General-purpose utility types

utilityTypes.ts

type Invert<T> = { [ P in keyof T as Extract<T[P], string | number | symbol> ] : P };
  
type test1 = Invert<{ a: "ia", b: "ib", c: 1 }>; //{ ia: "a"; ib: "b"; 1: "c"; }

type DeepPath<T extends string[], U> = T extends [infer F, ...infer L]
  ? F extends string
    ? { [P in F]: L extends string[] ? DeepPath<L,U> : never }
    : {}
  : U;

type test2 = DeepPath<["a", "b", "c"], ()=> number>; //{ a: { b: { c: () => number } } }

//dep: type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never
type MergeUnionOfObjects<U> = { [ P in keyof UnionToIntersection<U> ] : UnionToIntersection<U>[P] };

type test3 = MergeUnionOfObjects<{a: () => string} | {b: () => number}>; //{a: () => string; b: () => number;}

type RecursiveRecord<K extends keyof any, T> = {
  [ P in K ] : T | RecursiveRecord<K, T>;
}

let i : RecursiveRecord<string, string> = {
  "p": {
    "p": {
      "o": "ok", //ok
      "n": 1 //error
    }
  }
}