Utility types to improve JS's native `.split` and `.join`

split-and-join-utility-types.ts

// So let's say we have a string of known parts, Str
type Foo = 'foo'
type Bar = 'bar'
type Baz = 'baz'

type Str = `${Foo}.${Bar}.${Baz}`
const str: Str = 'foo.bar.baz'

// I want to split it into the known parts,
// Which I can do in a nice way with this Split type
type Split<S extends string, D extends string> = S extends ""
  ? []
  : S extends `${infer T}${D}${infer U}`
  ? [T, ...Split<U, D>]
  : [S];

// Now splitStr is typed as a tuple, which is nice
const splitStr = str.split('.') as Split<typeof str, '.'>

// The Join type joins an array to a string literal
type Join<Arr extends any[], Joiner extends string> = 
    Arr["length"] extends 1
        ? `${Arr[0]}`
        : Arr extends [string, ...infer R]
            ? `${Arr[0]}${Joiner}${Join<R, Joiner>}`
            : never

// joined now has type 'foo.bar.baz' instead of string that the native .join method gives us
const joined = splitStr.join('.') as Join<typeof splitStr, '.'>