TypeScript: Define a union type from an array of strings
TypescriptTypescript Problem Overview
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.
Typescript Solutions
Solution 1 - Typescript
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.
Solution 2 - Typescript
It can be done but first you need an extra function to help infer the string literal type for the array elements. By default Typescript will infer string[]
for an array even if it is a constant. After we have an array of string literal types we can just use a type query to get the desired type
function stringLiteralArray<T extends string>(a: T[]) {
return a;
}
const fruits = stringLiteralArray(["Apple", "Orange", "Pear"]);
type Fruits = typeof fruits[number]
Since 3.4 you can also use a const type assertion instead of the stringLiteralArray
function:
const fruits = ["Apple", "Orange", "Pear"] as const;
type Fruits = typeof fruits[number]