Using parameters to create unions in TypeScript

Is there a standard method to parametrically define a union? I'm searching for a way to generically define something like:

type U<K> = T<K1> | T<K2> | ... | T<Kn> // Where K === (K1 | ... | Kn)

Note: I'm encountering a situation where 

T<U | V> !== T<U> | T<V>
. (A bit opposite of #14107 and #16644)

Edit: When K is a combination of string literals, I've noticed the following pattern:

type U<K> = {[key in K]: T<key>}[K]

However, it doesn't work well with type inference when used as a function argument.

typescripttypes

Answer №1

With the release of TypeScript 2.8, a new method for achieving distribution is introduced using conditional types. The announcement includes an example showcasing this concept:

type Example<T> = T extends any ? T[] : never;

/**
 * Example distributes on 'string | number' to the type
 *
 *    (string extends any ? string[] : never) |
 *    (number extends any ? number[] : never)
 * 
 * which results in
 *
 *    string[] | number[]
 */
type Result = Example<string | number>;

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