Typecheck nested props

typelevel-props.ts

/**
 * Extract nested type from interface or object type.
 * 
 * Example usage:
 * ```ts
 * type result = CheckNested<["a", "b"], {a: {b: number}}>;
 * ```
 */
type ExtractNestedType<
  Props extends any[],
  Dict extends object,
> =
  Props extends []
    ? Dict :
  Props extends [infer Last]
    ? Last extends keyof Dict
      ? Dict[Last]
      : never :
  Props extends [infer Head, ...infer Tail]
    ? Head extends keyof Dict
      ? Dict[Head] extends object
        ? ExtractNestedType<Tail, Dict[Head]>
        : never
      : never :
  never;

type SubSplit<Props extends string> =
  Props extends `${infer Prop}.${infer Rest}`
    ? [Prop, ...SubSplit<Rest>]
    : [Props];

type Split<Props extends string> =
  Props extends `${infer Prop}|${infer Rest}`
    ? [SubSplit<Prop>, ...Split<Rest>]
    : [SubSplit<Props>];

type CheckPaths<
  Dict extends object,
  Paths extends any[],
> =
  Paths extends [infer Head, ...infer Tail]
    ? Head extends any[]
      ? ExtractNestedType<Head, Dict> extends never
        ? never
        : CheckPaths<Dict, Tail>
      : never :
    Dict;

type SatisfiesPaths<D, P> = 
  D extends object
    ? P extends string
      ? CheckPaths<D, Split<P>> extends never
        ? never
        : D
      : D
    : D;


type test = SatisfiesPaths<{a: {b: 5}; c: number}, 'a.b'>