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Oaphi/union-print.ts

Created October 2, 2021 04:12
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Print string union
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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";
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