TypeScript: Define a union type from an array of strings

Question

I can't be the first person coming across this, but my searches have not turned up any useful leads yet. Would greatly appreciate some expert TypeScript advice.

Say I have an array:

const fruits = ["Apple", "Orange", "Pear"];

and I want to define an object mapping each fruit to some fun facts about it:

interface Facts {
    color: string,
    typicalWeight: number
}

const fruitFacts: { [key: members of fruits]: Facts } = {
    "Apple": { color: "green", typicalWeight: 150 }
    //
}

How do I do that [key: members of fruits] part?

Bonus: How do I enforce that my fruitFacts object exhaust all the keys derived from the array as well, so that it specifies facts for Apples, Oranges, and Pears in the example above.

question from:https://stackoverflow.com/questions/52085454/typescript-define-a-union-type-from-an-array-of-strings

Answer 1

TypeScript 3.4 added const assertions which allow for writing this as:

const fruits = ["Apple", "Orange", "Pear"] as const;
type Fruits = typeof fruits[number]; // "Apple" | "Orange" | "Pear"

With as const TypeScript infers the type of fruits above as readonly["Apple", "Orange", "Pear"]. Previously, it would infer it as string[], preventing typeof fruits[number] from producing the desired union type.