Typescript: Create string type from Union of Array-CodePudding

Let's say that I have following type:

type ArrayType = ["AB"] | ["AB", "CD"] | ["X", "Y", "Z"];

I would like to transform it to the following type:

type TransformedArrayType = "AB" | "ABCD" | "XYZ";

How can I accomplish that in the Typescript?

CodePudding user response：

You can join strings recursively using template literal types:

type ArrayType = ["AB"] | ["AB", "CD"] | ["X", "Y", "Z"];

type JoinTuple<T> = T extends [infer Head, ...infer Rest]
  ? `${Head & string}${JoinTuple<Rest>}` : ''

type Test = JoinTuple<["AB", "CD"]> // "ABCD"

type TransformedArrayType = JoinTuple<ArrayType> // "AB" | "ABCD" | "XYZ"

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