typed-deep-path-access.ts

type GetArrayIndexValue<A extends [], K extends number | string> = K extends `.${infer I}` ? GetArrayIndexValue<A, I> : K extends number ? A[K] : never

type GetObjectPathValue<O extends object, P extends string> = P extends `${infer A}.${infer B}` ? A extends keyof O ? O[A] extends object ? GetObjectPathValue<O[A], B> : O[A] : never : P extends keyof O ? O[P] extends [] ? GetArrayIndexValue<O[P], P> : O[P] : never

const obj = {
  a: {
    b: 1,
    c: [1, 2],
    d: [
      [12],
      {
        e: 1,
        f: [
          'f'
        ]
      }
    ]
  }
} as const

type objType = typeof obj

/*{
  readonly b: 1;
  readonly c: readonly [1, 2];
  readonly d: readonly [readonly [12], {
    readonly e: 1;
    readonly f: readonly ["f"];
  }];
}*/
type a = GetObjectPathValue<objType, 'a'>

// 1
type b = GetObjectPathValue<objType, 'a.b'>

// readonly [1, 2]
type c = GetObjectPathValue<objType, 'a.c'>

// 1
type c0 = GetObjectPathValue<objType, 'a.c.0'>

// readonly [12]
type d0 = GetObjectPathValue<objType, 'a.d.0'>

// 12
type d00 = GetObjectPathValue<objType, 'a.d.0.0'>

// { readonly e: 1; readonly f: readonly ["f"]; }
type d1 = GetObjectPathValue<objType, 'a.d.1'>

// 1
type d1e = GetObjectPathValue<objType, 'a.d.1.e'>

// readonly ["f"]
type d1f = GetObjectPathValue<objType, 'a.d.1.f'>

// "f"
type d1f0 = GetObjectPathValue<objType, 'a.d.1.f.0'>