type-challenges-solutions/en/medium-join

[utterances bot]

Join

This project is aimed at helping you better understand how the type system works, writing your own utilities, or just having fun with the challenges.

https://ghaiklor.github.io/type-challenges-solutions/en/medium-join.html

[Alexey Berezin]

Great job! Thank you for the article!

TIL: I really like the way you mention references at the end of the article, really helpful for people who are lost with the solution because of some missed TS feature or term

[Alexey Berezin]

In case someone faces the problem with Type instantiation is excessively deep and possibly infinite, you will need to use Tail-Recursion Elimination on Conditional Types

The updated solution can look as following:

type JoinV2<T extends string[], U extends string | number, V extends string = ''> = T extends [
  infer S extends string,
  ...infer R extends string[]
]
  ? JoinV2<R, U, `${V}${V extends '' ? '' : U}${S}`>
  : V;

It also has its limitations around 998 recursive calls

[JanessaTech]

Here is my similar solution:

  type Join<T extends unknown[], U extends string | number = ',', acc extends string = ''> = T extends [infer F extends string, ...infer R]
  ? Join<R, U, `${acc}${F}${R extends [] ? '' : U}`>
  : acc