How to transform union type to tuple type

Question

For example i have type

type abc = \'a\' | \'b\' | \'c\';

How to make tuple type that contains all elements of the union in compile time?

Answer 1

It's easy to convert from a tuple type to a union type; for example, see this question. But the opposite, converting from a union to a tuple is one of those Truly Bad Ideas that you shouldn't try to do. Let's do it first and scold ourselves later:

// oh boy don't do this
type UnionToIntersection =
  (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never
type LastOf =
  UnionToIntersection T : never> extends () => (infer R) ? R : never

// TS4.0+
type Push = [...T, V];

// TS4.1+
type TuplifyUnion, N = [T] extends [never] ? true : false> =
  true extends N ? [] : Push>, L>

type abc = 'a' | 'b' | 'c';
type t = TuplifyUnion; // ["a", "b", "c"] 

Playground link

That kind of works, but I really really REALLY recommend not using it for any official purpose or in any production code. Here's why:

• You can't rely on the ordering of a union type. It's an implementation detail of the compiler; since X | Y is equivalent to Y | X, the compiler feels free to change one to the other. And sometimes it does:

    type TypeTrue1A = TuplifyUnion; // [true, 1, "a"]