TypeScript: Define a union type from an array of strings

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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.

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