[SOLVED] TypeScript array to string literal type

Issue

I currently have both an array of strings and a string literal union type containing the same strings:

const furniture = ['chair', 'table', 'lamp'];
type Furniture = 'chair' | 'table' | 'lamp';

I need both in my application, but I am trying to keep my code DRY. So is there any way to infer one from the other?

I basically want to say something like type Furniture = [any string in furniture array], so there are no duplicate strings.

Solution

TypeScript 3.4+

TypeScript version 3.4 has introduced so-called **const contexts**, which is a way to declare a tuple type as immutable and get the narrow literal type directly (without the need to call a function like shown above).

With this new syntax, we get this nice concise solution:

const furniture = ['chair', 'table', 'lamp'] as const;
type Furniture = typeof furniture[number];

More about the new const contexts is found in this PR as well as in the release notes.

TypeScript 3.0+

With the use of generic rest parameters, there is a way to correctly infer string[] as a literal tuple type and then get the union type of the literals.

It goes like this:

const tuple = <T extends string[]>(...args: T) => args;
const furniture = tuple('chair', 'table', 'lamp');
type Furniture = typeof furniture[number];

More about generic rest parameters

Answered By – ggradnig

Answer Checked By – Robin (BugsFixing Admin)