Derive union of string literal types with lookup types in Typescript

Recently I started playing with unions of string literals as an alternative to enums. I did not experience any issues while using enums. I just prefer the string literals as these, in contrast to enums, do not lead additional code generated during compilation.

One thing that has bothered me so far is that I could not find an easy way to store all strings of a string literal union in an array. This can be useful if you want to e.g. randomly select one of the string literals in the union as part of creating mock/fixture data.

Of course you can define both the unions of string literals and an array with the same strings:

type PostType = 'article' | 'podcast';
const postTypes = ['article', 'podcast'];

This duplication is error prone in case a new option needs to be added, removed and the like. So I was searching for a means to either derive the array from the type or the type from the array.

With lookup types this is exactly possible:

const postTypes = ['article', 'podcast'] as const;
type PostTypes = typeof postTypes[number];

First typeof in combination with as const is used to infer the type of the defined array. as const is important here as Typescript otherwise defines the type as Array<string> instead of an array of string literals.

Using indexed access types/lookup types gives the union of string literals. This is somehow equivalent to using an index in an "normal" Javascript/Typescript to get a specific element like list[0].

Lookup types can be used for more sophisticated use case e.g. like deriving all properties in an object the values of which are e.g. a string:

type Author = {
  firstName: string;
  lastName: string;
};
type Post = {
  title: string;
  description: string;
  views: number;
  author: Author;
};

type PostStringKeys = {
  [P in keyof Post]: Post[P] extends string ? P : never;
}[keyof Post];

Let's quickly break this down:

• P keyof Post gives all keys of Post (title and description).
• Post[P] extends string ? P : never checks if the value of property P in Post is of type string. If true the property name is set as value otherwise the property is not included in the newly created type.
• With the help of lookup types the union of property names/string literals is derived using keyof Post.
  • The set of keys in Post is a superset of the keys of the derived type and can therefor be used as an index

This can be made generic like so:

type KeysOfType<T, K> = { [P in keyof T]: T[P] extends K ? P : never }[keyof T];

type PostStringKeys = KeysOfType<Post, string>;
type PostNumberKeys = KeysOfType<Post, number>;

Compare to the previous example T == Post and K == string. This provides the additional possibility to include properties with different value types like string and Author using unions.

type PostStringAndAuthorKeys = KeysOfType<Post, number | Author>;

The code snippets can be found here.

That's it and as always, thanks for reading.