TypeScript snippets for nested object types

deepObjectType.ts

export type DeepKeys<T, K = keyof T, R extends string[] = []> = 
  K extends keyof T ? 
    T[K] extends object ? 
      T[K] extends unknown[] | [unknown, ...unknown[]] ? 
        [...R, K] | [...R, K, KeyOfArrayLike<T[K]>]
      : [...R, K] | [...R, K, ...DeepKeys<T[K], keyof T[K], R>]
    : [...R, K]
  : []

type KeyOfArrayLike<T> = 
  T extends [] ?
    never
  : T extends [unknown, ...infer R] ?
    R['length'] | KeyOfArrayLike<R>
  : T extends unknown[] ?
    number
  : never
  
export type DeepValue<T, P extends (string | number)[]> = 
  P extends [infer K, ...infer Rest extends (string | number)[]] ?
    K extends keyof T ?
      Rest extends [] ?
        T[K]
      : DeepValue<T[K], Rest>
    : never
  : never

DeepKeys<T> resolve every possible key path to the leaf properties in nested object.

type U = DeepKeys<{
    y: {
        z: 'a'
        h: [number, number]
        arr: string[]
    }
    a: number
}>
/*
  ["a"] | ["y"] | ["y", "z"] | ["y", "h"] | ["y", "h", 0 | 1] | ["y", "arr"] | ["y", "arr", number]
*/

DeepValues<T, P> resolve the type of value of nested properties of T with path of P.

interface X {
    y: {
        z: 'a'
        h: [number, 'b']
        arr: string[]
    }
    a: number
}

DeepValue<X, ['y', 'h', 0]> // number
DeepValue<X, ['y', 'arr', 0]> // string
DeepValue<X, ['y', 'z']> // a