TypeScript: Define a union type from an array of strings

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]

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.