Typescript: Create string type from Union of Array

Question

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?

Aleksey L. · Accepted Answer · 2021-12-16 14:14:28Z

2

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"

Playground

answered Dec 16, 2021 at 14:14

Aleksey L.

38.4k11 gold badges86 silver badges90 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

jsejcksn Over a year ago

Is there a way in TS to recursively iterate heterogeneous unions without chained conditionals?

Aleksey L. Over a year ago

@jsejcksn could you provide specific example of what you're trying to do?

jsejcksn Over a year ago

I don't have a case: it was just something I wondered in the moment. I think the answer is "no" because the serial conditionals are needed to discriminate and get to leaves.

Collectives™ on Stack Overflow

Typescript: Create string type from Union of Array

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related