Print string union

union-print.ts

type PrintStringUnion<T extends string, S extends string = "|", A extends string = ""> = {
    [P in T]: [T] extends [P] ? `${A}${P}` : PrintStringUnion<Exclude<T, P>, S, `${A}${P}${S}`>
}[T];

type TestUnion = "A" | "B" | "C" | "D" | "E";

type U = PrintStringUnion<TestUnion>;

const test1: U = "A|B|C|D|E"; // OK

// @ts-expect-error
const test2: U = "A|B|C";