Typescript helper types for working with nested object keys in dot notation (aka formik ass shit)

example-usage.ts

interface A {
  foo: {
    bar: number;
  };

  hee: {
    hoo?: string;
    haa?: {
      test: number;
    }[];
  };

  lol: boolean;
}


type T = NestedKeyOf<A>;
//   ^? type T = "lol" | "foo.bar" | "hee.hoo" | "hee.haa" | "hee.haa.length" | `hee.haa.${number}.test`

type E1 = NestedObjectKeyType<A, 'foo.bar'>;
//   ^? type E1 = number
type E2 = NestedObjectKeyType<A, 'hee.haa.3'>;
//   ^? type E2 = {  test: number;}[] | undefined
type E3 = NestedObjectKeyType<A, 'hee.haa.3.test'>;
//   ^? type E3 = number

nested-object-key.ts

type DotPrefix<T extends string> = T extends '' ? '' : `.${T}`;

type NestedKeyOf<T extends object> = (
  T extends object
    ? {
        // @ts-ignore-next-line
        [K in Exclude<keyof T, symbol>]: `${K}${DotPrefix<NestedKeyOf<T[K]>>}`;
      }[Exclude<keyof T, symbol>]
    : ''
) extends infer D
  ? Extract<D, string>
  : never;

type NestedObjectKeyType<
  T,
  K extends string,
> = K extends `${infer P}.${infer R}`
  ? P extends keyof T
    ? T[P] extends Array<infer U>
      ? NestedObjectKeyType<U, R>
      : NestedObjectKeyType<T[P], R>
    : P extends `${number}`
    ? T extends Array<infer U>
      ? NestedObjectKeyType<U, R>
      : NestedObjectKeyType<T, R>
    : never
  : K extends keyof T
  ? T[K]
  : K extends `${number}`
  ? T
  : never;