {
    [S in U]: Exclude<U, S> extends never ? [...R, S] : TupleUnion<Exclude<U, S>, [...R, S]>;
}[U];

export interface Constants {
  platformTypes: TupleUnion<PlatformType>; // how to declare a type for platformTypes?
}

export const constants: Constants = {
  platformTypes: ['em', 'ea', 'hi'] // ok
}

export const constants2: Constants = {
  platformTypes: ['em', 'ea'] // error
}

export const constants3: Constants = {
  platformTypes: ['em', 'ea', 'hi', 'extra'] // error
}

export const constants4: Constants = {
  platformTypes: ['em', 'em', 'ea', 'hi'] // error
}

Playground

Please keep in mind, that order of unions in typescript is tricky. Please see this question and this issue

You can find more examples in my blog

How to declare an array of exactly one alias declared in a type alias

1 Answers1