Is it possible to create a typescript type from an array?

Question

I often use code such as

export type Stuff = 'something' | 'else'
export const AVAILABLE_STUFF: Stuff[] = ['something', 'else']

This way I can both use the type Stuff, and iterate over all the available stuffs if need be.

This works, but it feels like repeating twice the information. And you have to be careful because an update of either Stuff or AVAILABLE_STUFF also requires an update of its counterpart.

Is there a better way to define the type from the Array, or even to somewhat use the array to type some data ?

Answer 1

One built-in option would be to use an enum instead of the type and array approach.

export enum Stuff {
    something = 'something',
    else = 'else',
}

export const AVAILABLE_STUFF: Stuff[] = Object.values(Stuff);

Another option is to extract the type from the type of AVAILABLE_STUFF. To do this we must force the compiler to infer a tuple of string literals for AVAILABLE_STUFF. This can be done in 3.4 with as const or before 3.4 using an extra function. After AVAILABLE_STUFF is a tuple type we can just use a type query to get the type of the elements:

export const AVAILABLE_STUFF = (<T extends string[]>(...o: T)=> o)('something', 'else'); // typed as ["something", "else"]
// export const AVAILABLE_STUFF = ['something', 'else'] as const; // typed as ["something", "else"]  in 3.4
export type Stuff = typeof AVAILABLE_STUFF[number] //"something" | "else"

A few explanations of the above code. typeof AVAILABLE_STUFF gives us the type of the constant (["something", "else"]) to get the [number] is called a type query and will give us the type of an item in the tuple.

The (<T extends string[]>(...o: T)=> o) is just an IIFE we use to force the compiler to infer a string literal tuple type. It has to be generic as the compiler will only infer literal types and tuples in certain cases (a type parameter with a constraint of string being one of them). The as const version is what I would recommend using when it becomes available as it is more readable.