typescript array of specific string values

Question

I have an array

const relations = ['profiles', 'locations', 'change_history']

If I want to create an interface like

interface IParams {
  id: number;
  relations: []string; // how to make this an array of those relations above?
}

How can I do that?

Answer 1

You basically have two options here:

const string enum

• https://www.typescriptlang.org/docs/handbook/enums.html#string-enums (for having the string values present for checks at runtime)
• https://www.typescriptlang.org/docs/handbook/enums.html#const-enums (to avoid some overhead concerning enums and replacing references to them in your code with the string literals)

You can define a const enum the following way:

const enum Relation {
  profiles = 'profiles', 
  locations = 'locations', 
  change_history = 'change_history'
}

string literal types

• https://www.typescriptlang.org/docs/handbook/advanced-types.html#string-literal-types

type Relation = 'profiles' | 'locations' | 'change_history';

and like @guijob already pointed out this would be your interface (in both cases):

interface IParams {
  id: number;
  relations: Relation[];
}

Of course you could also inline this string literal type definition

relations: ('profiles' | 'locations' | 'change_history')[];

But be aware that values are not checked at runtime!

So if you add data from a resource that is not checked at compile time (like an API or user input) there is no guarantee for only those values being present.

Answer 2

You can use as const assertions to effortless type "relations"

const relations = ['profiles', 'locations', 'change_history'] as const

interface IParams {
  id: number;
  relations: typeof relations;
}

As an alternative, you can use Array<T>

interface IParams {
  id: number;
  relations: Array<'profiles' | 'locations' | 'change_history'>
}

Or if you prefer the other syntax

interface IParams {
  id: number;
  relations: ('profiles' | 'locations' | 'change_history')[]
}